C Pointers

Example

int myAge = 43; // an int variable

printf("%d", myAge);  // Outputs the value of myAge (43)
printf("%p", &myAge); // Outputs the memory address of myAge (0x7ffe5367e044)

Example

int myAge = 43;     // An int variable
int* ptr = &myAge;  // A pointer variable, with the name ptr, that stores the address of myAge

// Output the value of myAge (43)
printf("%d\n", myAge);

// Output the memory address of myAge (0x7ffe5367e044)
printf("%p\n", &myAge);

// Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);

Example

int myAge = 43;     // Variable declaration
int* ptr = &myAge;  // Pointer declaration

// Reference: Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);

// Dereference: Output the value of myAge with the pointer (43)
printf("%d\n", *ptr);

Note that the * sign can be confusing here, as it does two different things in our code:

• When used in declaration (int* ptr), it creates a pointer variable.
• When not used in declaration, it act as a dereference operator.

Good To Know: There are two ways to declare pointer variables in C:

int* myNum;
int *myNum;

Notes on Pointers

Pointers are one of the things that make C stand out from other programming languages, like Python and Java.

They are important in C, because they allow us to manipulate the data in the computer's memory. This can reduce the code and improve the performance. If you are familiar with data structures like lists, trees and graphs, you should know that pointers are especially useful for implementing those. And sometimes you even have to use pointers, for example when working with files and memory management.

But be careful; pointers must be handled with care, since it is possible to damage data stored in other memory addresses.

Pointers & Arrays

You can also use pointers to access arrays.

Consider the following array of integers:

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

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

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

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

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

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

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:

// Create an int variable
int myInt;

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

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:

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

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

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:

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

// Get the memory address of the myNumbers array
printf("%p\n", myNumbers);

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

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:

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

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

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

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

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

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

// and so on..

Or loop through it:

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

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

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

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("%d\n", *myNumbers);

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