HTML Image Maps

With HTML image maps, you can create clickable areas on an image.

Image Maps

The HTML <map> tag defines an image map. An image map is an image with clickable areas. The areas are defined with one or more <area> tags.

Try to click on the computer, phone, or the cup of coffee in the image below:

Example

Here is the HTML source code for the image map above:

How Does it Work?

The idea behind an image map is that you should be able to perform different actions depending on where in the image you click.

To create an image map you need an image, and some HTML code that describes the clickable areas.

The Image

The image is inserted using the <img> tag. The only difference from other images is that you must add a usemap attribute:

The usemap value starts with a hash tag # followed by the name of the image map, and is used to create a relationship between the image and the image map.

Tip: You can use any image as an image map!

Create Image Map

Then, add a <map> element.

The <map> element is used to create an image map, and is linked to the image by using the required name attribute:

The name attribute must have the same value as the <img>'s usemap attribute .

The Areas

Then, add the clickable areas.

A clickable area is defined using an <area> element.

Shape

You must define the shape of the clickable area, and you can choose one of these values:

rect - defines a rectangular region
circle - defines a circular region
poly - defines a polygonal region
default - defines the entire region

You must also define some coordinates to be able to place the clickable area onto the image.

Shape="rect"

The coordinates for shape="rect" come in pairs, one for the x-axis and one for the y-axis.

So, the coordinates 34,44 is located 34 pixels from the left margin and 44 pixels from the top:

The coordinates 270,350 is located 270 pixels from the left margin and 350 pixels from the top:

Now we have enough data to create a clickable rectangular area:

Example

This is the area that becomes clickable and will send the user to the page "computer.htm":

Shape="circle"

To add a circle area, first locate the coordinates of the center of the circle:

337,300

Then specify the radius of the circle:

44 pixels

Now you have enough data to create a clickable circular area:

Example

This is the area that becomes clickable and will send the user to the page "coffee.htm":

Shape="poly"

The shape="poly" contains several coordinate points, which creates a shape formed with straight lines (a polygon).

This can be used to create any shape.

Like maybe a croissant shape!

How can we make the croissant in the image below become a clickable link?

We have to find the x and y coordinates for all edges of the croissant:

The coordinates come in pairs, one for the x-axis and one for the y-axis:

Example

This is the area that becomes clickable and will send the user to the page "croissant.htm":

Image Map and JavaScript

A clickable area can also trigger a JavaScript function.

Add a click event to the <area> element to execute a JavaScript function:

Example

Here, we use the onclick attribute to execute a JavaScript function when the area is clicked:

<map name="workmap">
<area shape="circle" coords="337,300,44" href="coffee.htm" onclick="myFunction()">
</map>

<script>
function myFunction() {
alert("You clicked the coffee cup!");
}
</script>

Chapter Summary

Use the HTML <map> element to define an image map
Use the HTML <area> element to define the clickable areas in the image map
Use the HTML usemap attribute of the <img> element to point to an image map

HTML Image Tags

Tag	Description
	Defines an image
	Defines an image map
	Defines a clickable area inside an image map
	Defines a container for multiple image resources

For a complete list of all available HTML tags, visit our .

Go is a popular programming language.

Go is used to create computer programs.

Examples in Each Chapter

Our "Try it Yourself" editor makes it easy to learn Go. You can edit Go code and view the result in your browser.

Example

package main
import ("fmt")

func main() {
fmt.Println("Hello World!")
}

Click on the "Try it Yourself" button to see how it works.Top of Form

est ourself With E

My Learning

Track your progress with the free "My Learning" program here at W3Schools.

This is an optional feature. You can study at W3Schools without using My Learning.

What is Go?

Go is a cross-platform, open source programming language
Go can be used to create high-performance applications
Go is a fast, statically typed, compiled language that feels like a dynamically typed, interpreted language
Go was developed at Google by Robert Griesemer, Rob Pike, and Ken Thompson in 2007
Go's syntax is similar to C++

What is Go Used For?

Web development (server-side)
Developing network-based programs
Developing cross-platform enterprise applications
Cloud-native development

Why Use Go?

Go is fun and easy to learn
Go has fast run time and compilation time
Go supports concurrency
Go has memory management
Go works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)

Go Compared to Python and C++

Go	Python	C++
Statically typed	Dynamically typed	Statically typed
Fast run time	Slow run time	Fast run time
Compiled	Interpreted	Compiled
Fast compile time	Interpreted	Slow compile time
Supports concurrency through goroutines and channel	No built-in concurrency mechanism	Supports concurrency through threads
Has automatic garbage collection	Has automatic garbage collection	Does not have automatic garbage collection
Does not support classes and objects	Has classes and objects	Has classes and objects
Does not support inheritance	Supports inheritance	Supports inheritance

Notes:

· Compilation time refers to translating the code into an executable program

· Concurrency is performing multiple things out-of-order, or at the same time, without affecting the final outcome

· Statically typed means that the variable types are known at compile time

Get Started

This tutorial will teach you the basics of Go.

It is not necessary to have any prior programming experience.

Go Getting Started

Go Get Started

To start using Go, you need two things:

A text editor, like VS Code, to write Go code
A compiler, like GCC, to translate the Go code into a language that the computer will understand

There are many text editors and compilers to choose from. In this tutorial, we will use an IDE (see below).

Go Install

You can find the relevant installation files at .

Follow the instructions related to your operating system. To check if Go was installed successfully, you can run the following command in a terminal window:

go version

Which should show the version of your Go installation.

Go Install IDE

An IDE (Integrated Development Environment) is used to edit AND compile the code.

Popular IDE's include Visual Studio Code (VS Code), Vim, Eclipse, and Notepad. These are all free, and they can be used to both edit and debug Go code.

Note: Web-based IDE's can work as well, but functionality is limited.

We will use VS Code in our tutorial, which we believe is a good place to start.

You can find the latest version of VS Code at .

Go Quickstart

Let's create our first Go program.

Launch the VS Code editor
Open the extension manager or alternatively, press Ctrl + Shift + x
In the search box, type "go" and hit enter
Find the Go extension by the GO team at Google and install the extension
After the installation is complete, open the command palette by pressing Ctrl + Shift + p
Run the Go: Install/Update Tools command
Select all the provided tools and click OK

VS Code is now configured to use Go.

Open up a terminal window and type:

go mod init example.com/hello

Do not worry if you do not understand why we type the above command. Just think of it as something that you always do, and that you will learn more about in a later chapter.

Create a new file (File > New File). Copy and paste the following code and save the file as helloworld.go (File > Save As):

helloworld.go

package main
import ("fmt")

func main() {
fmt.Println("Hello World!")
}

Now, run the code: Open a terminal in VS Code and type:

go run .helloworld.go


      Hello World!

Congratulations! You have now written and executed your first Go program.

If you want to save the program as an executable, type and run:

go build .helloworld.go

Learning Go At W3Schools

When learning Go at W3Schools.com, you can use our "Try it Yourself" tool. It shows both the code and the result. This will make it easier for you to understand every part as we move forward:

helloworld.go

Code:

package main
import ("fmt")

func main() {
fmt.Println("Hello World!")
}

Result:


        Hello World!

Go Syntax

A Go file consists of the following parts:

Package declaration
Import packages
Functions
Statements and expressions

Look at the following code, to understand it better:

Example

package main
import ("fmt")

func main() {
fmt.Println("Hello World!")
}

Example explained

Line 1: In Go, every program is part of a package. We define this using the package keyword. In this example, the program belongs to the main package.

Line 2: import ("fmt") lets us import files included in the fmt package.

Line 3: A blank line. Go ignores white space. Having white spaces in code makes it more readable.

Line 4: func main() {} is a function. Any code inside its curly brackets {} will be executed.

Line 5: fmt.Println() is a function made available from the fmt package. It is used to output/print text. In our example it will output "Hello World!".

Note: In Go, any executable code belongs to the main package.

Go Statements

fmt.Println("Hello World!") is a statement.

In Go, statements are separated by ending a line (hitting the Enter key) or by a semicolon ";".

Hitting the Enter key adds ";" to the end of the line implicitly (does not show up in the source code).

The left curly bracket { cannot come at the start of a line.

Run the following code and see what happens:

Example

package main
import ("fmt")

func main()
{
fmt.Println("Hello World!")
}

Go Compact Code

You can write more compact code, like shown below (this is not recommended because it makes the code more difficult to read):

Example

package main; import ("fmt"); func main() { fmt.Println("Hello World!");}

Go Comments

A comment is a text that is ignored upon execution.

Comments can be used to explain the code, and to make it more readable.

Comments can also be used to prevent code execution when testing an alternative code.

Go supports single-line or multi-line comments.

Go Single-line Comments

Single-line comments start with two forward slashes (//).

Any text between // and the end of the line is ignored by the compiler (will not be executed).

Example

// This is a comment
package main
import ("fmt")

func main() {
// This is a comment
fmt.Println("Hello World!")
}

The following example uses a single-line comment at the end of a code line:

Example

package main
import ("fmt")

func main() {
fmt.Println("Hello World!") // This is a comment
}

Go Multi-line Comments

Multi-line comments start with /* and ends with */.

Any text between /* and */ will be ignored by the compiler:

Example

package main
import ("fmt")

func main() {
  /* The code below will print Hello World
  to the screen, and it is amazing */
  fmt.Println("Hello World!")
}

Tip: It is up to you which you want to use. Normally, we use // for short comments, and /* */ for longer comments.

Comment to Prevent Code Execution

You can also use comments to prevent the code from being executed.

The commented code can be saved for later reference and troubleshooting.

Example

package main
import ("fmt")

func main() {
fmt.Println("Hello World!")
// fmt.Println("This line does not execute")
}

Variables are containers for storing data values.

Go Variable Types

In Go, there are different types of variables, for example:

int- stores integers (whole numbers), such as 123 or -123
float32- stores floating point numbers, with decimals, such as 19.99 or -19.99
string - stores text, such as "Hello World". String values are surrounded by double quotes
bool- stores values with two states: true or false

More about different variable types, will be explained in the chapter.

