Arrays and Pointers

Big idea

An array's name works like a pointer to its first element, and arr[i] is really "start at arr, move i elements along". Pointers and arrays are two views of the same memory.

The idea

int a[4] = {10, 20, 30, 40};
int *p = a;        // same as  int *p = &a[0];

Adding to a pointer moves it by whole elements, not bytes. If p points at a[0], then p + 2 points at a[2] — C multiplies by sizeof(int) for you.

In fact a[i] is defined as *(a + i).

Walking an array with a pointer

*p++ reads the value then moves the pointer — ++ on the pointer, used after *.

Passing arrays to functions

When you pass an array to a function, C passes the address of its first element — not a copy of the whole array. Two consequences:

1. The function can change the caller's array.
2. The function can't tell how long the array is, so you pass the length too.

#include <stdio.h>

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

int main(void) {
    int nums[5] = {1, 2, 3, 4, 5};
    double_all(nums, 5);
    for (int i = 0; i < 5; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n");
    return 0;
}

output
2 4 6 8 10

int *arr and int arr[] mean exactly the same thing in a parameter list.

Watch the array change

Think of it like

Houses on a street. The street name gets you to the first house; "3 doors down" gets you to the fourth. a + 3 is "3 doors down from the start".

🕷️ Spidey-sense: Swinging down 5th Avenue, I don't count bricks — I count buildings. p + 1 is the next building, however wide it is. C does the brick-counting for me.

Key terms

• Pointer arithmetic — Adding to or subtracting from a pointer moves it by whole elements.
• Array decay — An array name becoming a pointer to its first element when used in an expression or passed to a function.