Control Flow in C Explained — if, else, loops and switch with Real Examples

#include <stdio.h>

int main(void) {
    int temperature = 28; /* degrees Celsius — change this to test different branches */

    if (temperature > 30) {
        /* This block only runs when temperature is strictly greater than 30 */
        printf("It's scorching! Turn the air conditioning on.\n");
    } else if (temperature >= 20) {
        /* Runs when temperature is between 20 and 30 (inclusive of 20) */
        printf("It's comfortable. No action needed.\n");
    } else if (temperature >= 10) {
        /* Runs when temperature is between 10 and 19 */
        printf("A bit chilly. Maybe grab a jacket.\n");
    } else {
        /* Catches everything below 10 — the 'default' safety net */
        printf("It's freezing! Turn the heating on.\n");
    }

    return 0; /* 0 tells the operating system the program finished successfully */
}

#include <stdio.h>

int main(void) {

    /* ── FOR LOOP ──────────────────────────────────────────────
       We know exactly how many students we have (5),
       so a for loop is the cleanest choice here.
       Breakdown of: for (initializer; condition; update)
         - int student = 1      → start counting at student 1
         - student <= 5         → keep going while we haven't exceeded 5
         - student++            → after each loop body, add 1 to student
    ────────────────────────────────────────────────────────── */
    printf("--- Roll Call (for loop) ---\n");
    for (int student = 1; student <= 5; student++) {
        printf("Calling student number %d\n", student);
    }

    /* ── WHILE LOOP ────────────────────────────────────────────
       We don't know how many attempts the user will need,
       so while is more natural. Here we simulate a countdown
       where fuel_level drives when we stop.
    ────────────────────────────────────────────────────────── */
    printf("\n--- Rocket Fuel Countdown (while loop) ---\n");
    int fuel_level = 5;
    while (fuel_level > 0) {
        printf("Fuel level: %d — burning...\n", fuel_level);
        fuel_level--; /* subtract 1 from fuel each iteration */
    }
    printf("Fuel exhausted. Engine off.\n");

    /* ── DO-WHILE LOOP ─────────────────────────────────────────
       The menu always needs to display at least once before
       we can check what the user chose. do-while guarantees
       the body runs before the condition is ever evaluated.
    ────────────────────────────────────────────────────────── */
    printf("\n--- Vending Machine Menu (do-while loop) ---\n");
    int user_choice;
    do {
        printf("Press 1 for Coffee, 2 for Tea, 0 to Exit: ");
        /* Hardcoded for demo — in a real program this would be scanf() */
        user_choice = 1;
        printf("%d\n", user_choice); /* echo the simulated input */

        if (user_choice == 1) {
            printf("Dispensing coffee. Enjoy!\n");
        } else if (user_choice == 2) {
            printf("Dispensing tea. Enjoy!\n");
        }
    } while (user_choice != 0); /* keep looping until user chooses Exit */
    /* NOTE: Because user_choice is hardcoded to 1 above, this loops once
       and then re-evaluates. In a real program, scanf would update it. */

    return 0;
}

#include <stdio.h>

int main(void) {
    int day_number = 3; /* 1 = Monday, 2 = Tuesday, ... 7 = Sunday */

    printf("Day %d is: ", day_number);

    switch (day_number) {
        case 1:
            printf("Monday\n");
            break; /* break jumps execution to after the closing } of switch */

        case 2:
            printf("Tuesday\n");
            break;

        case 3:
            printf("Wednesday\n");
            break;

        case 4:
            printf("Thursday\n");
            break;

        case 5:
            printf("Friday\n");
            break;

        case 6:  /* Intentional fall-through — Saturday AND Sunday share the same message */
        case 7:  /* No break on case 6, so C slides down into case 7's body */
            printf("Weekend! No work today.\n");
            break; /* break HERE stops us falling into default */

        default:
            /* Catches any value outside 1-7 — always include this guard */
            printf("Invalid day number. Please use 1 to 7.\n");
            break;
    }

    return 0;
}

#include <stdio.h>

int main(void) {

    /* ── BREAK EXAMPLE ────────────────────────────────────────
       We're scanning a list of exam scores to find the first
       student who failed (scored below 50). Once we find one,
       there's no need to keep scanning — we break out.
    ────────────────────────────────────────────────────────── */
    int exam_scores[] = {78, 91, 65, 43, 88, 55}; /* array of 6 scores */
    int total_students = 6;

    printf("--- Scanning for first failing score (break demo) ---\n");
    for (int index = 0; index < total_students; index++) {
        if (exam_scores[index] < 50) {
            /* Found a failing score — report it and stop searching */
            printf("First fail found at index %d: score %d\n", index, exam_scores[index]);
            break; /* exit the for loop immediately */
        }
        printf("Score %d passed.\n", exam_scores[index]);
    }

    /* ── CONTINUE EXAMPLE ─────────────────────────────────────
       We want to print only the ODD numbers from 1 to 10.
       When we hit an even number, we skip (continue) to the
       next iteration rather than printing it.
    ────────────────────────────────────────────────────────── */
    printf("\n--- Printing only odd numbers 1-10 (continue demo) ---\n");
    for (int number = 1; number <= 10; number++) {
        if (number % 2 == 0) {
            /* % is the modulus operator — it gives the remainder.
               If remainder after dividing by 2 is 0, the number is even. */
            continue; /* skip the printf below and go to next iteration */
        }
        /* This line only executes for odd numbers */
        printf("%d is odd\n", number);
    }

    return 0;
}