Declaring (Creating) Variables

In Go, there are two ways to declare a variable:

1. With the `var` keyword:

Use the var keyword, followed by variable name and type:

Syntax

var variablename type = value

Note: You always have to specify either type or value (or both).

2. With the `:=` sign:

Use the := sign, followed by the variable value:

Syntax

variablename := value

Note: In this case, the type of the variable is inferred from the value (means that the compiler decides the type of the variable, based on the value).

Note: It is not possible to declare a variable using :=, without assigning a value to it.

Variable Declaration With Initial Value

If the value of a variable is known from the start, you can declare the variable and assign a value to it on one line:

Example

package main
import ("fmt")

func main() {
  var student1 string = "John" //type is string
  var student2 = "Jane" //type is inferred
  x := 2 //type is inferred

  fmt.Println(student1)
  fmt.Println(student2)
  fmt.Println(x)
}

Note: The variable types of student2 and x is inferred from their values.

Variable Declaration Without Initial Value

In Go, all variables are initialized. So, if you declare a variable without an initial value, its value will be set to the default value of its type:

Example

package main
import ("fmt")

func main() {
  var a string
  var b int
  var c bool

  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Example explained

In this example there are 3 variables:

These variables are declared but they have not been assigned initial values.

By running the code, we can see that they already have the default values of their respective types:

a is ""
b is 0
c is false

Value Assignment After Declaration

It is possible to assign a value to a variable after it is declared. This is helpful for cases the value is not initially known.

Example

package main
import ("fmt")

func main() {
  var student1 string
  student1 = "John"
  fmt.Println(student1)
}

Note: It is not possible to declare a variable using ":=" without assigning a value to it.

Difference Between var and :=

There are some small differences between the var var :=:

var	:=
Can be used inside and outside of functions	Can only be used inside functions
Variable declaration and value assignment can be done separately	Variable declaration and value assignment cannot be done separately (must be done in the same line)

Example

This example shows declaring variables outside of a function, with the var keyword:

package main
import ("fmt")

var a int
var b int = 2
var c = 3

func main() {
  a = 1
  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Example

Since := is used outside of a function, running the program results in an error.

package main
import ("fmt")

a := 1

func main() {
fmt.Println(a)
}

Result:


          ./prog.go:5:1: syntax error: non-declaration statement outside function body

Variables are containers for storing data values.

Go Variable Types

In Go, there are different types of variables, for example:

int- stores integers (whole numbers), such as 123 or -123
float32- stores floating point numbers, with decimals, such as 19.99 or -19.99
string - stores text, such as "Hello World". String values are surrounded by double quotes
bool- stores values with two states: true or false

More about different variable types, will be explained in the chapter.

Declaring (Creating) Variables

In Go, there are two ways to declare a variable:

1. With the `var` keyword:

Use the var keyword, followed by variable name and type:

Syntax

var variablename type = value

Note: You always have to specify either type or value (or both).

2. With the `:=` sign:

Use the := sign, followed by the variable value:

Syntax

variablename := value

Note: In this case, the type of the variable is inferred from the value (means that the compiler decides the type of the variable, based on the value).

Note: It is not possible to declare a variable using :=, without assigning a value to it.

Variable Declaration With Initial Value

If the value of a variable is known from the start, you can declare the variable and assign a value to it on one line:

Example

Note: The variable types of student2 and x is inferred from their values.

Variable Declaration Without Initial Value

In Go, all variables are initialized. So, if you declare a variable without an initial value, its value will be set to the default value of its type:

Example

package main
import ("fmt")

func main() {
  var a string
  var b int
  var c bool

  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Example explained

In this example there are 3 variables:

These variables are declared but they have not been assigned initial values.

By running the code, we can see that they already have the default values of their respective types:

a is ""
b is 0
c is false

Value Assignment After Declaration

It is possible to assign a value to a variable after it is declared. This is helpful for cases the value is not initially known.

Example

package main
import ("fmt")

func main() {
  var student1 string
  student1 = "John"
  fmt.Println(student1)
}

Note: It is not possible to declare a variable using ":=" without assigning a value to it.

Difference Between var and :=

There are some small differences between the var var :=:

var	:=
Can be used inside and outside of functions	Can only be used inside functions
Variable declaration and value assignment can be done separately	Variable declaration and value assignment cannot be done separately (must be done in the same line)

Example

This example shows declaring variables outside of a function, with the var keyword:

package main
import ("fmt")

var a int
var b int = 2
var c = 3

func main() {
  a = 1
  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Example

Since := is used outside of a function, running the program results in an error.

package main
import ("fmt")

a := 1

func main() {
fmt.Println(a)
}

Result:


          ./prog.go:5:1: syntax error: non-declaration statement outside function body

Go Exercises

Test Yourself With Exercises

Exercise:

Create a variable named myNum and assign the value 50 to it.

package main   
import ("fmt") 

func main() {
  var  = 
  fmt.Println()
}

Go Multiple Variable Declaration

In Go, it is possible to declare multiple variables in the same line.

Example

This example shows how to declare multiple variables in the same line:

package main
import ("fmt")

func main() {
  var a, b, c, d int = 1, 3, 5, 7

  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
  fmt.Println(d)
}

Note: If you use the type keyword, it is only possible to declare one type of variable per line.

If the type keyword is not specified, you can declare different types of variables in the same line:

Example

package main
import ("fmt")

func main() {
  var a, b = 6, "Hello"
  c, d := 7, "World!"

  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
  fmt.Println(d)
}

Go Variable Declaration in a Block

Multiple variable declarations can also be grouped together into a block for greater readability:

Example

package main
import ("fmt")

func main() {
   var (
     a int
     b int = 1
     c string = "hello"
   )

  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Go Variable Naming Rules

A variable can have a short name (like x and y) or a more descriptive name (age, price, carname, etc.).

Go variable naming rules:

A variable name must start with a letter or an underscore character (_)
A variable name cannot start with a digit
A variable name can only contain alpha-numeric characters and underscores (a-z, A-Z, 0-9, and _ )
Variable names are case-sensitive (age, Age and AGE are three different variables)
There is no limit on the length of the variable name
A variable name cannot contain spaces
The variable name cannot be any Go keywords

Multi-Word Variable Names

Variable names with more than one word can be difficult to read.

There are several techniques you can use to make them more readable:

Camel Case

Each word, except the first, starts with a capital letter:

myVariableName = "John"

Pascal Case

Each word starts with a capital letter:

MyVariableName = "John"

Snake Case

Each word is separated by an underscore character:

my_variable_name = "John"

Go Constants

If a variable should have a fixed value that cannot be changed, you can use the const keyword.

The const keyword declares the variable as "constant", which means that it is unchangeable and read-only.

Syntax

const CONSTNAME type = value

Note: The value of a constant must be assigned when you declare it.

Declaring a Constant

Here is an example of declaring a constant in Go:

Example

package main
import ("fmt")

const PI = 3.14

func main() {
fmt.Println(PI)
}

Constant Rules

Constant names follow the same naming rules as
Constant names are usually written in uppercase letters (for easy identification and differentiation from variables)
Constants can be declared both inside and outside of a function

Constant Types

There are two types of constants:

Typed constants
Untyped constants

Typed Constants

Typed constants are declared with a defined type:

Example

package main
import ("fmt")

const A int = 1

func main() {
fmt.Println(A)
}

Untyped Constants

Untyped constants are declared without a type:

Example

package main
import ("fmt")

const A = 1

func main() {
fmt.Println(A)
}

Note: In this case, the type of the constant is inferred from the value (means the compiler decides the type of the constant, based on the value).

Constants: Unchangeable and Read-only

When a constant is declared, it is not possible to change the value later:

Example

package main
import ("fmt")

func main() {
  const A = 1
  A = 2
  fmt.Println(A)
}

Result:


        ./prog.go:8:7: cannot assign to A

Multiple Constants Declaration

Multiple constants can be grouped together into a block for readability:

Example

package main
import ("fmt")

const (
  A int = 1
  B = 3.14
  C = "Hi!"
)

func main() {
  fmt.Println(A)
  fmt.Println(B)
  fmt.Println(C)
}

Go has three functions to output text:

Print()
Println()
Printf()

The Print() Function

The Print() function prints its arguments with their default format.

Example

Print the values of i and j:

package main
import ("fmt")

func main() {
  var i,j string = "Hello","World"

  fmt.Print(i)
  fmt.Print(j)
}

Result:


      HelloWorld

Example

If we want to print the arguments in new lines, we need to use n.

package main
import ("fmt")

func main() {
  var i,j string = "Hello","World"

  fmt.Print(i, "n")
  fmt.Print(j, "n")
}

Result:


      Hello
 
      World

Tip: n creates new lines.

Example

It is also possible to only use one Print() for printing multiple variables.

package main
import ("fmt")

func main() {
var i,j string = "Hello","World"

fmt.Print(i, "n",j)
}

Result:


      Hello
 
      World

Example

If we want to add a space between string arguments, we need to use " ":

package main
import ("fmt")

func main() {
var i,j string = "Hello","World"

fmt.Print(i, " ", j)
}

Result:


      Hello World

Example

Print() inserts a space between the arguments if neither are strings:

package main
import ("fmt")

func main() {
var i,j = 10,20

fmt.Print(i,j)
}

Result:


      10 20

The Println() Function

The Println() function is similar to Print() with the difference that a whitespace is added between the arguments, and a newline is added at the end:

Example

package main
import ("fmt")

func main() {
var i,j string = "Hello","World"

fmt.Println(i,j)
}

Result:


      Hello World

The Printf() Function

The Printf() function first formats its argument based on the given formatting verb and then prints them.

Here we will use two formatting verbs:

%v is used to print the value of the arguments
%T is used to print the type of the arguments

Example

package main
import ("fmt")

func main() {
var i string = "Hello"
var j int = 15

fmt.Printf("i has value: %v and type: %Tn", i, i)
fmt.Printf("j has value: %v and type: %T", j, j)
}

Result:

i has value: Hello and type: string
j has value: 15 and type: int

Tip: Look at all the formatting verbs in the chapter.

Go has three functions to output text:

Print()
Println()
Printf()

The Print() Function

The Print() function prints its arguments with their default format.

