1-D arrays: store, index, and traverse

A row of boxes

• An array is a row of variables of the same type, kept side by side in memory.
• int a[5]; makes room for five ints. They are one block, reached by number.
• You can fill them when you create them: int a[] = {10, 20, 30}; makes an array of three.

Indexing from 0

• Each item has an index (a position). The first index is 0, not 1.
• a[0] is the first item, a[1] the second, and a[n - 1] is the last in an array of n items.
• Reading or writing outside the array (like a[5] in a 5-item array) is a serious bug.

Traversing with a loop

• To visit every item, run an index from 0 up to n - 1 with a for loop.
• Inside the loop, use a[i] to read or change the item at position i.
• This is how you sum, count, or search an array.

Arrays don't know their own length

• An array does not carry its own size. A function cannot ask "how long are you?".
• So you always pass the length next to the array: sum(a, n).
• Use const int a[] in a parameter to promise you will only read the array, not change it.

#include <stdio.h>

int main(void) {
    int a[] = {10, 20, 30, 40};
    int n = 4, total = 0;
    for (int i = 0; i < n; i++) {
        total += a[i];
    }
    printf("%d\n", total);   // 100
    return 0;
}

Common mistakes

• C does NOT check array bounds: a[n] in a size-n array is undefined.
• Valid indexes run 0 to n - 1.

Now you try

• Pass both the array and its length n, and loop the index from 0 to n - 1.
• For the function tasks, do not write a main — the checker provides one.