C Loops: for, while, do-while, break, continue & Control Flow

⚡ C (C17 / C23 Standard) 🟢 Lesson 8 📂 Phase 05: Loops & Iterations 📅 2026 Edition

📌 Covered in this in-depth guide: Why Loops are Needed · 3 Pillars of a Loop · for Loop Mechanics · while vs do-while · break & continue · Infinite Loops · Array & String Traversal

Welcome to Phase 5 (Part 1): C Loops, Iteration Mechanics & Control Flow Masterclass! In programming, repetitive manual execution violates the core DRY (Don't Repeat Yourself) principle. Loops allow your CPU to execute a block of instructions millions of times with microscopic precision and high performance. In this comprehensive guide, you will master the 3 structural pillars of every loop, the internal mechanics of the for loop, entry-controlled while loops vs exit-controlled do-while loops, control jump statements (break and continue), diagnosing infinite loop bugs, and traversing memory arrays and string buffers.

1Why Loops are Needed & The 3 Structural Pillars of Every Loop

Oka task ni 1,000 times manually repeat cheyyakunda, automated loop construct dwara single line tho execute cheyyavachu. World lo prati loop (regardless of programming language) 3 Mandatory Components meedha run avthundhi:

1. Loop Initialization: Loop counter variable starting value ni set cheyyadam (e.g. int i = 1;).
2. Loop Condition: Prati iteration mundhu check chese boundary criteria (e.g. i <= 10;). Condition TRUE unnantha varaku loop run avthundhi.
3. Loop Update (Increment / Decrement): Counter variable ni target value vaipu move cheyyadam (e.g. i++). Missing update causes an Infinite Loop Bug!

The 4-Step Execution Cycle of a for Loop:
for ( [1. Init] ; [2. Condition Check] ; [4. Update Counter] ) {
[3. Execute Loop Body Statements];
}

Flow Order: [1. Init] ──► [2. Check] (TRUE) ──► [3. Run Body] ──► [4. Update] ──► [2. Check again...]

2The for Loop (Deterministic Iteration)

Number of iterations mundhe thelisinappudu for loop best choice. C99 standard nunchi loop counter variable ni directly for header loni declare cheyyavachu (Block Scope):

C — User Curriculum Example ▶ Run in C Compiler

#include <stdio.h>

int main(void) {
    // Prints numbers from 1 to 5
    for (int number = 1; number <= 5; number++) {
        printf("%d\n", number);
    }

    return 0;
}

3while (Entry-Controlled) vs do-while (Exit-Controlled) Loops

Loop Construct	Condition Check Timing	Minimum Executions Guaranteed	Typical Real-World Use Case
`while` Loop	Entry-Controlled: Loop body execute avvaka mundhe condition check avthundhi.	0 times (Condition first time fail ayithe body zero times run avthundhi).	File reading, network packet streaming, unknown iteration counts.
`do-while` Loop	Exit-Controlled: Loop body execute ayina tharvatha condition check avthundhi.	1 time guaranteed (Even if condition is completely false!).	Interactive Console Menus (Prompt user at least once before checking choice).

C — while vs do-while Comparison ▶ Run Code

#include <stdio.h>

int main(void) {
    int count = 10;

    // while loop: condition is false (10 < 5), so body executes 0 times!
    while (count < 5) {
        printf("This while loop will NEVER print.\n");
        count++;
    }

    // do-while loop: body executes 1 time BEFORE checking condition!
    int val = 10;
    do {
        printf("do-while executes at least once! (val = %d)\n", val);
        val++;
    } while (val < 5); // ⚠️ Note the mandatory semicolon ';' at the end!

    return 0;
}

4break vs continue & Traversing Arrays and Strings

⚡ break vs continue Control Jump Commands

• break;: Loop execution ni ventane terminate chesi loop outer scope ki jump chesthundhi (Search element dorikinappudu loop nunchi exit avvadaniki).
• continue;: Current iteration lo kindha unna code ni skip chesi, ventane next iteration (update step) ki jump chesthundhi (Even numbers filter cheyyadaniki).

C — String & Array Traversal with Loops ▶ Run Code

#include <stdio.h>

int main(void) {
    char message[] = "Hello C!";
    int scores[] = {85, 92, 40, 78, 95};
    int size = sizeof(scores) / sizeof(scores[0]);

    // 1. Looping through string until null-terminator '\0'
    printf("Characters in message: ");
    for (int i = 0; message[i] != '\0'; i++) {
        printf("[%c] ", message[i]);
    }
    printf("\n");

    // 2. Looping through array with continue (skip failing grades < 50)
    printf("Passing Scores: ");
    for (int i = 0; i < size; i++) {
        if (scores[i] < 50) {
            continue; // Skip failing score
        }
        printf("%d ", scores[i]);
    }
    printf("\n");

    return 0;
}

💻 Try It Yourself — Test Loops in Live C Compiler

Run this 1 to 20 even number accumulator loop in our online C compiler:

C (GCC Standard) ▶ Open C Compiler

#include <stdio.h>

int main(void) {
    int sum = 0;
    for (int i = 1; i <= 20; i++) {
        if (i % 2 == 0) {
            sum += i;
        }
    }
    printf("Sum of even numbers (1 to 20) = %d\n", sum);
    return 0;
}

Open in Online C Compiler →