Iterative statements: while, do-while, for loop

📖 Theory

Sixty students, one program run

The marks program handles one student beautifully. The class has sixty.

Your options today: run the program sixty times (the peon's method), copy-paste the code sixty times (the disaster method), or tell C: "repeat this block until I say stop".

That third option is iteration, the last of your three structured-programming shapes, and it comes in three flavours whose differences are pure exam gold.

📖 Theory

Three kinds of repetition in real life

while is airport security: the guard checks BEFORE you enter, and if you fail the first check, you never enter at all (zero entries possible). do-while is tasting dal while cooking: you taste FIRST, then decide whether to adjust again, the first taste always happens. for is a teacher taking attendance: roll 1 to 60, she knows the count before starting and steps through it mechanically.

📖 Theory

The three loops, formally

• while (condition) { }: test first, run if true, repeat. Fails immediately? The body runs zero times (entry-controlled).
• do { } while (condition);: run first, test after, so the body always runs at least once (exit-controlled). Note the closing semicolon.
• for (init; condition; update) { }: the counted loop. init runs once, condition is tested before each round, update runs after each round:

for (i = 1; i <= 60; i++) reads: start at student 1, continue while within 60, step by one.

📊 At a glance

Choosing your loop

⌨️ Practical

The whole class, one loop

#include <stdio.h>

int main() {
    int i, marks, total = 0;

    for (i = 1; i <= 5; i++) {
        printf("Marks of student %d: ", i);
        scanf("%d", &marks);
        total = total + marks;
    }
    printf("Class total: %d\n", total);
    return 0;
}

This example runs in Gri-Learn on the web, where you can edit it and see the output.

🧩 Quiz

The condition of a while loop is false the very first time. The same condition sits in a do-while. How many times does each body run?

1. while: 0 times, do-while: 1 time
2. Both run 0 times
3. Both run 1 time
4. while: 1 time, do-while: 0 times

💭 Think first

Trace the loop

In the code above, the five entered marks are 70, 80, 60, 90, 75. Walk the loop in your head: what is total after each round, and what prints at the end?

⚠️ Watch out

The three loop killers

The stray semicolon: for (i = 1; i <= 5; i++); ends the loop with an empty body; the block below runs once. Same disease as if's semicolon. The infinite loop: forget the update (i++) and the condition never turns false; the program hangs. Off-by-one: i < 5 runs five times starting at 0, i <= 5 runs six starting at 0, count on your fingers in exams, every time.

📖 Theory

All three shapes acquired

Sequence, selection, and now iteration: the marks program uses every structured-programming shape, exactly as promised in Unit 1. Two refinements complete the loop story: loops INSIDE loops (the pattern-printing questions exams adore), and the emergency exits break and continue. Both are next.

✅ Summary

Key takeaways

• while: test before, may run zero times (entry-controlled).
• do-while: test after, always runs at least once (exit-controlled), note the semicolon.
• for (init; condition; update): the counted loop for known repetitions.
• Killers: semicolon after for(...), missing update (infinite loop), off-by-one bounds.
• The running-total accumulator is the loop pattern of a lifetime.
• Memory hook: guard checks first (while), taste first (do-while), attendance count (for).