C - Pointers and Variables

In this tutorial we will learn to manipulate variables using pointers in C programming language.

We already know from the Pointers tutorial how to create a pointer variable and store address of a variable in it.

Now let us go ahead and create an integer int variable and manipulate it via an integer pointer variable ptr.

Creating an integer variable

This is fairly simple. All we have to do is declare a variable using the int data type.

int num = 10;

Three things will happen for the above line of code.

• A memory location is located to store integer value.
• The value 10 is saved in that memory location.
• We can refer the memory location using the variable name num.

Creating integer pointer variable

Now we will create an integer pointer variable ptr that will store the address of the integer variable num.

To get the address of a variable we use the address of & operator.

int *ptr = #

We can represent the integer variable num and pointer variable ptr as follows.

So, we can see that variable num was allocated memory location 8280 (which will change the next time the program is executed). In that location value 10 was stored.

Similarly, variable ptr was allocated memory location 8272 and it holds the value 8280 which is the memory location of variable num. So, ptr is pointing at the num variable.

Accessing value of a variable via pointer

To access the value stored in the variable num via the pointer variable ptr we have to use the value at the address of * operator.

We already have the address of variable num stored in variable ptr. So, using *ptr will give use the value stored at the address i.e., value stored in num variable.

//value of num via ptr
printf("num value via ptr: %d\n", *ptr);

Updating the value of a variable via pointer

To update the value of a variable via pointer we have to first get the address of the variable and then set the new value in that memory address.

We get the address via address of & operator and then we set the value using the value at the address of * operator.

// updating the value of num via ptr
*ptr = 20;

Complete code:

#include <stdio.h>

int main(void) {
  
  // num variable
  int num = 10;

  // ptr pointer variable
  int *ptr = NULL;

  // assigning the address of num to ptr
  ptr = &num;

  // printing the value of num - Output: 10
  printf("num: %d\n", num);
  printf("num via ptr: %d\n", *ptr);
 
  // updating the value of num via ptr
  printf("Updating value of num via ptr...\n");
  *ptr = 20;

  // printing the new value of num - Output: 20
  printf("num: %d\n", num);
  printf("num via ptr: %d\n", *ptr);

  return 0;
}

Output:

num: 10
num via ptr: 10
Updating value of num via ptr...
num: 20
num via ptr: 20