Loops: Repetition Control

Loops are used to repeat a block of code multiple times. This is essential for tasks like processing arrays, printing patterns, or running simulations.

8.1 The while Loop

A pre-test loop. It checks the condition before executing the block.

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

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8.2 The do-while Loop

A post-test loop. It executes the block at least once before checking the condition.

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

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8.3 The for Loop

The most compact loop, combining initialization, condition, and increment/decrement. Syntax: for (init; condition; update) { ... }

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

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8.4 Loop Control Statements

break

Immediately terminates the loop and jumps to the statement following the loop.

for (int i = 1; i <= 10; i++) {
    if (i == 5) break; // Stops at 5
    printf("%d ", i);
}

continue

Skips the rest of the current iteration and jumps to the next update/condition check.

for (int i = 1; i <= 5; i++) {
    if (i == 3) continue; // Skips printing 3
    printf("%d ", i);
}

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8.5 Nested Loops

Placing one loop inside another. Commonly used for multi-dimensional data or patterns.

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

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Example: Factorial Calculation

#include <stdio.h>

int main() {
    int n, i;
    unsigned long long fact = 1;
    printf("Enter an integer: ");
    scanf("%d", &n);

    if (n < 0)
        printf("Error! Negative numbers don't have factorials.");
    else {
        for (i = 1; i <= n; ++i) {
            fact *= i;
        }
        printf("Factorial of %d = %llu", n, fact);
    }

    return 0;
}

Refer to the Lecture Slides for a comparison between while and for loops.