Label the boxes with a loop

Three boxes need labels. You could write three printf calls, but a for loop keeps the changing number in one place. In for (int box = 1; box <= 3; box++), C initializes box once, checks the condition before each pass, runs the body when it is true, and increments afterward. box++ adds one. When the condition fails, execution continues after the loop.

Trace the arrows with box = 1: print 1, then update to 2. The same happens for 2 and 3. At 4, 4 <= 3 is false, so nothing else prints from the loop. Without an update, a condition that stays true can keep the program running forever.

Run the box labels and check where the final message appears:

#include <stdio.h>

int main(void) {
    for (int box = 1; box <= 3; box++) {
        printf("Box %d\n", box);
    }
    printf("All boxes labeled\n");
    return 0;
}

Set the upper limit to 2 and count the box lines. For a running total, keep a variable outside the loop and update it inside:

#include <stdio.h>

int main(void) {
    int sum = 0;
    for (int number = 1; number <= 4; number++) {
        sum = sum + number;
    }
    printf("Sum: %d\n", sum);
    return 0;
}

sum starts at zero, then keeps its previous value between passes. Trace it: as number goes 1, 2, 3, 4, sum becomes 1, 3, 6, 10.

Three passes, two starting points

Labels 1 through 3 use box <= 3. Array indexes for three elements are 0 through 2, so they use i < 3. Both loops run three times; the starting values differ. Keep that distinction in mind for the accumulator exercise.