7.2 Pointers and Arrays

An array name is essentially a pointer to the first element. So arr and &arr[0] are the same address, and you can even take the array's address without writing &.

int arr[] = {4, 1, 6, 7};
int *p = arr;             // same as &arr[0]

printf("%d\n", *p);       // 4
printf("%d\n", *(p + 2)); // 6

Pointer arithmetic

When you add to or subtract from a pointer into an array (such as p++ or p -= 2), it does not move the address by one byte; it moves by the size of the pointer's type. For example, if p is an int * and an int takes 4 bytes, then p + 1 increases the address by 4.

So *(p + i) equals arr[i], and you can also write p[i]. You can walk the whole array with a pointer:

for (int *it = arr; it < arr + 4; it++) {
    printf("%d ", *it);
}

Arrays and functions

When you pass an array to a function, what you actually pass is the address of the first element, so the function receives a pointer. Inside the function, sizeof gives the size of a pointer, not the array length. That is why you usually pass the length as a separate parameter.

int search(int *arr, int n, int key) {
    for (int i = 0; i < n; i++) {
        if (arr[i] == key) {
            return i;
        }
    }
    return -1;   // not found
}

int main(void) {
    int arr[] = {4, 7, 1, 3, 9, 2};
    int n = sizeof(arr) / sizeof(arr[0]);
    printf("%d\n", search(arr, n, 3));   // 3
    return 0;
}

sizeof(arr) / sizeof(arr[0]) only works where the array itself lives; once the array decays to a pointer, it no longer gives the length.