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C Pointers and Arrays

Pointers & Arrays

You can also use pointers to access .

Consider the following array of integers:

Example

int myNumbers[4] = {25, 50, 75, 100};

You learned from the that you can loop through the array elements with a for loop:

Example

int myNumbers[4] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
  printf("%dn", myNumbers[i]);
}

Result:

25
50
75
100

Instead of printing the value of each array element, let's print the memory address of each array element:

Example

int myNumbers[4] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
  printf("%pn", &myNumbers[i]);
}

Result:

0x7ffe70f9d8f0
0x7ffe70f9d8f4
0x7ffe70f9d8f8
0x7ffe70f9d8fc

Note that the last number of each of the elements' memory address is different, with an addition of 4.

It is because the size of an int type is typically 4 bytes, remember:

Example

// Create an int variable
int myInt;

// Get the memory size of an int
printf("%lu", sizeof(myInt));

Result:

4

So from the "memory address example" above, you can see that the compiler reserves 4 bytes of memory for each array element, which means that the entire array takes up 16 bytes (4 * 4) of memory storage:

Example

int myNumbers[4] = {25, 50, 75, 100};

// Get the size of the myNumbers array
printf("%lu", sizeof(myNumbers));

Result:

16

How Are Pointers Related to Arrays

Ok, so what's the relationship between pointers and arrays? Well, in C, the name of an array, is actually a pointer to the first element of the array.

Confused? Let's try to understand this better, and use our "memory address example" above again.

The memory address of the first element is the same as the name of the array:

Example

int myNumbers[4] = {25, 50, 75, 100};

// Get the memory address of the myNumbers array
printf("%pn", myNumbers);

// Get the memory address of the first array element
printf("%pn", &myNumbers[0]);

Result:

0x7ffe70f9d8f0
0x7ffe70f9d8f0

This basically means that we can work with arrays through pointers!

How? Since myNumbers is a pointer to the first element in myNumbers, you can use the * operator to access it:

Example

int myNumbers[4] = {25, 50, 75, 100};

// Get the value of the first element in myNumbers
printf("%d", *myNumbers);

Result:

25

To access the rest of the elements in myNumbers, you can increment the pointer/array (+1, +2, etc):

Example

int myNumbers[4] = {25, 50, 75, 100};

// Get the value of the second element in myNumbers
printf("%dn", *(myNumbers + 1));

// Get the value of the third element in myNumbers
printf("%d", *(myNumbers + 2));

// and so on..

Result:

50
75

Or loop through it:

Example

int myNumbers[4] = {25, 50, 75, 100};
int *ptr = myNumbers;
int i;

for (i = 0; i < 4; i++) {
  printf("%dn", *(ptr + i));
}

Result:

25
50
75
100

It is also possible to change the value of array elements with pointers:

Example

int myNumbers[4] = {25, 50, 75, 100};

// Change the value of the first element to 13
*myNumbers = 13;

// Change the value of the second element to 17
*(myNumbers +1) = 17;

// Get the value of the first element
printf("%dn", *myNumbers);

// Get the value of the second element
printf("%dn", *(myNumbers + 1));

Result:

13
17

This way of working with arrays might seem a bit excessive. Especially with simple arrays like in the examples above. However, for large arrays, it can be much more efficient to access and manipulate arrays with pointers.

It is also considered faster and easier to access with pointers.

And since strings are actually arrays, you can also use pointers to access .

For now, it's great that you know how this works. But like we specified in the previous chapter; pointers must be handled with care, since it is possible to overwrite other data stored in memory.


Pointers & Arrays
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