Understanding Pointers to Arrays in C

This section explores how arrays in C are treated as pointers, specifically focusing on how the array name serves as a constant pointer to its first element. Gain insights into pointer arithmetic, array manipulation, and effective memory management in your C programs.

Pointer to an Array in C

An array name in C is treated as a constant pointer to the first element of the array. For example, in the declaration:

Code Example


int balance[5];

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

Example

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

Code Example


#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


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

In all three cases, you will get the same output. If you fetch the value stored at the address that ptr points to (i.e., *ptr), it will return 1.

Array Names as Constant Pointers

It is valid to use array names as constant pointers, and vice versa. For instance, *(balance + 4) is a legitimate way to access 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 the concepts discussed above:

Code Example


#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


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 this example, ptr is a pointer that can store the address of a variable of type double. Once the address is stored in ptr, *ptr will give you the value available at that address.