Example

Print the values of i and j:

package main
import ("fmt")

func main() {
  var i,j string = "Hello","World"

  fmt.Print(i)
  fmt.Print(j)
}

Result:


      HelloWorld

Example

If we want to print the arguments in new lines, we need to use n.

package main
import ("fmt")

func main() {
  var i,j string = "Hello","World"

  fmt.Print(i, "n")
  fmt.Print(j, "n")
}

Result:


      Hello
 
      World

Tip: n creates new lines.

Example

It is also possible to only use one Print() for printing multiple variables.

package main
import ("fmt")

func main() {
var i,j string = "Hello","World"

fmt.Print(i, "n",j)
}

Result:


      Hello
 
      World

Example

If we want to add a space between string arguments, we need to use " ":

package main
import ("fmt")

func main() {
var i,j string = "Hello","World"

fmt.Print(i, " ", j)
}

Result:


      Hello World

Example

Print() inserts a space between the arguments if neither are strings:

package main
import ("fmt")

func main() {
var i,j = 10,20

fmt.Print(i,j)
}

Result:


      10 20

The Println() Function

The Println() function is similar to Print() with the difference that a whitespace is added between the arguments, and a newline is added at the end:

Example

package main
import ("fmt")

func main() {
var i,j string = "Hello","World"

fmt.Println(i,j)
}

Result:


      Hello World

The Printf() Function

The Printf() function first formats its argument based on the given formatting verb and then prints them.

Here we will use two formatting verbs:

%v is used to print the value of the arguments
%T is used to print the type of the arguments

Example

package main
import ("fmt")

func main() {
var i string = "Hello"
var j int = 15

fmt.Printf("i has value: %v and type: %Tn", i, i)
fmt.Printf("j has value: %v and type: %T", j, j)
}

Result:

i has value: Hello and type: string
j has value: 15 and type: int

Tip: Look at all the formatting verbs in the chapter.

Variables are containers for storing data values.

Go Variable Types

In Go, there are different types of variables, for example:

int- stores integers (whole numbers), such as 123 or -123
float32- stores floating point numbers, with decimals, such as 19.99 or -19.99
string - stores text, such as "Hello World". String values are surrounded by double quotes
bool- stores values with two states: true or false

More about different variable types, will be explained in the Go Data Types chapter.

Declaring (Creating) Variables

In Go, there are two ways to declare a variable:

1. With the var keyword:

Use the var keyword, followed by variable name and type:

Syntax

var variablename type = value

Note: You always have to specify either type or value (or both).

2. With the := sign:

Use the := sign, followed by the variable value:

Syntax

variablename := value

Note: In this case, the type of the variable is inferred from the value (means that the compiler decides the type of the variable, based on the value).

Note: It is not possible to declare a variable using :=, without assigning a value to it.

Variable Declaration With Initial Value

If the value of a variable is known from the start, you can declare the variable and assign a value to it on one line:

Example

Try it Yourself »

Note: The variable types of student2 and x is inferred from their values.

Variable Declaration Without Initial Value

In Go, all variables are initialized. So, if you declare a variable without an initial value, its value will be set to the default value of its type:

Example

package main
import ("fmt")

func main() {
  var a string
  var b int
  var c bool

  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Try it Yourself »

Example explained

In this example there are 3 variables:

These variables are declared but they have not been assigned initial values.

By running the code, we can see that they already have the default values of their respective types:

a is ""
b is 0
c is false

Value Assignment After Declaration

It is possible to assign a value to a variable after it is declared. This is helpful for cases the value is not initially known.

Example

package main
import ("fmt")

func main() {
  var student1 string
  student1 = "John"
  fmt.Println(student1)
}

Note: It is not possible to declare a variable using ":=" without assigning a value to it.

Difference Between var and :=

There are some small differences between the var var :=:

var	:=
Can be used inside and outside of functions	Can only be used inside functions
Variable declaration and value assignment can be done separately	Variable declaration and value assignment cannot be done separately (must be done in the same line)

Example

This example shows declaring variables outside of a function, with the var keyword:

package main
import ("fmt")

var a int
var b int = 2
var c = 3

func main() {
  a = 1
  fmt.Println(a)
  fmt.Println(b)
  fmt.Println(c)
}

Example

Since := is used outside of a function, running the program results in an error.

package main
import ("fmt")

a := 1

func main() {
fmt.Println(a)
}

Result:

./prog.go:5:1: syntax error: non-declaration statement outside function body

Go Data Types

Data type is an important concept in programming. Data type specifies the size and type of variable values.

Go is statically typed, meaning that once a variable type is defined, it can only store data of that type.

Go has three basic data types:

bool: represents a boolean value and is either true or false
Numeric: represents integer types, floating point values, and complex types
string: represents a string value

Example

This example shows some of the different data types in Go:

package main
import ("fmt")

func main() {
  var a bool = true     // Boolean
  var b int = 5         // Integer
  var c float32 = 3.14 // Floating point number
  var d string = "Hi!"  // String

  fmt.Println("Boolean: ", a)
  fmt.Println("Integer: ", b)
  fmt.Println("Float:   ", c)
  fmt.Println("String: ", d)
}

Go Data Types

Data type is an important concept in programming. Data type specifies the size and type of variable values.

Go is statically typed, meaning that once a variable type is defined, it can only store data of that type.

Go has three basic data types:

bool: represents a boolean value and is either true or false
Numeric: represents integer types, floating point values, and complex types
string: represents a string value

Example

This example shows some of the different data types in Go:

Go Exercises

Test Yourself With Exercises

Exercise:

Add the correct data type for the following variables:

package main   
import ("fmt") 

func main() {
  var myNum   = 90
  var myWord   = "Hello"
  var myBool   = true
}

Boolean Data Type

A boolean data type is declared with the bool keyword and can only take the values true or false.

The default value of a boolean data type is false.

Example

This example shows some different ways to declare Boolean variables:

package main
import ("fmt")

func main() {
  var b1 bool = true // typed declaration with initial value
  var b2 = true // untyped declaration with initial value
  var b3 bool // typed declaration without initial value
  b4 := true // untyped declaration with initial value

  fmt.Println(b1) // Returns true
  fmt.Println(b2) // Returns true
  fmt.Println(b3) // Returns false
  fmt.Println(b4) // Returns true
}

Note: Boolean values are mostly used for conditional testing which you will learn more about in the chapter.

Go Integer Data Types

Integer data types are used to store a whole number without decimals, like 35, -50, or 1345000.

The integer data type has two categories:

Signed integers - can store both positive and negative values
Unsigned integers - can only store non-negative values

Tip: The default type for integer is int. If you do not specify a type, the type will be int.

Signed Integers

Signed integers, declared with one of the int keywords, can store both positive and negative values:

Example

package main
import ("fmt")

func main() {
  var x int = 500
var y int = -4500
  fmt.Printf("Type: %T, value: %v", x, x)
  fmt.Printf("Type: %T, value: %v", y, y)
}

Go has five keywords/types of signed integers:

Type	Size	Range
`int`	Depends on platform: 32 bits in 32 bit systems and 64 bit in 64 bit systems	-2147483648 to 2147483647 in 32 bit systems and -9223372036854775808 to 9223372036854775807 in 64 bit systems
`int8`	8 bits/1 byte	-128 to 127
`int16`	16 bits/2 byte	-32768 to 32767
`int32`	32 bits/4 byte	-2147483648 to 2147483647
`int64`	64 bits/8 byte	-9223372036854775808 to 9223372036854775807

Unsigned Integers

Unsigned integers, declared with one of the uint keywords, can only store non-negative values:

Example

package main
import ("fmt")

func main() {
  var x uint = 500
var y uint = 4500
  fmt.Printf("Type: %T, value: %v", x, x)
  fmt.Printf("Type: %T, value: %v", y, y)
}

Go has five keywords/types of unsigned integers:

Type	Size	Range
`uint`	Depends on platform: 32 bits in 32 bit systems and 64 bit in 64 bit systems	0 to 4294967295 in 32 bit systems and 0 to 18446744073709551615 in 64 bit systems
`uint8`	8 bits/1 byte	0 to 255
`uint16`	16 bits/2 byte	0 to 65535
`uint32`	32 bits/4 byte	0 to 4294967295
`uint64`	64 bits/8 byte	0 to 18446744073709551615

Which Integer Type to Use?

The type of integer to choose, depends on the value the variable has to store.

Example

This example will result in an error because 1000 is out of range for int8 (which is from -128 to 127):

package main
import ("fmt")

func main() {
var x int8 = 1000
fmt.Printf("Type: %T, value: %v", x, x)
}

Result:


      ./prog.go:5:7: constant 1000 overflows int8

Go Float Data Types

The float data types are used to store positive and negative numbers with a decimal point, like 35.3, -2.34, or 3597.34987.

The float data type has two keywords:

Type	Size	Range
`float32`	32 bits	-3.4e+38 to 3.4e+38.
`float64`	64 bits	-1.7e+308 to +1.7e+308.

Tip: The default type for float is float64. If you do not specify a type, the type will be float64.

The float32 Keyword

Example

This example shows how to declare some variables of type float32:

package main
import ("fmt")

func main() {
var x float32 = 123.78
var y float32 = 3.4e+38
fmt.Printf("Type: %T, value: %vn", x, x)
fmt.Printf("Type: %T, value: %v", y, y)
}

The float64 Keyword

The float64 data type can store a larger set of numbers than float32.

Example

This example shows how to declare a variable of type float64:

package main
import ("fmt")

func main() {
var x float64 = 1.7e+308
fmt.Printf("Type: %T, value: %v", x, x)
}

Which Float Type to Use?

The type of float to choose, depends on the value the variable has to store.

Example

This example will result in an error because 3.4e+39 is out of range for float32:

package main
import ("fmt")

func main() {
var x float32= 3.4e+39
fmt.Println(x)
}

Result:


      ./prog.go:5:7: constant 3.4e+39 overflows float32

String Data Type

The string data type is used to store a sequence of characters (text). String values must be surrounded by double quotes:

Example

package main
import ("fmt")

func main() {
  var txt1 string = "Hello!"
  var txt2 string
  txt3 := "World 1"

  fmt.Printf("Type: %T, value: %vn", txt1, txt1)
  fmt.Printf("Type: %T, value: %vn", txt2, txt2)
  fmt.Printf("Type: %T, value: %vn", txt3, txt3)
}

Result:


      Type: string, value: Hello! 

