Pointer to an Array in C

An array name is a constant pointer to the first element of the array. Therefore, in this declaration,

int balance[5];

balance is a pointer to &balance[0], which is the address of the first element of the array.

Example

In this code, we have a pointer ptr that points to the address of the first element of an integer array called balance.

#include <stdio.h>

int main(){

   int *ptr;
   int balance[5] = {1, 2, 3, 4, 5};

   ptr = balance;

   printf("Pointer 'ptr' points to the address: %d", ptr);
   printf("\nAddress of the first element: %d", balance);
   printf("\nAddress of the first element: %d", &balance[0]);

   return 0;
}

Output

In all the three cases, you get the same output −

Pointer 'ptr' points to the address: 647772240
Address of the first element: 647772240
Address of the first element: 647772240

If you fetch the value stored at the address that ptr points to, that is *ptr, then it will return 1.

Array Names as Constant Pointers

It is legal to use array names as constant pointers and vice versa. Therefore, *(balance + 4) is a legitimate way of accessing the data at balance[4].

Once you store the address of the first element in "ptr", you can access the array elements using *ptr, *(ptr + 1), *(ptr + 2), and so on.

Example

The following example demonstrates all the concepts discussed above −

#include <stdio.h>

int main(){

   /* an array with 5 elements */
   double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};
   double *ptr;
   int i;

   ptr = balance;
 
   /* output each array element's value */
   printf("Array values using pointer: \n");
	
   for(i = 0; i < 5; i++){
      printf("*(ptr + %d): %f\n",  i, *(ptr + i));
   }

   printf("\nArray values using balance as address:\n");
	
   for(i = 0; i < 5; i++){
      printf("*(balance + %d): %f\n",  i, *(balance + i));
   }
 
   return 0;
}

Output

When you run this code, it will produce the following output −

Array values using pointer:
*(ptr + 0): 1000.000000
*(ptr + 1): 2.000000
*(ptr + 2): 3.400000
*(ptr + 3): 17.000000
*(ptr + 4): 50.000000

Array values using balance as address:
*(balance + 0): 1000.000000
*(balance + 1): 2.000000
*(balance + 2): 3.400000
*(balance + 3): 17.000000
*(balance + 4): 50.000000

In the above example, ptr is a pointer that can store the address of a variable of double type. Once we have the address in ptr, *ptr will give us the value available at the address stored in ptr.