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

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...

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.

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

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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)`?