      Type: string, value:  

      Type: string, value: World 1

Go Arrays

Arrays are used to store multiple values of the same type in a single variable, instead of declaring separate variables for each value.

Declare an Array

In Go, there are two ways to declare an array:

1. With the `var` keyword:

Syntax

var array_name = [length]datatype{values} // here length is defined

or

var array_name = [...]datatype{values} // here length is inferred

2. With the `:=` sign:

Syntax

array_name := [length]datatype{values} // here length is defined

or

array_name := [...]datatype{values} // here length is inferred

Note: The length specifies the number of elements to store in the array. In Go, arrays have a fixed length. The length of the array is either defined by a number or is inferred (means that the compiler decides the length of the array, based on the number of values).

Array Examples

Example

This example declares two arrays (arr1 and arr2) with defined lengths:

package main
import ("fmt")

func main() {
  var arr1 = [3]int{1,2,3}
  arr2 := [5]int{4,5,6,7,8}

  fmt.Println(arr1)
  fmt.Println(arr2)
}

Result:


    [1 2 3] 

    [4 5 6 7 8]

Example

This example declares two arrays (arr1 and arr2) with inferred lengths:

package main
import ("fmt")

func main() {
  var arr1 = [...]int{1,2,3}
  arr2 := [...]int{4,5,6,7,8}

  fmt.Println(arr1)
  fmt.Println(arr2)
}

Result:


    [1 2 3] 

    [4 5 6 7 8]

Example

This example declares an array of strings:

package main
import ("fmt")

func main() {
var cars = [4]string{"Volvo", "BMW", "Ford", "Mazda"}
fmt.Print(cars)
}

Result:


    [Volvo BMW Ford Mazda]

Access Elements of an Array

You can access a specific array element by referring to the index number.

In Go, array indexes start at 0. That means that [0] is the first element, [1] is the second element, etc.

Example

This example shows how to access the first and third elements in the prices array:

package main
import ("fmt")

func main() {
  prices := [3]int{10,20,30}

  fmt.Println(prices[0])
  fmt.Println(prices[2])
}

Result:


    10 

    30

Change Elements of an Array

You can also change the value of a specific array element by referring to the index number.

Example

This example shows how to change the value of the third element in the prices array:

package main
import ("fmt")

func main() {
  prices := [3]int{10,20,30}

  prices[2] = 50
  fmt.Println(prices)
}

Result:


    [10 20 50]

Array Initialization

If an array or one of its elements has not been initialized in the code, it is assigned the default value of its type.

Tip: The default value for int is 0, and the default value for string is "".

Example

package main
import ("fmt")

func main() {
  arr1 := [5]int{} //not initialized
  arr2 := [5]int{1,2} //partially initialized
  arr3 := [5]int{1,2,3,4,5} //fully initialized

  fmt.Println(arr1)
  fmt.Println(arr2)
  fmt.Println(arr3)
}

Result:


    [0 0 0 0 0] 

    [1 2 0 0 0] 

    [1 2 3 4 5]

Initialize Only Specific Elements

It is possible to initialize only specific elements in an array.

Example

This example initializes only the second and third elements of the array:

package main
import ("fmt")

func main() {
arr1 := [5]int{1:10,2:40}

fmt.Println(arr1)
}

Result:


    [0 10 40 0 0]

Example Explained

The array above has 5 elements.

1:10 means: assign 10 to array index 1 (second element).
2:40 means: assign 40 to array index 2 (third element).

Find the Length of an Array

The len() function is used to find the length of an array:

Example

package main
import ("fmt")

func main() {
  arr1 := [4]string{"Volvo", "BMW", "Ford", "Mazda"}
  arr2 := [...]int{1,2,3,4,5,6}

  fmt.Println(len(arr1))
  fmt.Println(len(arr2))
}

Result:


    4 

    6

Go Slices

Slices are similar to arrays, but are more powerful and flexible.

Like arrays, slices are also used to store multiple values of the same type in a single variable.

However, unlike arrays, the length of a slice can grow and shrink as you see fit.

In Go, there are several ways to create a slice:

Using the []datatype{values} format
Create a slice from an array
Using the make() function

Create a Slice With []datatype{values}

Syntax

slice_name := []datatype{values}

A common way of declaring a slice is like this:

myslice := []int{}

The code above declares an empty slice of 0 length and 0 capacity.

To initialize the slice during declaration, use this:

myslice := []int{1,2,3}

The code above declares a slice of integers of length 3 and also the capacity of 3.

In Go, there are two functions that can be used to return the length and capacity of a slice:

len() function - returns the length of the slice (the number of elements in the slice)
cap() function - returns the capacity of the slice (the number of elements the slice can grow or shrink to)

Example

This example shows how to create slices using the []datatype{values} format:

package main
import ("fmt")

func main() {
myslice1 := []int{}
fmt.Println(len(myslice1))
fmt.Println(cap(myslice1))
fmt.Println(myslice1)

myslice2 := []string{"Go", "Slices", "Are", "Powerful"}
fmt.Println(len(myslice2))
fmt.Println(cap(myslice2))
fmt.Println(myslice2)
}

Result:


    0
0
[]
4
4
[Go Slices Are Powerful]

In the example above, we see that in the first slice (myslice1), the actual elements are not specified, so both the length and capacity of the slice will be zero. In the second slice (myslice2), the elements are specified, and both length and capacity is equal to the number of actual elements specified.

Create a Slice From an Array

You can create a slice by slicing an array:

Syntax

var myarray = [length]datatype{values} // An array
myslice := myarray[start:end] // A slice made from the array

Example

This example shows how to create a slice from an array:

package main
import ("fmt")

func main() {
arr1 := [6]int{10, 11, 12, 13, 14,15}
myslice := arr1[2:4]

fmt.Printf("myslice = %vn", myslice)
fmt.Printf("length = %dn", len(myslice))
fmt.Printf("capacity = %dn", cap(myslice))
}

Result:


    myslice = [12 13]
length = 2
capacity = 4

In the example above myslice is a slice with length 2. It is made from arr1 which is an array with length 6.

The slice starts from the second element of the array which has value 12. The slice can grow to the end of the array. This means that the capacity of the slice is 4.

If myslice started from element 0, the slice capacity would be 6.

Create a Slice With The make() Function

The make() function can also be used to create a slice.

Syntax

slice_name := make([]type, length, capacity)

Note: If the capacity parameter is not defined, it will be equal to length.

Example

This example shows how to create slices using the make() function:

package main
import ("fmt")

func main() {
myslice1 := make([]int, 5, 10)
fmt.Printf("myslice1 = %vn", myslice1)
fmt.Printf("length = %dn", len(myslice1))
fmt.Printf("capacity = %dn", cap(myslice1))

// with omitted capacity
myslice2 := make([]int, 5)
fmt.Printf("myslice2 = %vn", myslice2)
fmt.Printf("length = %dn", len(myslice2))
fmt.Printf("capacity = %dn", cap(myslice2))
}

Result:


    myslice1 = [0 0 0 0 0]
length = 5
capacity = 10
myslice2 = [0 0 0 0 
    0]
length = 5
capacity = 5

Go Slices

Slices are similar to arrays, but are more powerful and flexible.

Like arrays, slices are also used to store multiple values of the same type in a single variable.

However, unlike arrays, the length of a slice can grow and shrink as you see fit.

In Go, there are several ways to create a slice:

Using the []datatype{values} format
Create a slice from an array
Using the make() function

Create a Slice With []datatype{values}

Syntax

slice_name := []datatype{values}

A common way of declaring a slice is like this:

myslice := []int{}

The code above declares an empty slice of 0 length and 0 capacity.

To initialize the slice during declaration, use this:

myslice := []int{1,2,3}

The code above declares a slice of integers of length 3 and also the capacity of 3.

In Go, there are two functions that can be used to return the length and capacity of a slice:

len() function - returns the length of the slice (the number of elements in the slice)
cap() function - returns the capacity of the slice (the number of elements the slice can grow or shrink to)

Example

This example shows how to create slices using the []datatype{values} format:

Result:


    0
0
[]
4
4
[Go Slices Are Powerful]

Create a Slice From an Array

You can create a slice by slicing an array:

Syntax

var myarray = [length]datatype{values} // An array
myslice := myarray[start:end] // A slice made from the array

Example

This example shows how to create a slice from an array:

Result:


    myslice = [12 13]
length = 2
capacity = 4

In the example above myslice is a slice with length 2. It is made from arr1 which is an array with length 6.

The slice starts from the second element of the array which has value 12. The slice can grow to the end of the array. This means that the capacity of the slice is 4.

If myslice started from element 0, the slice capacity would be 6.

Create a Slice With The make() Function

The make() function can also be used to create a slice.

Syntax

slice_name := make([]type, length, capacity)

Note: If the capacity parameter is not defined, it will be equal to length.

Example

This example shows how to create slices using the make() function:

Result:


    myslice1 = [0 0 0 0 0]
length = 5
capacity = 10
myslice2 = [0 0 0 0 
    0]
length = 5
capacity = 5

Access Elements of a Slice

You can access a specific slice element by referring to the index number.

In Go, indexes start at 0. That means that [0] is the first element, [1] is the second element, etc.

Example

This example shows how to access the first and third elements in the prices slice:

package main
import ("fmt")

func main() {
  prices := []int{10,20,30}

  fmt.Println(prices[0])
  fmt.Println(prices[2])
}

Result:


    10 

    30

Change Elements of a Slice

You can also change a specific slice element by referring to the index number.

Example

This example shows how to change the third element in the prices slice:

package main
import ("fmt")

func main() {
  prices := []int{10,20,30}
prices[2] = 50
  fmt.Println(prices[0])
  fmt.Println(prices[2])
}

Result:


    10 

    50

Append Elements To a Slice

You can append elements to the end of a slice using the append()function:

Syntax

slice_name = append(slice_name, element1, element2, ...)

