Arrays

One name, many boxes

int marks[5]; makes 5 int boxes in a row: marks[0] to marks[4]. Positions start at 0, so the last one is size − 1.

C doesn't check positions for you. Reading marks[5] doesn't stop with an error: it quietly reads whatever memory comes next. Keep every index between 0 and size − 1.

Fill it, then use it

#include <stdio.h>

int main() {
    int n, a[100], sum = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++) scanf("%d", &a[i]);
    for (int i = 0; i < n; i++) sum += a[i];
    printf("first %d, last %d, sum %d\n", a[0], a[n - 1], sum);
    return 0;
}

1. Line 5: n is 4: we'll use a[0] to a[3] of the 100 boxes.
2. Line 6: Read 4 numbers into positions 0, 1, 2, 3.
3. Line 7: Add them up: 3 + 8 + 1 + 6.
4. Line 8: The last element is at n − 1, not n.

Shifting things around

#include <stdio.h>

int main() {
    int a[4] = {5, 6, 7, 8};
    for (int i = 0; i < 3; i++) a[i] = a[i + 1];
    printf("%d %d %d %d\n", a[0], a[1], a[2], a[3]);
    return 0;
}

What does this print?

• 6 7 8 8
• 5 5 6 7
• 6 7 8 5

Exercise: Print it backwards

Read n (1 to 100), then n numbers. Print them in reverse order, separated by spaces.

Exercise: Above average

Read n (1 to 100) and n marks. Print how many marks are strictly above the average.