Pointers and Arrays

An array's name, used on its own, decays into a pointer to its first element — which is why array indexing and pointer arithmetic are two views of the exact same thing:

int nums[3] = {10, 20, 30};
int *p = nums;         // same as &nums[0]

printf("%d\n", nums[1]); // 20
printf("%d\n", *(p + 1)); // 20 — identical to nums[1]

This is also why an array parameter is really just a pointer in disguise, and sizeof behaves differently on it than on the original array:

void printSize(int arr[]) {
    printf("%zu\n", sizeof(arr)); // size of a POINTER, e.g. 8 — NOT the array!
}

int main(void) {
    int nums[5] = {1, 2, 3, 4, 5};
    printf("%zu\n", sizeof(nums)); // 20 — 5 ints, the real array size
    printSize(nums);
    return 0;
}

Common mistakes

• Using sizeof(arr) / sizeof(arr[0]) inside a function that received the array as a parameter — it silently computes the wrong number, since arr there is just a pointer. Pass the length as a separate parameter instead.