Example

This example shows how to append elements to the end of a slice:

package main
import ("fmt")

func main() {
myslice1 := []int{1, 2, 3, 4, 5, 6}
fmt.Printf("myslice1 = %vn", myslice1)
fmt.Printf("length = %dn", len(myslice1))
fmt.Printf("capacity = %dn", cap(myslice1))

myslice1 = append(myslice1, 20, 21)
fmt.Printf("myslice1 = %vn", myslice1)
fmt.Printf("length = %dn", len(myslice1))
fmt.Printf("capacity = %dn", cap(myslice1))
}

Result:


    myslice1 = [1 2 3 4 5 6]
length = 6
capacity = 6
myslice1 = [1 2 3 
    4 5 6 20 21]
length = 8
capacity = 12

Append One Slice To Another Slice

To append all the elements of one slice to another slice, use the append()function:

Syntax

slice3 = append(slice1, slice2...)

Note: The '...' after slice2 is necessary when appending the elements of one slice to another.

Example

This example shows how to append one slice to another slice:

package main
import ("fmt")

func main() {
myslice1 := []int{1,2,3}
myslice2 := []int{4,5,6}
myslice3 := append(myslice1, myslice2...)

fmt.Printf("myslice3=%vn", myslice3)
fmt.Printf("length=%dn", len(myslice3))
fmt.Printf("capacity=%dn", cap(myslice3))
}

Result:


    myslice3=[1 2 3 4 5 6]
length=6
capacity=6

Change The Length of a Slice

Unlike arrays, it is possible to change the length of a slice.

Example

This example shows how to change the length of a slice:

package main
import ("fmt")

func main() {
arr1 := [6]int{9, 10, 11, 12, 13, 14} // An array
myslice1 := arr1[1:5] // Slice array
fmt.Printf("myslice1 = %vn", myslice1)
fmt.Printf("length = %dn", len(myslice1))
fmt.Printf("capacity = %dn", cap(myslice1))

myslice1 = arr1[1:3] // Change length by re-slicing the array
fmt.Printf("myslice1 = %vn", myslice1)
fmt.Printf("length = %dn", len(myslice1))
fmt.Printf("capacity = %dn", cap(myslice1))

myslice1 = append(myslice1, 20, 21, 22, 23) // Change length by appending items
fmt.Printf("myslice1 = %vn", myslice1)
fmt.Printf("length = %dn", len(myslice1))
fmt.Printf("capacity = %dn", cap(myslice1))
}

Result:


    myslice1 = [10 11 12 13]
length = 4
capacity = 5
myslice1 = [10 11]

    length = 2
capacity = 5
myslice1 = [10 11 20 21 22 23]
length = 6

    capacity = 10

Memory Efficiency

When using slices, Go loads all the underlying elements into the memory.

If the array is large and you need only a few elements, it is better to copy those elements using the copy() function.

The copy() function creates a new underlying array with only the required elements for the slice. This will reduce the memory used for the program.

Syntax

copy(dest, src)

The copy() function takes in two slices dest and src, and copies data from src to dest. It returns the number of elements copied.

Example

This example shows how to use the copy() function:

package main
import ("fmt")

func main() {
numbers := []int{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}
// Original slice
fmt.Printf("numbers = %vn", numbers)
fmt.Printf("length = %dn", len(numbers))
fmt.Printf("capacity = %dn", cap(numbers))

// Create copy with only needed numbers
neededNumbers := numbers[:len(numbers)-10]
numbersCopy := make([]int, len(neededNumbers))
copy(numbersCopy, neededNumbers)

fmt.Printf("numbersCopy = %vn", numbersCopy)
fmt.Printf("length = %dn", len(numbersCopy))
fmt.Printf("capacity = %dn", cap(numbersCopy))
}

Result:


    // Original slice
numbers = [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15]
length = 15
capacity = 
    15
// New slice
numbersCopy = [1 2 3 4 5]
length = 5
capacity = 5

The capacity of the new slice is now less than the capacity of the original slice because the new underlying array is smaller.

Go Operators

Operators are used to perform operations on variables and values.

The + operator adds together two values, like in the example below:

Example

package main
import ("fmt")

func main() {
var a = 15 + 25
fmt.Println(a)
}

Although the + operator is often used to add together two values, it can also be used to add together a variable and a value, or a variable and another variable:

Example

package main
import ("fmt")

func main() {
  var (
    sum1 = 100 + 50 // 150 (100 + 50)
    sum2 = sum1 + 250 // 400 (150 + 250)
    sum3 = sum2 + sum2 // 800 (400 + 400)
  )
  fmt.Println(sum3)

}

Go divides the operators into the following groups:

Arithmetic operators
Assignment operators
Comparison operators
Logical operators
Bitwise operators

Go Operators

Operators are used to perform operations on variables and values.

The + operator adds together two values, like in the example below:

Example

package main
import ("fmt")

func main() {
var a = 15 + 25
fmt.Println(a)
}

Although the + operator is often used to add together two values, it can also be used to add together a variable and a value, or a variable and another variable:

Example

package main
import ("fmt")

func main() {
  var (
    sum1 = 100 + 50 // 150 (100 + 50)
    sum2 = sum1 + 250 // 400 (150 + 250)
    sum3 = sum2 + sum2 // 800 (400 + 400)
  )
  fmt.Println(sum3)
}

Arithmetic Operators

Arithmetic operators are used to perform common mathematical operations.

Operator	Name	Description	Example
+	Addition	Adds together two values	x + y
-	Subtraction	Subtracts one value from another	x - y
*	Multiplication	Multiplies two values	x * y
/	Division	Divides one value by another	x / y
%	Modulus	Returns the division remainder	x % y
++	Increment	Increases the value of a variable by 1	x++
--	Decrement	Decreases the value of a variable by 1	x--

Go Exercises

Test Yourself With Exercises

Exercise:

Multiply 10 with 5, and print the result.

package main   
import ("fmt") 

func main() {
  fmt.Print(105)
}

Assignment Operators

Assignment operators are used to assign values to variables.

In the example below, we use the assignment operator (=) to assign the value 10 to a variable called x:

Example

package main
import ("fmt")

func main() {
var x = 10
fmt.Println(x)
}

The addition assignment operator (+=) adds a value to a variable:

Example

package main
import ("fmt")

func main() {
  var x = 10
  x +=5
  fmt.Println(x)
}

A list of all assignment operators:

Operator	Example	Same As	Try it
=	x = 5	x = 5	Try it »
+=	x += 3	x = x + 3	Try it »
-=	x -= 3	x = x - 3	Try it »
*=	x *= 3	x = x * 3	Try it »
/=	x /= 3	x = x / 3	Try it »
%=	x %= 3	x = x % 3	Try it »
&=	x &= 3	x = x & 3	Try it »
\|=	x \|= 3	x = x \| 3	Try it »
^=	x ^= 3	x = x ^ 3	Try it »
>>=	x >>= 3	x = x >> 3	Try it »
<<=	x <<= 3	x = x << 3

Comparison Operators

Comparison operators are used to compare two values.

Note: The return value of a comparison is either true (1) or false (0).

In the following example, we use the greater than operator (>) to find out if 5 is greater than 3:

Example

package main
import ("fmt")

func main() {
  var x = 5
  var y = 3
  fmt.Println(x>y) // returns 1 (true) because 5 is greater than 3
}

A list of all comparison operators:

Operator	Name	Example	Try it
==	Equal to	x == y	Try it »
!=	Not equal	x != y	Try it »
>	Greater than	x > y	Try it »
<	Less than	x < y	Try it »
>=	Greater than or equal to	x >= y	Try it »
<=	Less than or equal to	x <= y	Try it »

You will learn more about comparison operators and how to use them in the Go Conditions chapter.

Logical Operators

Logical operators are used to determine the logic between variables or values:

Operator	Name	Description	Example	Try it
&&	Logical and	Returns true if both statements are true	x < 5 && x < 10	Try it »
\|\|	Logical or	Returns true if one of the statements is true	x < 5 \|\| x < 4	Try it »
!	Logical not	Reverse the result, returns false if the result is true	!(x < 5 && x < 10)	Try it »

Logical Operators

Logical operators are used to determine the logic between variables or values:

Operator	Name	Description	Example	Try it
&&	Logical and	Returns true if both statements are true	x < 5 && x < 10	Try it »
\|\|	Logical or	Returns true if one of the statements is true	x < 5 \|\| x < 4	Try it »
!	Logical not	Reverse the result, returns false if the result is true	!(x < 5 && x < 10)	Try it »

Conditional statements are used to perform different actions based on different conditions.

Go Conditions

A condition can be either true or false.

Go supports the usual from mathematics:

Less than <
Less than or equal <=
Greater than >
Greater than or equal >=
Equal to ==
Not equal to !=

Additionally, Go supports the usual :

Logical AND &&
Logical OR ||
Logical NOT !

You can use these operators or their combinations to create conditions for different decisions.

Example	Try it
x > y
x != y
(x > y) && (y > z)
(x == y) \|\| z

Go has the following conditional statements:

Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed

Conditional statements are used to perform different actions based on different conditions.

Go Conditions

A condition can be either true or false.

Go supports the usual from mathematics:

Less than <
Less than or equal <=
Greater than >
Greater than or equal >=
Equal to ==
Not equal to !=

Additionally, Go supports the usual :

Logical AND &&
Logical OR ||
Logical NOT !

You can use these operators or their combinations to create conditions for different decisions.

Example	Try it
x > y
x != y
(x > y) && (y > z)
(x == y) \|\| z

Go has the following conditional statements:

Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed

The if Statement

Use the if statement to specify a block of Go code to be executed if a condition is true.

Syntax

if condition {
// code to be executed if condition is true
}

Note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.

In the example below, we test two values to find out if 20 is greater than 18. If the condition is true, print some text:

Example

package main
import ("fmt")

func main() {
  if 20 > 18 {
    fmt.Println("20 is greater than 18")
  }
}

We can also test variables:

Example

package main
import ("fmt")

func main() {
  x:= 20
  y:= 18
  if x > y {
    fmt.Println("x is greater than y")
  }
}

Example explained

In the example above we use two variables, x and y, to test whether x is greater than y (using the > operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".

