Looping

Programming Language & Its Applications — Introduction to C Programming, NEC licence examination syllabus (Nepal Engineering Council).

Looping: three forms, one purpose

for, while, do-while — and knowing which fits saves you from off-by-one errors.

All three loops repeat a block, and any one can be rewritten as the others. But they express different intentions: for says "a known number of iterations", while says "until a condition changes", and do-while says "at least once, then check". Matching the loop to the intention is what makes code readable — and prevents the off-by-one mistakes that plague loop-heavy exam questions.

🌍 Where this lives: the infinite loop for(;;) at the end of this topic is what every embedded device runs. The firmware in a microwave, a router, an Arduino or a pacemaker has a main that never returns — it loops forever reading sensors and driving outputs, because there is no operating system to return to. Search "embedded superloop architecture" and you will find this exact pattern in production code controlling real machinery.

The three forms

for (init; condition; update) { body; } → condition tested BEFORE the first iteration → body may run ZERO times while (condition) { body; } → condition tested BEFORE each iteration → body may run ZERO times do { body; } while (condition); ← note the semicolon → condition tested AFTER each iteration → body ALWAYS runs at least ONCE

three_loops.c

#include <stdio.h>

int main(void) {
    /* for - counted iteration */
    for (int i = 1; i <= 5; i++) printf("%d ", i);
    printf("\n");

    /* while - same thing, more verbose */
    int j = 1;
    while (j <= 5) { printf("%d ", j); j++; }
    printf("\n");

    /* do-while - runs even when the condition is false */
    int k = 10;
    do { printf("ran once with k=%d\n", k); } while (k < 5);

    /* while with the same false condition: never runs */
    int m = 10;
    while (m < 5) { printf("never printed\n"); }
    return 0;
}
Output:
1 2 3 4 5
1 2 3 4 5
ran once with k=10

The do-while distinction is not academic. Menu loops and input validation genuinely need "do the thing, then decide whether to repeat" — you must show the menu before you can know whether the user wants to quit. Using a while loop there forces you to duplicate the menu code before the loop.

break and continue

break → exit the loop entirely continue → skip the rest of THIS iteration, go to the next In a for loop, continue still executes the UPDATE part. In a while loop it does NOT — which is a classic infinite loop: int i = 0; while (i < 10) { if (i == 5) continue; ← i never increments: HANGS printf("%d ", i); i++; }

break_continue.c

#include <stdio.h>

int main(void) {
    /* skip multiples of 3, stop at 13 */
    for (int i = 1; i <= 20; i++) {
        if (i % 3 == 0) continue;   /* skip this one   */
        if (i > 13)     break;      /* leave the loop  */
        printf("%d ", i);
    }
    printf("\n");

    /* nested loops: break exits only the INNER one */
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            if (j == 2) break;   /* only escapes the j loop */
            printf("(%d,%d) ", i, j);
        }
    }
    printf("\n");
    return 0;
}
Output:
1 2 4 5 7 8 10 11 13
(1,1) (2,1) (3,1)

Worked example 1 — patterns with nested loops

pyramid.c

#include <stdio.h>

int main(void) {
    int n = 5;

    /* right triangle */
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= i; j++) printf("* ");
        printf("\n");
    }
    printf("\n");

    /* centred pyramid: spaces then stars */
    for (int i = 1; i <= n; i++) {
        for (int s = n - i; s > 0; s--) printf(" ");
        for (int j = 1; j <= 2*i - 1; j++) printf("*");
        printf("\n");
    }
    return 0;
}
Output:
*
* *
* * *
* * * *
* * * * *

    *
   ***
  *****
 *******
*********

The pyramid arithmetic, worth deriving rather than memorising: row i (1-based, n rows total): leading spaces = n − i stars = 2i − 1 Check row 3 of 5: spaces = 2, stars = 5 ✔ Total stars in the pyramid = n² (sum of odd numbers) for n=5: 1+3+5+7+9 = 25 = 5² ✔

Worked example 2 — the off-by-one boundary

int a[5]; /* valid indices are 0,1,2,3,4 */ for (int i = 0; i < 5; i++) ✔ 5 iterations: 0-4 for (int i = 0; i <= 5; i++) ✗ 6 iterations: 0-5 → a[5] is OUT OF BOUNDS for (int i = 1; i <= 5; i++) ✗ skips a[0], overruns The safe idiom for an array of n elements: for (int i = 0; i < n; i++) Count of iterations for for (i = a; i < b; i++) is b − a Count for for (i = a; i <= b; i++) is b − a + 1 That "+1" is where off-by-one bugs come from.

Worked example 3 — infinite loop forms

infinite.c

#include <stdio.h>

int main(void) {
    int count = 0;

    /* all three are the same infinite loop */
    /* for (;;) { }      while (1) { }     do { } while(1); */

    for (;;) {                    /* the idiomatic C form */
        count++;
        if (count >= 3) break;    /* exit condition inside */
        printf("iteration %d\n", count);
    }
    printf("exited after %d\n", count);

    /* An accidental infinite loop - float comparison */
    /* for (float f = 0.0f; f != 1.0f; f += 0.1f) ...
       NEVER terminates: 0.1 is not exact in binary, so
       the sum never equals exactly 1.0                  */
    return 0;
}
Output:
iteration 1
iteration 2
exited after 3

That commented-out float loop is a genuinely important trap: never test floating-point values for exact equality. Use f < 1.0f or compare against a tolerance.

💡 Exam angle: pattern-printing programs (pyramids, triangles, number patterns) are extremely common — derive the spaces and stars formulas rather than guessing. Know that do-while always executes once, and that continue in a while loop skips the increment, causing a hang. For output-tracing questions, count iterations carefully: i < n gives n iterations, i <= n gives n+1.

Syllabus points

for, while, do-while; break/continue

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