Chapter 825 min read

C Loops & Iterative Control Flow

Master repetitive execution using while loops, do-while loops, and break/continue control statements.

Released1972

Core MottoAutomate Repetitive Logic Efficiently

CreatorsDennis Ritchie / Bell Labs

1. Why Loops? Loop Architecture

Loops allow executing a single block of code repeatedly without writing duplicate statements.

Conditions determine whether the loop body continues running or exits.

The Problem with Repetitive Code

Problem Statement:Printing a message 5 times sequentially requires writing 5 separate `printf` statements. Doing this a thousand times is impractical.

Solution:Programming languages provide loop statements to repeat a block of code until a specified stopping condition is reached.

#include <stdio.h>

int main() {
    // Sequential repetitive print (Without Loops)
    printf("Hello, World!\n");
    printf("Hello, World!\n");
    printf("Hello, World!\n");
    printf("Hello, World!\n");
    printf("Hello, World!\n");
    return 0;
}

Basic Loop Logic in C

Loop Control Variable:A counter variable (e.g., `int i = 0`) tracks iteration state.

Condition Evaluation:The loop checks if `i < 5`. If true, the body executes.

Increment Step:Updating `i = i + 1` ensures the loop eventually terminates and avoids an infinite execution cycle.

#include <stdio.h>

int main() {
    int i = 0;
    
    // Iterative approach using a while loop
    while (i < 5) {
        printf("Hello, World!\n");
        i = i + 1;
    }
    return 0;
}

2. The while Loop

A pre-test loop that evaluates the condition before executing the code block.

If the initial condition is false, the loop body never runs.

while Loop Syntax & Mechanics

Keyword & Condition:Starts with `while` followed by a boolean condition in parentheses `( condition )`.

Body Enclosure:Code statements are enclosed in curly braces `{...}`.

Truth Values:Non-zero values are treated as `true`, while `0` is treated as `false`.

Exit Mechanism:As soon as the condition evaluates to false, execution resumes at the statement following the loop body.

#include <stdio.h>

int main() {
    int count = 0;

    while (count < 5) {
        printf("Hello, World!\n");
        count++;
    }
    return 0;
}

3. The do...while Loop

A post-test loop that evaluates its condition after executing the body.

Guarantees that the loop statements execute at least once.

do...while Syntax & Mechanics

Guarantee:Executes the loop body first, then checks the condition at the bottom.

Mandatory Semicolon:Note the semicolon after the condition: `while ( condition );`.

Use Cases:Ideal for menu systems, input validation, or tasks that must execute at least once regardless of initial state.

#include <stdio.h>

int main() {
    int i = 0;

    do {
        printf("Hello, World!\n");
        i = i + 1;
    } while (i < 5);

    return 0;
}

4. Control Statements: break & continue

The break statement exits a loop immediately.

The continue statement skips the current iteration and jumps to the next evaluation.

The break Statement

Immediate Termination:Forces an immediate exit from the innermost active loop.

Execution Resumption:Program flow jumps directly to the statement immediately following the loop body.

#include <stdio.h>

int main() {
    int i = 0;

    do {
        printf("Hello, World!\n");
        i = i + 1;
        
        // Exits early after 3 iterations
        if (i == 3) {
            break;
        }
    } while (i < 5);

    return 0;
}

The continue Statement

Skip Iteration:Bypasses the rest of the code in the current iteration without exiting the loop entirely.

Next Evaluation:Re-evaluates the condition for `while` / `do-while` loops or advances the update step in `for` loops.

#include <stdio.h>

int main() {
    int i = 0;

    do {
        // Skips printing when i equals 3
        if (i == 3) {
            i = i + 1;
            continue;
        }
        printf("Hello, World!\n");
        i = i + 1;
    } while (i < 5);

    return 0;
}

Classroom REMINDER

How does a while loop differ from a do...while loop when the initial condition is false?

Takeaway Goal: A while loop tests its condition before executing and will run zero times if the condition is false initially. A do...while loop checks its condition at the end, guaranteeing that the body executes at least once.