Go Exercises

Test Yourself With Exercises

Exercise:

Print "Hello World" if x is greater than y.

package main   
import ("fmt") 

func main() {
  var x = 50
  var y = 10
   x  y {
  fmt.Print("Hello World")
  }
}

The else Statement

Use the else statement to specify a block of code to be executed if the condition is false.

Syntax

if condition {
// code to be executed if condition is true
} else {
// code to be executed if condition is false
}

Using The if else Statement

Example

In this example, time (20) is greater than 18, so the if condition is false. Because of this, we move on to the else condition and print to the screen "Good evening". If the time was less than 18, the program would print "Good day":

package main
import ("fmt")

func main() {
  time := 20
  if (time < 18) {
    fmt.Println("Good day.")
  } else {
    fmt.Println("Good evening.")
  }
}

Example

In this example, the temperature is 14 so the condition for if is false so the code line inside the else statement is executed:

package main
import ("fmt")

func main() {
  temperature := 14
  if (temperature > 15) {
    fmt.Println("It is warm out there")
  } else {
    fmt.Println("It is cold out there")
  }
}

The brackets in the else statement should be like } else {:

Example

Having the else brackets in a different line will raise an error:

package main
import ("fmt")

func main() {
  temperature := 14
  if (temperature > 15) {
    fmt.Println("It is warm out there.")
  } // this raises an error
  else {
    fmt.Println("It is cold out there.")
  }
}

Result:


          ./prog.go:9:3: syntax error: unexpected else, expecting }

The else if Statement

Use the else if statement to specify a new condition if the first condition is false.

Syntax

if condition1 {
   // code to be executed if condition1 is true
} else if condition2 {
   // code to be executed if condition1 is false and condition2 is true
} else {
   // code to be executed if condition1 and condition2 are both false
}

Using The else if Statement

Example

This example shows how to use an else if statement.

package main
import ("fmt")

func main() {
  time := 22
  if time < 10 {
    fmt.Println("Good morning.")
  } else if time < 20 {
    fmt.Println("Good day.")
  } else {
    fmt.Println("Good evening.")
  }
}

Result:

Good evening.

Example explained

In the example above, time (22) is greater than 10, so the first condition is false. The next condition, in the else if statement, is also false, so we move on to else condition since condition1 and condition2 are both false - and print to the screen "Good evening".

However, if the time was 14, our program would print "Good day."

Example

Another example for the use of else if.

package main
import ("fmt")

func main() {
  a := 14
  b := 14
  if a < b {
    fmt.Println("a is less than b.")
  } else if a > b {
    fmt.Println("a is more than b.")
  } else {
    fmt.Println("a and b are equal.")
  }
}

Result:

a and b are equal.

Example

Note: If condition1 and condition2 are BOTH true, only the code for condition1 are executed:

package main
import ("fmt")

func main() {
  x := 30
  if x >= 10 {
    fmt.Println("x is larger than or equal to 10.")
  } else if x > 20 {
    fmt.Println("x is larger than 20.")
  } else {
    fmt.Println("x is less than 10.")
  }
}

Result:


      x is larger than or equal to 10.

The Nested if Statement

You can have if statements inside if statements, this is called a nested if.

Syntax

if condition1 {
   // code to be executed if condition1 is true
  if condition2 {
     // code to be executed if both condition1 and condition2 are true
}
}

Example

This example shows how to use nested if statements:

package main
import ("fmt")

func main() {
  num := 20
  if num >= 10 {
    fmt.Println("Num is more than 10.")
    if num > 15 {
      fmt.Println("Num is also more than 15.")
     }
  } else {
    fmt.Println("Num is less than 10.")
  }
}

Result:


      Num is more than 10.

      Num is also more than 15.

The switch Statement

Use the switch statement to select one of many code blocks to be executed.

The switch statement in Go is similar to the ones in C, C++, Java, JavaScript, and PHP. The difference is that it only runs the matched case so it does not need a break statement.

Single-Case switch Syntax

Syntax

switch expression {
case x:
   // code block
case y:
   // code block
case z:
...
default:
   // code block
}

This is how it works:

The expression is evaluated once
The value of the switch expression is compared with the values of each case
If there is a match, the associated block of code is executed
The default keyword is optional. It specifies some code to run if there is no case match

Single-Case switch Example

The example below uses a weekday number to calculate the weekday name:

Example

package main
import ("fmt")

func main() {
  day := 4

  switch day {
  case 1:
    fmt.Println("Monday")
  case 2:
    fmt.Println("Tuesday")
  case 3:
    fmt.Println("Wednesday")
  case 4:
    fmt.Println("Thursday")
  case 5:
    fmt.Println("Friday")
  case 6:
    fmt.Println("Saturday")
  case 7:
    fmt.Println("Sunday")
  }
}

Result:


      Thursday

The default Keyword

The default keyword specifies some code to run if there is no case match:

Example

package main
import ("fmt")

func main() {
  day := 8

  switch day {
  case 1:
    fmt.Println("Monday")
  case 2:
    fmt.Println("Tuesday")
  case 3:
    fmt.Println("Wednesday")
  case 4:
    fmt.Println("Thursday")
  case 5:
    fmt.Println("Friday")
  case 6:
    fmt.Println("Saturday")
  case 7:
    fmt.Println("Sunday")
  default:
    fmt.Println("Not a weekday")
  }
}

Result:

Not a weekday

All the case values should have the same type as the switch expression. Otherwise, the compiler will raise an error:

Example

package main
import ("fmt")

func main() {
  a := 3

  switch a {
  case 1:
    fmt.Println("a is one")
  case "b":
    fmt.Println("a is b")
  }
}

Result:


      ./prog.go:11:2: cannot use "b" (type untyped string) as type int

The switch Statement

Use the switch statement to select one of many code blocks to be executed.

The switch statement in Go is similar to the ones in C, C++, Java, JavaScript, and PHP. The difference is that it only runs the matched case so it does not need a break statement.

Single-Case switch Syntax

Syntax

switch expression {
case x:
   // code block
case y:
   // code block
case z:
...
default:
   // code block
}

This is how it works:

The expression is evaluated once
The value of the switch expression is compared with the values of each case
If there is a match, the associated block of code is executed
The default keyword is optional. It specifies some code to run if there is no case match

Single-Case switch Example

The example below uses a weekday number to calculate the weekday name:

Example

Result:


      Thursday

The default Keyword

The default keyword specifies some code to run if there is no case match:

Example

Result:

Not a weekday

All the case values should have the same type as the switch expression. Otherwise, the compiler will raise an error:

Example

package main
import ("fmt")

func main() {
  a := 3

  switch a {
  case 1:
    fmt.Println("a is one")
  case "b":
    fmt.Println("a is b")
  }
}

Result:


      ./prog.go:11:2: cannot use "b" (type untyped string) as type int

Go Exercises

Test Yourself With Exercises

Exercise:

Insert the missing parts to complete the following switch statement.

package main   
import ("fmt") 

func main() {
  var day = 2
  switch  {
  (1):
    fmt.Print("Saturday")
  (2):
    fmt.Print("Sunday")    
  }
}

The Multi-case switch Statement

It is possible to have multiple values for each case in the switch statement:

Syntax

switch expression {
case x,y:
   // code block if expression is evaluated to x or y
case v,w:
   // code block if expression is evaluated to v or w
case z:
...
default:
   // code block if expression is not found in any cases
}

Multi-case switch Example

The example below uses the weekday number to return different text:

Example

package main
import ("fmt")

func main() {
   day := 5

   switch day {
   case 1,3,5:
    fmt.Println("Odd weekday")
   case 2,4:
     fmt.Println("Even weekday")
   case 6,7:
    fmt.Println("Weekend")
  default:
    fmt.Println("Invalid day of day number")
}
}

Result:

Odd weekday

The for loop loops through a block of code a specified number of times.

The for loop is the only loop available in Go.

Go for Loop

Loops are handy if you want to run the same code over and over again, each time with a different value.

Each execution of a loop is called an iteration.

The for loop can take up to three statements:

Syntax

for statement1; statement2; statement3 {
// code to be executed for each iteration
}

statement1 Initializes the loop counter value.

statement2 Evaluated for each loop iteration. If it evaluates to TRUE, the loop continues. If it evaluates to FALSE, the loop ends.

statement3 Increases the loop counter value.

Note: These statements don't need to be present as loops arguments. However, they need to be present in the code in some form.

for Loop Examples

Example 1

This example will print the numbers from 0 to 4:

package main
import ("fmt")

func main() {
  for i:=0; i < 5; i++ {
    fmt.Println(i)
  }
}

Example 1 explained

i:=0; - Initialize the loop counter (i), and set the start value to 0
i < 5; - Continue the loop as long as i is less than 5
i++ - Increase the loop counter value by 1 for each iteration

Example 2

This example counts to 100 by tens:

package main
import ("fmt")

func main() {
  for i:=0; i <= 100; i+=10 {
    fmt.Println(i)
  }
}

Example 2 explained

i:=0; - Initialize the loop counter (i), and set the start value to 0
i <= 100; - Continue the loop as long as i is less than or equal to 100
i+=10 - Increase the loop counter value by 10 for each iteration

The continue Statement

The continue statement is used to skip one or more iterations in the loop. It then continues with the next iteration in the loop.

Example

This example skips the value of 3:

package main
import ("fmt")

func main() {
for i:=0; i < 5; i++ {
    if i == 3 {
      continue
    }
   fmt.Println(i)
  }
}

The break Statement

The break statement is used to break/terminate the loop execution.

Example

This example breaks out of the loop when i is equal to 3:

package main
import ("fmt")

func main() {
  for i:=0; i < 5; i++ {
    if i == 3 {
      break
    }
   fmt.Println(i)
  }
}

Note: continue and break are usually used with conditions.

Nested Loops

It is possible to place a loop inside another loop.

