C Pointers and Arrays

In C, pointers and arrays are very closely related. The name of an array is the address of its first element.

Array name as pointer

#include <stdio.h>

int main(void) {
    int a[] = {10, 20, 30};

    printf("%d\n", *a);          // a[0]
    printf("%d\n", *(a + 1));    // a[1]
    printf("%d\n", *(a + 2));    // a[2]
    return 0;
}

Output:

The expression a[i] is exactly the same as *(a + i). The compiler converts the first form to the second.

Store the array address in a pointer

#include <stdio.h>

int main(void) {
    int a[] = {10, 20, 30};
    int *p = a;

    for (int i = 0; i < 3; i++) {
        printf("%d %d\n", p[i], *(p + i));
    }
    return 0;
}

Output:

Difference between array and pointer

Array	Pointer
Fixed block of memory	Variable that stores an address
sizeof(a) is the whole array	sizeof(p) is size of the pointer
Cannot be changed to point somewhere else (a++ is an error)	Can point to any place (p++ is fine)

#include <stdio.h>

int main(void) {
    int a[5] = {1, 2, 3, 4, 5};
    int *p = a;

    printf("%zu\n", sizeof(a));
    printf("%zu\n", sizeof(p));
    return 0;
}

Output:

Next: C Pointers and Functions.

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