Control Structures in C

Control structures determine the flow of execution of a program.

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1. if Statement

if (condition) {
    /* executes if condition is true */
}

2. if-else Statement

if (condition) {
    /* true block */
} else {
    /* false block */
}

3. if-else-if Ladder

if (marks >= 90) {
    printf("Grade A");
} else if (marks >= 75) {
    printf("Grade B");
} else if (marks >= 60) {
    printf("Grade C");
} else {
    printf("Fail");
}

4. Nested if

if (a > 0) {
    if (b > 0) {
        printf("Both positive");
    }
}

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5. switch Statement

switch (expression) {
    case value1:
        /* statements */
        break;
    case value2:
        /* statements */
        break;
    default:
        /* default statements */
}

Example:

int day = 3;
switch (day) {
    case 1: printf("Monday"); break;
    case 2: printf("Tuesday"); break;
    case 3: printf("Wednesday"); break;
    default: printf("Other day");
}

Note: Without break, execution falls through to the next case.

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6. while Loop

while (condition) {
    /* loop body */
}

int i = 1;
while (i <= 5) {
    printf("%d ", i);
    i++;
}
/* Output: 1 2 3 4 5 */

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7. do-while Loop

do {
    /* loop body — executes at least once */
} while (condition);

int i = 1;
do {
    printf("%d ", i);
    i++;
} while (i <= 5);

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8. for Loop

for (initialisation; condition; update) {
    /* loop body */
}

for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}

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Comparison: while vs do-while vs for

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Nested Loops Example — Multiplication Table

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        printf("%d\t", i * j);
    }
    printf("\n");
}