Here, the "inner loop" will be executed one time for each iteration of the "outer loop":

Example

package main
import ("fmt")

func main() {
  adj := [2]string{"big", "tasty"}
  fruits := [3]string{"apple", "orange", "banana"}
  for i:=0; i < len(adj); i++ {
    for j:=0; j < len(fruits); j++ {
      fmt.Println(adj[i],fruits[j])
    }
  }
}

The Range Keyword

The range keyword is used to more easily iterate over an array, slice or map. It returns both the index and the value.

The range keyword is used like this:

Syntax

for index, value := array|slice|map {
// code to be executed for each iteration
}

Example

This example uses range to iterate over an array and print both the indexes and the values at each (idx stores the index, val stores the value):

package main
import ("fmt")

func main() {
  fruits := [3]string{"apple", "orange", "banana"}
  for idx, val := range fruits {
     fmt.Printf("%vt%vn", idx, val)
  }
}

Tip: To only show the value or the index, you can omit the other output using an underscore (_).

Example

Here, we want to omit the indexes (idx stores the index, val stores the value):

package main
import ("fmt")

func main() {
  fruits := [3]string{"apple", "orange", "banana"}
  for _, val := range fruits {
     fmt.Printf("%vn", val)
  }
}

Result:


      apple 

      orange 

      banana

Example

Here, we want to omit the values (idx stores the index, val stores the value):

package main
import ("fmt")

func main() {
  fruits := [3]string{"apple", "orange", "banana"}

  for idx, _ := range fruits {
     fmt.Printf("%vn", idx)
  }
}

A function is a block of statements that can be used repeatedly in a program.

A function will not execute automatically when a page loads.

A function will be executed by a call to the function.

Create a Function

To create (often referred to as declare) a function, do the following:

Use the func keyword.
Specify a name for the function, followed by parentheses ().
Finally, add code that defines what the function should do, inside curly braces {}.

Syntax

func FunctionName() {
// code to be executed
}

Call a Function

Functions are not executed immediately. They are "saved for later use", and will be executed when they are called.

In the example below, we create a function named "myMessage()". The opening curly brace ( { ) indicates the beginning of the function code, and the closing curly brace ( } ) indicates the end of the function. The function outputs "I just got executed!". To call the function, just write its name followed by two parentheses ():

Example

package main
import ("fmt")

func myMessage() {
fmt.Println("I just got executed!")
}

func main() {
myMessage() // call the function
}

Result:


      I just got executed!

A function can be called multiple times.

Example

package main
import ("fmt")

func myMessage() {
  fmt.Println("I just got executed!")
}

func main() {
  myMessage()
  myMessage()
  myMessage()
}

Result:


      I just got executed! 

      I just got executed! 

      I just got executed!

Naming Rules for Go Functions

A function name must start with a letter
A function name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
Function names are case-sensitive
A function name cannot contain spaces
If the function name consists of multiple words, techniques introduced for can be used

Tip: Give the function a name that reflects what the function does!

A function is a block of statements that can be used repeatedly in a program.

A function will not execute automatically when a page loads.

A function will be executed by a call to the function.

Create a Function

To create (often referred to as declare) a function, do the following:

Use the func keyword.
Specify a name for the function, followed by parentheses ().
Finally, add code that defines what the function should do, inside curly braces {}.

Syntax

func FunctionName() {
// code to be executed
}

Call a Function

Functions are not executed immediately. They are "saved for later use", and will be executed when they are called.

Example

package main
import ("fmt")

func myMessage() {
fmt.Println("I just got executed!")
}

func main() {
myMessage() // call the function
}

Result:


      I just got executed!

A function can be called multiple times.

Example

package main
import ("fmt")

func myMessage() {
  fmt.Println("I just got executed!")
}

func main() {
  myMessage()
  myMessage()
  myMessage()
}

Result:


      I just got executed! 

      I just got executed! 

      I just got executed!

Naming Rules for Go Functions

A function name must start with a letter
A function name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
Function names are case-sensitive
A function name cannot contain spaces
If the function name consists of multiple words, techniques introduced for can be used

Tip: Give the function a name that reflects what the function does!

Go Exercises

Parameters and Arguments

Information can be passed to functions as a parameter. Parameters act as variables inside the function.

Parameters and their types are specified after the function name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma:

Syntax

func FunctionName(param1 type, param2 type, param3 type) {
// code to be executed
}

Function With Parameter Example

The following example has a function with one parameter (fname) of type string. When the familyName() function is called, we also pass along a name (e.g. Liam), and the name is used inside the function, which outputs several different first names, but an equal last name:

Example

package main
import ("fmt")

func familyName(fname string) {
  fmt.Println("Hello", fname, "Refsnes")
}

func main() {
  familyName("Liam")
  familyName("Jenny")
  familyName("Anja")
}

Result:


      Hello Liam Refsnes 

      Hello Jenny Refsnes 

      Hello Anja Refsnes

Note: When a parameter is passed to the function, it is called an argument. So, from the example above: fname is a parameter, while Liam, Jenny and Anja are arguments.

Multiple Parameters

Inside the function, you can add as many parameters as you want:

Example

package main
import ("fmt")

func familyName(fname string, age int) {
  fmt.Println("Hello", age, "year old", fname, "Refsnes")
}

func main() {
  familyName("Liam", 3)
  familyName("Jenny", 14)
  familyName("Anja", 30)
}

Result:


    Hello 3 year old Liam Refsnes

    Hello 14 year old Jenny Refsnes

    Hello 30 year old Anja Refsnes

Note: When you are working with multiple parameters, the function call must have the same number of arguments as there are parameters, and the arguments must be passed in the same order.

Return Values

If you want the function to return a value, you need to define the data type of the return value (such as int, string, etc), and also use the return keyword inside the function:

Syntax

func FunctionName(param1 type, param2 type) type {
// code to be executed
return output
}

Function Return Example

Example

Here, myFunction() receives two integers (x and y) and returns their addition (x + y) as integer (int):

package main
import ("fmt")

func myFunction(x int, y int) int {
return x + y
}

func main() {
fmt.Println(myFunction(1, 2))
}

Result:

Named Return Values

In Go, you can name the return values of a function.

Example

Here, we name the return value as result (of type int), and return the value with a naked return (means that we use the return statement without specifying the variable name):

package main
import ("fmt")

func myFunction(x int, y int) (result int) {
  result = x + y
  return
}

func main() {
  fmt.Println(myFunction(1, 2))
}

Result:

The example above can also be written like this. Here, the return statement specifies the variable name:

Example

package main
import ("fmt")

func myFunction(x int, y int) (result int) {
  result = x + y
  return result
}

func main() {
  fmt.Println(myFunction(1, 2))
}

Store the Return Value in a Variable

You can also store the return value in a variable, like this:

Example

Here, we store the return value in a variable called total:

package main
import ("fmt")

func myFunction(x int, y int) (result int) {
  result = x + y
  return
}

func main() {
  total := myFunction(1, 2)
  fmt.Println(total)
}

Multiple Return Values

Go functions can also return multiple values.

Example

Here, myFunction() returns one integer (result) and one string (txt1):

package main
import ("fmt")

func myFunction(x int, y string) (result int, txt1 string) {
  result = x + x
  txt1 = y + " World!"
  return
}

func main() {
  fmt.Println(myFunction(5, "Hello"))
}

Result:


    10 Hello World!

Example

Here, we store the two return values into two variables (a and b):

package main
import ("fmt")

func myFunction(x int, y string) (result int, txt1 string) {
  result = x + x
  txt1 = y + " World!"
  return
}

func main() {
  a, b := myFunction(5, "Hello")
  fmt.Println(a, b)
}

Result:


    10 Hello World!

If we (for some reason) do not want to use some of the returned values, we can add an underscore (_), to omit this value.

Example

Here, we want to omit the first returned value (result - which is stored in variable a):

package main
import ("fmt")

func myFunction(x int, y string) (result int, txt1 string) {
  result = x + x
  txt1 = y + " World!"
  return
}

func main() {
   _, b := myFunction(5, "Hello")
  fmt.Println(b)
}

Result:


    Hello World!

Example

Here, we want to omit the second returned value (txt1 - which is stored in variable b):

package main
import ("fmt")

func myFunction(x int, y string) (result int, txt1 string) {
  result = x + x
  txt1 = y + " World!"
  return
}

func main() {
   a, _ := myFunction(5, "Hello")
  fmt.Println(a)
}

Result:

Recursion Functions

Go accepts recursion functions. A function is recursive if it calls itself and reaches a stop condition.

In the following example, testcount() is a function that calls itself. We use the x variable as the data, which increments with 1 (x + 1) every time we recurse. The recursion ends when the x variable equals to 11 (x == 11).

Example

package main
import ("fmt")

func testcount(x int) int {
if x == 11 {
return 0
}
fmt.Println(x)
return testcount(x + 1)
}

func main(){
testcount(1)
}

Recursion is a common mathematical and programming concept. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be careful with recursion functions as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In the following example, factorial_recursion() is a function that calls itself. We use the x variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

Example

package main
import ("fmt")

func factorial_recursion(x float64) (y float64) {
  if x > 0 {
     y = x * factorial_recursion(x-1)
  } else {
     y = 1
  }
  return
}

func main() {
  fmt.Println(factorial_recursion(4))
}

Go Structures

A struct (short for structure) is used to create a collection of members of different data types, into a single variable.

While arrays are used to store multiple values of the same data type into a single variable, structs are used to store multiple values of different data types into a single variable.

A struct can be useful for grouping data together to create records.

Declare a Struct

To declare a structure in Go, use the type and struct keywords:

Syntax

type struct_name struct {
member1 datatype;
member2 datatype;
member3 datatype;
...
}

Example

Here we declare a struct type Person with the following members: name, age, job and salary:

type Person struct {
name string
age int
job string
salary int
}

Tip: Notice that the struct members above have different data types. name and job is of type string, while age and salary is of type int.

