C Loops Tutorial | for, while, and do-while Explained

📖 What to learn on this page

✅ Must-know essentials

for (init; cond; update) { ... }
while (cond) { ... } runs while cond is true
break exits; continue skips to next iteration

⭐ Read if you have time

do-while runs at least once
Infinite loops: for (;;) / while (1)
Nested loops and complexity

Why Do We Need Loops? — The Pain of Repetition

What if loops didn't exist? Let's try writing "print 1 to 5" without any loop, and see how painful programming would be.

❌ Without loops

#include <stdio.h>

int main(void) {
    printf("1\n");
    printf("2\n");
    printf("3\n");
    printf("4\n");
    printf("5\n");
    return 0;
}

Five times is barely manageable, but… what if you needed to print 1 to 100?

Imagine this: You're told to print 1 to 100. Would you really write printf("1\n"); printf("2\n"); ... printf("100\n"); for 100 lines? And if the requirement changed to "print only multiples of 3", you'd have to rewrite everything.

✅ With a loop

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 5; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Just 3 lines! And if you want to go up to 100, just change 5 to 100. Even 10,000 iterations fit in the same amount of code.

Feel the power of loops with this slider

Change the number of iterations. See how many lines you'd need without a loop.

Iterations N = 5

❌ Without loop

5 lines

You need to write printf N times

✅ With loop

3 lines

Same code no matter what N is

It gets worse: what if the logic changes?

Computing "the sum of 1 to 10" without a loop is also painful.

// Without loop: manually add each number
int sum = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10;

// With loop
int sum = 0;
for (int i = 1; i <= 10; i++) sum += i;

The essence of loops: "Repeat the same pattern, with a slightly different value each time." It's one of the most important concepts in programming — you'll see it in almost every program.

Even worse: If the number of iterations depends on user input (e.g., "sum from 1 to N where N is entered at runtime"), you can't write it without a loop. You don't know how many printf to write until the program runs.

for loop — a fixed number of iterations

for (int i = 0; i < 5; i++) {
  printf("i = %d\n", i);
}

Iteration-count visualizer

N = 5

Current i

—

Iterations

while loop

int n = 1;
while (n <= 100) {
  n = n * 2;
}
printf("n = %d\n", n); // -> 128

Caution: If the condition stays true forever, you get an infinite loop!

Step execution — for loop

for_demo.c

Variable state

Name	Type	Value

Standard output

do-while — always runs at least once

while checks the condition before the body; do-while checks it after. As a result, the body of a do-while runs at least once.

while (pre-test)

while (condition) {
    body;  // 0 iterations if false
}

do-while (post-test)

do {
    body;  // runs at least once
} while (condition);

Watch the trailing semicolon! Forgetting the ; after } while (condition); is a compile error.

Typical use: re-prompt on invalid input

"Enter 1-3" → re-prompt on bad input. do-while is ideal here.

int choice;
do {
    printf("Pick 1-3> ");
    scanf("%d", &choice);
    if (choice < 1 || choice > 3)
        printf("Invalid input\n");
} while (choice < 1 || choice > 3);
printf("You picked %d\n", choice);

Which loop to use

Construct	Test timing	Min iterations	When to use
`for`	Pre-test	0	Known number of iterations
`while`	Pre-test	0	Unknown count, stop on condition
`do-while`	Post-test	1	Must run at least once (input validation, etc.)

break & continue — loop control

break

Exits the loop immediately. All remaining iterations are skipped.

for (int i=0; i<10; i++) {
    if (i == 5) break;
    printf("%d ", i);
}
// Output: 0 1 2 3 4

continue

Skips the rest of the current iteration and goes on to the next one. The loop itself keeps running.

for (int i=0; i<5; i++) {
    if (i == 2) continue;
    printf("%d ", i);
}
// Output: 0 1 3 4 (skips 2)

Caution: break and continue only affect the innermost loop. break inside a nested loop does not exit the outer loop.

Try it yourself — loops

my_loop.c

Output

Click "Run" to execute...

💡 Try these ideas too

Print only evens, or reverse order (10, 9, ..., 1)
Nested for to print a multiplication table
Use break vs continue appropriately
Convert to do-while and observe the guaranteed-once behavior

Review Quiz

Check your understanding of this lesson!

Q1. How many times does `for (int i=0; i<5; i++)` execute?

4 times

5 times

6 times

i takes the five values 0, 1, 2, 3, 4, so the body runs 5 times.

Q2. What happens with `while(1)`?

Runs exactly once

Infinite loop

Compile error

The condition is always 1 (true), so the loop keeps running forever unless broken out of by break or return.

Q3. What is the difference between do-while and while?

No difference

do-while runs at least once

do-while is faster

do-while tests the condition at the end of the loop, so the body always runs at least once. while tests first, so it can run zero times.

Recommended Books to Deepen Your Understanding

Combine this interactive site with books for more practice.

📘

Painfully Learning C

by MMGames

A classic C book for beginners. Builds solid fundamentals with clear explanations.

View on Amazon

📗

New Clear C Language (Beginner)

by Bohyoh Shibata

Plenty of diagrams and exercises. Widely used as a university textbook.

View on Amazon

📙

The C Programming Language, 2nd Ed.

by Brian Kernighan & Dennis Ritchie

Known as K&R. The original C book. Ideal after completing the basics.

View on Amazon

* Links above are affiliate links. Purchases help support this site.

Lesson 10: Loops (for, while, do-while)

Why Do We Need Loops? — The Pain of Repetition

❌ Without loops

✅ With a loop

Feel the power of loops with this slider

It gets worse: what if the logic changes?

for loop — a fixed number of iterations

Iteration-count visualizer

while loop

Step execution — for loop

Variable state

Standard output

do-while — always runs at least once

Typical use: re-prompt on invalid input

Which loop to use

break & continue — loop control

Try it yourself — loops

Related Lessons

Review Quiz

Q1. How many times does `for (int i=0; i<5; i++)` execute?

Q2. What happens with `while(1)`?

Q3. What is the difference between do-while and while?

Recommended Books to Deepen Your Understanding

Why Do We Need Loops? — The Pain of Repetition

❌ Without loops

✅ With a loop

Feel the power of loops with this slider

It gets worse: what if the logic changes?

for loop — a fixed number of iterations

Iteration-count visualizer

while loop

Step execution — for loop

Variable state

Standard output

do-while — always runs at least once

Typical use: re-prompt on invalid input

Which loop to use

break & continue — loop control

Try it yourself — loops

Related Lessons

Review Quiz

Q1. How many times does for (int i=0; i<5; i++) execute?

Q2. What happens with while(1)?

Q3. What is the difference between do-while and while?

Recommended Books to Deepen Your Understanding

Q1. How many times does `for (int i=0; i<5; i++)` execute?

Q2. What happens with `while(1)`?