Tuesday, December 1, 2020

pointer and array in C

Today in this tutorial we will learn about pointer and array which is the most important topic in C language, But I will recommend you to refer array and pointer before going further in this tutorial, so that you could understand the topic easily as discussed below. Now let's go ahead...

Example 1: pointer and array

There is an int array named arr[] = {2, 4, 6, 8, 10}; with initialized values {2, 4, 6, 8, 10}. Let's see how to access the array elements as well as address of each elements using pointer.

#include <stdio.h>
int main
{
 int arr[] = {2, 4, 6, 8, 10};
 int i, *ptr;
 
 /* Assigning the address of 1st element (also known as base address) 
  * to a pointer (i.e. Initialization of pointer) */
 ptr = &arr[0]; 
 
 /*Printing array elements and address of each elements using pointer */
 for(i=0; i<5; i++)
 {
   printf("arr[%d] = %d and address of arr[%d] = %p", i, *ptr, ptr);
   /* Since, *ptr pointing to the base address (i.e. &arr[0]) therefore,
    * we need to increment the pointer so that it points to the next
    * elements on each iteration like ptr + 1.*/
 
   ptr++
 }
 return 0;
}

Note: You can use the format specifiers either %u, %p or %x for printing the address of elements in hex format.

Output

arr[0] = 2  and address of arr[0] = 0x9ff02
arr[1] = 4  and address of arr[1] = 0x9ff04
arr[2] = 6  and address of arr[2] = 0x9ff06
arr[3] = 8  and address of arr[3] = 0x9ff08
arr[4] = 10 and address of arr[4] = 0x9ff0A

Pointers arithmetic:

Memory representation of array elements

We can add and subtract an integer to/from an address. However, We can get address of next element by incrementing pointers with ++ (i.e. ptr++). as we know that pointer contains the address, so the result of an arithmetic operation performed on the pointer will be a pointer. Have a look at the below example how to find the address of next elements using pointers arithmetic.

int arr[5], i, *ptr;

/* Since we know, array name represent base address (i.e. &arr[0]).
 * So you can initialize pointer by simply like this: */
ptr = arr; 

/* Now let's see how to perform pointers arithmetic? i.e. 
 * (addition and subtraction an integer to/from a pointer)
 * You can use formula to find the address of next elements*/
 
Formula: 
  Pointer + n = value of pointer + sizeof (pointer’s type) * n 

For Example:
  ptr + 0 = 1000 + 4 * 0 = 1000  //address of 1st element i.e. &arr[0]
  ptr + 1 = 1000 + 4 * 1 = 1004  //address of 2nd element i.e. &arr[1]
  ptr + 2 = 1000 + 4 * 2 = 1008  //address of 3rd element i.e. &arr[2]
  ptr + 3 = 1000 + 4 * 3 = 1012  //address of 4th element i.e. &arr[3]
  ptr + 4 = 1000 + 4 * 4 = 1016  //address of 5th element i.e. &arr[4]
  
/* As you know that, to access elements of an array using pointer 
 * asterisk * symbol is used with pointer name (i.e. *(ptr+i)) */
 
  *(ptr + 0) == arr[0]  //to access the 1st element
  *(ptr + 1) == arr[1]  //to access the 2nd element
  *(ptr + 2) == arr[2]  //to access the 3rd element
  *(ptr + 3) == arr[3]  //to access the 4th element
  *(ptr + 4) == arr[4]  //to access the 5th element

Note: Incrementing pointers with ++, and decrementing with -- is useful when iterating over each elements in an array of elements.

We can access the memory location of next element by incrementing pointers with ++ (i.e. ptr++). As you can see in above example.

Example 2: Pointer and array

Let's rewrite the above code using pointers arithmetic & expression. This way it would be more better than 1st example1.

#include <stdio.h>
int main
{
 int arr[] = {2, 4, 6, 8, 10};
 int i, *ptr;
 
 /* Assigning base address to a pointer of arr[] (i.e. Initialization of pointer) 
  * array name 'arr' represent the base address i.e. arr == &arr[0] */   
 ptr = arr; 
 
 /*Printing array elements and address of each elements using pointers*/
 for(i=0; i<5; i++)
 {
    printf("arr[%d] = %d and address of arr[%d] = %p", i, *(ptr+i), ptr+i);
 }
 return 0;
}

Output

arr[0] = 2  and address of arr[0] = 0x9ff02
arr[1] = 4  and address of arr[1] = 0x9ff04
arr[2] = 6  and address of arr[2] = 0x9ff06
arr[3] = 8  and address of arr[3] = 0x9ff08
arr[4] = 10 and address of arr[4] = 0x9ff0A