Pointers and Arrays

In C, pointers and arrays are close friends. The name of an array, used on its own, is the address of its first element. So you can store it in a pointer, without &:

#include <stdio.h>

int main(void) {
    int numbers[] = {3, 7, 1};
    int *p = numbers;
    printf("%d\n", *p);
    printf("%d\n", p[1]);
    return 0;
}

This prints:

3
7

int *p = numbers; means the same as int *p = &numbers[0];, so *p is the first element. And you can use square brackets with a pointer, just like with the array: p[1] is the element right after the one p points to, here 7.

The elements of an array lie right next to each other in memory. p + 1 is the address of the next element, and p[1] is a shorter way to write *(p + 1). This course uses the square brackets, but you'll see the other form in other C programs.

Remember the error from the lesson on arrays: printing a whole array with printf("%d\n", scores); doesn't compile, and the message says the argument has type 'int *'. Now you know why: the name of the array is an address, of the type int *, and not a number for %d.

This also explains something you saw in the lesson on functions. When you pass an array to a function, C doesn't copy it. It passes only the address of its first element. A parameter int numbers[] is in fact a pointer, so you can also write it as int *numbers. Both mean exactly the same:

#include <stdio.h>

void double_all(int *numbers, int count) {
    for (int i = 0; i < count; i++) {
        numbers[i] *= 2;
    }
}

int main(void) {
    int prices[] = {10, 25, 40};
    double_all(prices, 3);
    printf("%d %d %d\n", prices[0], prices[1], prices[2]);
    return 0;
}

This prints:

20 50 80

Through the address, the function works on the caller's array, so its changes stay. That's also why the function needs the count: it gets only the address of the first element, and it can't tell where the array ends. Even sizeof(numbers) / sizeof(numbers[0]) doesn't help inside the function. There, numbers is a pointer, so sizeof gives the size of the pointer, and the result is wrong (here it's 1). Only where the array was created, as in main, does sizeof see the whole array. So always pass the count as a parameter.