Control Flow in C — if-else, switch, for, while, do-while, break, continue

Control statements decide which code runs and how many times.

1. if, else if, else

#include <stdio.h>

int main(void) {
    int marks = 67;
    if (marks >= 75) {
        printf("Distinction\n");
    } else if (marks >= 60) {
        printf("First class\n");
    } else if (marks >= 40) {
        printf("Pass\n");
    } else {
        printf("Fail\n");
    }
    return 0;
}

First class

In C, any non-zero value is true and 0 is false. So if (5) runs and if (0) doesn't.

Trap: if (x = 0) assigns 0 to x and is always false; if (x = 5) is always true. You meant ==.

2. switch

#include <stdio.h>

int main(void) {
    char op = '*';
    int a = 6, b = 3;
    switch (op) {
        case '+': printf("%d\n", a + b); break;
        case '-': printf("%d\n", a - b); break;
        case '*': printf("%d\n", a * b); break;
        case '/': printf("%d\n", a / b); break;
        default:  printf("unknown operator\n");
    }
    return 0;
}

18

• switch works with integers and characters (not strings or floats).
• Without break, execution falls through into the next case — a favourite output question.

3. The three loops

#include <stdio.h>

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

    int n = 4521, rev = 0;
    while (n > 0) {
        rev = rev * 10 + n % 10;
        n /= 10;
    }
    printf("reversed = %d\n", rev);

    int k = 100;
    do {
        printf("do-while runs once (k = %d)\n", k);
    } while (k < 10);
    return 0;
}

1 2 3 4 5
reversed = 1254
do-while runs once (k = 100)

4. break and continue

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; i++) {
        if (i % 2 == 0) continue;   /* skip even numbers */
        if (i > 7) break;           /* stop completely */
        printf("%d ", i);
    }
    printf("\n");
    return 0;
}

1 3 5 7

5. Nested loops and patterns

#include <stdio.h>

int main(void) {
    int n = 4;
    for (int i = 1; i <= n; i++) {
        for (int s = 1; s <= n - i; s++) printf(" ");
        for (int j = 1; j <= 2 * i - 1; j++) printf("*");
        printf("\n");
    }
    return 0;
}

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

The outer loop picks the row; inner loops print spaces, then stars for that row.

Prime numbers up to 30

#include <stdio.h>

int main(void) {
    for (int n = 2; n <= 30; n++) {
        int prime = 1;
        for (int d = 2; d * d <= n; d++) {
            if (n % d == 0) { prime = 0; break; }
        }
        if (prime) printf("%d ", n);
    }
    printf("\n");
    return 0;
}

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Common mistakes

• A semicolon right after the loop: for (i = 0; i < 5; i++); — the body runs once, after the loop.
• Forgetting break in switch cases.
• Infinite while loops (the variable never changes).

Quick recap

• Non-zero is true, 0 is false; beware = vs ==.
• switch on int/char; break to stop fall-through.
• for (known count), while (unknown), do-while (at least once).
• break exits the loop; continue skips to the next round.