Access Struct Members

To access any member of a structure, use the dot operator (.) between the structure variable name and the structure member:

Example

package main
import ("fmt")

type Person struct {
name string
age int
job string
salary int
}

func main() {
var pers1 Person
var pers2 Person

// Pers1 specification
pers1.name = "Hege"
pers1.age = 45
pers1.job = "Teacher"
pers1.salary = 6000

// Pers2 specification
pers2.name = "Cecilie"
pers2.age = 24
pers2.job = "Marketing"
pers2.salary = 4500

// Access and print Pers1 info
fmt.Println("Name: ", pers1.name)
fmt.Println("Age: ", pers1.age)
fmt.Println("Job: ", pers1.job)
fmt.Println("Salary: ", pers1.salary)

// Access and print Pers2 info
fmt.Println("Name: ", pers2.name)
fmt.Println("Age: ", pers2.age)
fmt.Println("Job: ", pers2.job)
fmt.Println("Salary: ", pers2.salary)
}

Result:


    Name: Hege
Age: 45
Job: Teacher
Salary: 6000
Name: Cecilie
Age: 
    24
Job: Marketing
Salary: 4500

Pass Struct as Function Arguments

You can also pass a structure as a function argument, like this:

Example

package main
import ("fmt")

type Person struct {
name string
age int
job string
salary int
}

func main() {
var pers1 Person
var pers2 Person

// Pers1 specification
pers1.name = "Hege"
pers1.age = 45
pers1.job = "Teacher"
pers1.salary = 6000

// Pers2 specification
pers2.name = "Cecilie"
pers2.age = 24
pers2.job = "Marketing"
pers2.salary = 4500

// Print Pers1 info by calling a function
printPerson(pers1)

// Print Pers2 info by calling a function
printPerson(pers2)
}

func printPerson(pers Person) {
fmt.Println("Name: ", pers.name)
fmt.Println("Age: ", pers.age)
fmt.Println("Job: ", pers.job)
fmt.Println("Salary: ", pers.salary)
}

Result:


    Name: Hege
Age: 45
Job: Teacher
Salary: 6000
Name: Cecilie
Age: 
    24
Job: Marketing
Salary: 4500

Go Maps

Maps are used to store data values in key:value pairs.

Each element in a map is a key:value pair.

A map is an unordered and changeable collection that does not allow duplicates.

The length of a map is the number of its elements. You can find it using the len() function.

The default value of a map is nil.

Maps hold references to an underlying hash table.

Go has multiple ways for creating maps.

Creating Maps Using `var` and `:=`

Syntax

var a = map[KeyType]ValueType{key1:value1, key2:value2,...}
b := map[KeyType]ValueType{key1:value1, key2:value2,...}

Example

This example shows how to create maps in Go. Notice the order in the code and in the output

package main
import ("fmt")

func main() {
  var a = map[string]string{"brand": "Ford", "model": "Mustang", "year": "1964"}
  b := map[string]int{"Oslo": 1, "Bergen": 2, "Trondheim": 3, "Stavanger": 4}

  fmt.Printf("at%vn", a)
  fmt.Printf("bt%vn", b)
}

Result:


    a   map[brand:Ford model:Mustang year:1964]

    b   map[Bergen:2 Oslo:1 Stavanger:4 Trondheim:3]

Note: The order of the map elements defined in the code is different from the way that they are stored. The data are stored in a way to have efficient data retrieval from the map.

Creating Maps Using Using `make()`Function:

Syntax

var a = make(map[KeyType]ValueType)
b := make(map[KeyType]ValueType)

Example

This example shows how to create maps in Go using the make()function.

package main
import ("fmt")

func main() {
  var a = make(map[string]string) // The map is empty now
  a["brand"] = "Ford"
  a["model"] = "Mustang"
  a["year"] = "1964"
                                 // a is no longer empty
  b := make(map[string]int)
  b["Oslo"] = 1
  b["Bergen"] = 2
  b["Trondheim"] = 3
  b["Stavanger"] = 4

  fmt.Printf("at%vn", a)
  fmt.Printf("bt%vn", b)
}

Result:


    a   map[brand:Ford model:Mustang year:1964]

    b   map[Bergen:2 Oslo:1 Stavanger:4 Trondheim:3]

Creating an Empty Map

There are two ways to create an empty map. One is by using the make()function and the other is by using the following syntax.

Syntax

var a map[KeyType]ValueType

Note: The make()function is the right way to create an empty map. If you make an empty map in a different way and write to it, it will causes a runtime panic.

Example

This example shows the difference between declaring an empty map using with the make()function and without it.

package main
import ("fmt")

func main() {
  var a = make(map[string]string)
  var b map[string]string

  fmt.Println(a == nil)
  fmt.Println(b == nil)
}

Result:


    false

    true

Allowed Key Types

The map key can be of any data type for which the equality operator (==) is defined. These include:

Booleans
Numbers
Strings
Arrays
Pointers
Structs
Interfaces (as long as the dynamic type supports equality)

Invalid key types are:

Slices
Maps
Functions

These types are invalid because the equality operator (==) is not defined for them.

Allowed Value Types

The map values can be any type.

Accessing Map Elements

You can access map elements by:

Syntax

value = map_name[key]

Example

package main
import ("fmt")

func main() {
  var a = make(map[string]string)
  a["brand"] = "Ford"
  a["model"] = "Mustang"
  a["year"] = "1964"

  fmt.Printf(a["brand"])
}

Result:


    Ford

Updating and Adding Map Elements

Updating or adding an elements are done by:

Syntax

map_name[key] = value

Example

This example shows how to update and add elements to a map.

package main
import ("fmt")

func main() {
  var a = make(map[string]string)
  a["brand"] = "Ford"
  a["model"] = "Mustang"
  a["year"] = "1964"

  fmt.Println(a)

  a["year"] = "1970" // Updating an element
  a["color"] = "red" // Adding an element

  fmt.Println(a)
}

Result:


    map[brand:Ford model:Mustang year:1964]

    map[brand:Ford color:red model:Mustang year:1970]

Remove Element from Map

Removing elements is done using the delete() function.

Syntax

delete(map_name, key)

Example

package main
import ("fmt")

func main() {
  var a = make(map[string]string)
  a["brand"] = "Ford"
  a["model"] = "Mustang"
  a["year"] = "1964"

  fmt.Println(a)

  delete(a,"year")

  fmt.Println(a)
}

Result:


    map[brand:Ford model:Mustang year:1964]

    map[brand:Ford model:Mustang]

Check For Specific Elements in a Map

You can check if a certain key exists in a map using:

Syntax

val, ok :=map_name[key]

If you only want to check the existence of a certain key, you can use the blank identifier (_) in place of val.

Example

package main
import ("fmt")

func main() {
  var a = map[string]string{"brand": "Ford", "model": "Mustang", "year": "1964", "day":""}

  val1, ok1 := a["brand"] // Checking for existing key and its value
  val2, ok2 := a["color"] // Checking for non-existing key and its value
  val3, ok3 := a["day"]   // Checking for existing key and its value
  _, ok4 := a["model"]    // Only checking for existing key and not its value

  fmt.Println(val1, ok1)
  fmt.Println(val2, ok2)
  fmt.Println(val3, ok3)
  fmt.Println(ok4)
}

Result:


    Ford true 

     false 

     true 

    true

Example Explained

In this example, we checked for existence of different keys in the map.

The key "color" does not exist in the map. So the value is an empty string ('').

The ok2 variable is used to find out if the key exist or not. Because we would have got the same value if the value of the "color" key was empty. This is the case for val3.

Maps Are References

Maps are references to hash tables.

If two map variables refer to the same hash table, changing the content of one variable affect the content of the other.

Example

package main
import ("fmt")

func main() {
  var a = map[string]string{"brand": "Ford", "model": "Mustang", "year": "1964"}
  b := a

  fmt.Println(a)
  fmt.Println(b)

  b["year"] = "1970"
  fmt.Println("After change to b:")

  fmt.Println(a)
  fmt.Println(b)
}

Result:


    map[brand:Ford model:Mustang year:1964]
map[brand:Ford model:Mustang 
    year:1964]
After change to b:
map[brand:Ford model:Mustang year:1970]

    map[brand:Ford model:Mustang year:1970]

Iterating Over Maps

You can use range to iterate over maps.

Example

This example shows how to iterate over the elements in a map. Note the order of the elements in the output.

package main
import ("fmt")

func main() {
  a := map[string]int{"one": 1, "two": 2, "three": 3, "four": 4}

  for k, v := range a {
    fmt.Printf("%v : %v, ", k, v)
  }
}

Result:


    two : 2, three : 3, four : 4, one : 1,

Iterate Over Maps in a Specific Order

Maps are unordered data structures. If you need to iterate over a map in a specific order, you must have a separate data structure that specifies that order.

Example

package main
import ("fmt")

func main() {
  a := map[string]int{"one": 1, "two": 2, "three": 3, "four": 4}

  var b []string             // defining the order
  b = append(b, "one", "two", "three", "four")

  for k, v := range a {        // loop with no order
    fmt.Printf("%v : %v, ", k, v)
  }

  fmt.Println()

  for _, element := range b {  // loop with the defined order
    fmt.Printf("%v : %v, ", element, a[element])
  }
}

Result:


    two : 2, three : 3, four : 4, one : 1, 

    one : 1, two : 2, three : 3, four : 4,

Go Exercises

You can test your Go skills with W3Schools' Exercises.

Exercises

We have gathered a variety of Go exercises (with answers) for each Go Chapter.

Try to solve an exercise by editing some code, or show the answer to see what you've done wrong.

Count Your Score

You will get 1 point for each correct answer. Your score and total score will always be displayed.

Start Go Exercises

Good luck!

If you don't know Go, we suggest that you read our from scratch.

Go Compiler

Go Compiler (Editor)

With our online Go compiler, you can edit Go code, and view the result in your browser.

Example

package main
import ("fmt")

func main() {
fmt.Println("Hello World!")
}

Hello World!

Click on the "Try it Yourself" button to see how it works.

Go Compiler Explained

The window to the left is editable - edit the code and click on the "Run" button to view the result in the right window.

The icons are explained in the table below:

Icon	Description
	Go to www.w3schools.com
	Menu button for more options
	Change orientation (horizontally or vertically)
	Change color theme (dark or light)

If you don't know Go, we suggest that you read our from scratch.

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