C Programming #48: Pointers and Array - brothers

In this article we will discuss, how pointer [C Programming #41: Pointer - introduction ] and array [C Programming #39: Array - single dimension] are related.




Let me first give an example and then explain it.

#include <stdio.h>
int main()
{
   int arr[4] = {11, 34, 71, 2};
   int *p;
   
   printf("Array arr is %d %d %d %d\n", arr[0], arr[1], arr[2], arr[3]);
   printf("Address of arr[0] is %p\n", &arr[0]);
   printf("Address of arr[1] is %p\n", &arr[1]);
   printf("Address of arr[2] is %p\n", &arr[2]);
   printf("Address of arr[3] is %p\n", &arr[3]);

   p = &arr[0];
   printf("Pointer p address is %p value is %d\n", p, *p);

   p = &arr[2];
   printf("Pointer p address is %p value is %d\n", p, *p);
   
   p++;
   printf("Pointer p address is %p value is %d\n", p, *p);
   
   p-=3;
   printf("Pointer p address is %p value is %d\n", p, *p);

   /* Accessing the whole array using pointer */
   printf("Using p arr value is %d %d %d %d\n", *p, *(p+1), *(p+2), *(p+3));

   /* Accessing whole array using p and array notation */
   printf("Using p (index notation) arr value is %d %d %d %d\n", p[0], p[1], p[2], p[3]);

   /* Using arr with pointer notation*/
   printf("arr value using pointer notation is %d %d %d %d\n", *arr, *(arr+1), *(arr+2), *(arr+3));

   return 0;
}

Array arr is 11 34 71 2
Address of arr[0] is 0xbf90dbcc
Address of arr[1] is 0xbf90dbd0
Address of arr[2] is 0xbf90dbd4
Address of arr[3] is 0xbf90dbd8
Pointer p address is 0xbf90dbcc value is 11
Pointer p address is 0xbf90dbd4 value is 71
Pointer p address is 0xbf90dbd8 value is 2
Pointer p address is 0xbf90dbcc value is 11
Using p arr value is 11 34 71 2
Using p (index notation) arr value is 11 34 71 2
arr value using pointer notation is 11 34 71 2

Lessons learnt from the above program

1. &arr[0] is address of a integer, hence it can be assigned to integer pointer.
2. Pointer arithmetic makes so much sense when it comes to array.
1. ++ increment of the pointer makes it to point to next element of the array.
2. Other short hand operator can be used with pointer to move though the array very easily.
3. Pointer and array notation are interchangeable.
1. Array can used with pointer notation, e.g *(arr+2) so on.
2. Pointer can be used with array notation, e.g. p[2].
4. NOTE array and pointer are very different.
1. sizeof(arr) would return 16 (assuming int is 4 bytes), while the sizeof(p) is always 4 (assuming 32 bit machine).
2. Pointer can point to different array. e.g. p = &brr[0];, But same cannot be done to array.
3. Address of the array is fixed and cannot be changed.

Hence by now you should be very familiar with array and pointer and how they are related. I always call them as brother, lot of things in conman, but both are not same.

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