C while Loop & do-while Loop | Condition-driven Iteration

while — repeat while the condition is true

while fits when the iteration count is not known in advance. "Keep going as long as this condition holds." In plain English: "while … do …".

while (condition) {
    // runs over and over while condition is true
}

Example: how many doublings to pass 100?

int n = 1;
int count = 0;
while (n <= 100) {
    n = n * 2;
    count++;
}
printf("%d steps to reach %d\n", count, n);
// -> 7 steps to reach 128

We can't predict the iteration count up front, so while fits better than for. The loop repeats: check condition → body → check condition → …

The evaluation order (pre-test)

1. Check n <= 100
2. If true, run the { ... } body
3. At }, go back to step 1
4. When the condition becomes false, the loop ends

Key: Because the check happens first, if the condition is already false, the body runs zero times.

for vs while — which should I use?

Mechanically, for and while can be converted into each other. Both work — pick the one that expresses your intent better.

Same task, two forms

for (count-based)

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

while (same logic)

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

for makes the "init / condition / update" triple explicit, so use for whenever the iteration count is fixed.

When to use which

Situation	Best fit	Example
Known iteration count	`for`	walk over an array 0..N-1
Stop on a condition (count unknown)	`while`	read until user types "q"
Must run at least once	`do-while`	prompt for input, re-prompt on invalid
Infinite loop (exit via `break`)	`while(1)` / `for(;;)`	event / main loop

⚠️ Easy mistake with while: forgetting to update the condition variable inside the body. for forces you to write the update in the header, so this class of bug is rarer.

do-while — guaranteed to run at least once

while checks the condition before the body (pre-test); do-while checks it after (post-test). That's why the do-while body always executes at least once.

while (pre-test)

while (cond) {
    body;  // 0 times if cond is false
}

If the condition is false initially, body runs zero times.

do-while (post-test)

do {
    body;  // always 1+ times
} while (cond);

Run the body first, then check — at least one iteration guaranteed.

⚠️ Don't forget the trailing semicolon! } while (cond); must end with ; or the compiler errors out. Unique to do-while.

Typical use: prompt and re-prompt for valid input

"Enter 1–3, and if invalid keep asking" — you must show the prompt at least once, so do-while fits perfectly.

int choice;
do {
    printf("Choose 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);

while vs do-while — zero-iteration case

while, condition false up front

int i = 10;
while (i < 5) {
    printf("x");
}

→ 0 iterations (nothing printed)

do-while, condition false up front

int i = 10;
do {
    printf("x");
} while (i < 5);

→ 1 iteration (one "x" is printed)

Infinite-loop traps — forgetting to update

The most common bug with while / do-while is an infinite loop — forgetting to change the condition variable inside the body.

🔴 Trap 1: forgotten counter update

int i = 0;
while (i < 5) {
    printf("%d\n", i);
    // forgot i++!
}

What happens: i never changes, so i < 5 is always true — an infinite loop printing 0 forever.

Fix: always double-check that the while body has something that moves toward ending the loop. for makes this harder to miss because the update clause is part of the syntax.

💡 Intentional infinite loop + break

When you want "keep running, exit on some event", use while (1) and break to leave.

while (1) {          // 1 is always true -> infinite
    int n;
    scanf("%d", &n);
    if (n == 0) break;  // leave on 0
    printf("Got: %d\n", n);
}

Common in event loops and interactive programs, where break / return is the planned way out.

⚠️ do-while shares the same risk

int choice;
do {
    printf("1-3 > ");
    // forgot scanf! choice stays uninitialized
} while (choice < 1 || choice > 3);

choice never changes, so depending on its initial value the loop can run forever. Make sure the variable the condition tests is updated every iteration.

If you hit one: Ctrl + C stops a terminal program. In Visual Studio, use the "Stop Debugging" button.

Try it yourself — while loop

my_while.c

Output

Press "Run" to execute...

💡 Try these ideas too

Rewrite with for and verify same output
Change to n = n * 3 and see the step count
Convert to a do-while
Use while(1) + break to read numbers until 0
Intentionally forget the counter update to see an infinite loop (Ctrl+C to stop)

Quiz

Check your understanding.

Q1. What does `while(1)` do?

Runs once

Infinite loop

Compile error

The condition is always 1 (true), so it loops forever unless a break or return exits the loop.

Q2. Difference between `do-while` and `while`?

No difference

do-while runs at least once

do-while is faster

do-while checks the condition after the body, so the body always runs at least once. while checks first, so the body can run zero times.

Q3. How many times does this print?

int i = 10;
while (i < 5) {
    printf("x");
    i++;
}

0 (nothing)

Once

5 times

Infinite loop

while is pre-test. i=10 fails i<5 immediately, so the body runs zero times.

Q4. Same condition but as `do-while`?

int i = 10;
do {
    printf("x");
    i++;
} while (i < 5);

Once

5 times

Infinite loop

do-while is post-test: run once, then check. After the first iteration i=11, and 11 < 5 is false, so the loop ends. One iteration — the classic one-more-than-while behaviour.

Q5. What does this code do?

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

Prints 0..4

Prints 0 once

Prints 0 forever (infinite loop)

Compile error

i++ is missing. i stays 0 and the condition never becomes false — infinite loop printing 0. The most common while bug. Ctrl+C to stop.

Q6. What's the syntax detail you must not forget in `do-while`?

Parentheses around the condition

Braces {}

The trailing semicolon ;

} while (cond); requires the trailing ; or the compiler rejects it. A do-while-only rule.

Lesson 12: while & do-while Loop

while — repeat while the condition is true

Example: how many doublings to pass 100?

The evaluation order (pre-test)

for vs while — which should I use?

Same task, two forms

When to use which

do-while — guaranteed to run at least once

Typical use: prompt and re-prompt for valid input

while vs do-while — zero-iteration case

Infinite-loop traps — forgetting to update

Try it yourself — while loop

Related Lessons

Quiz

Q1. What does `while(1)` do?

Q2. Difference between `do-while` and `while`?

Q3. How many times does this print?

Q4. Same condition but as `do-while`?

Q5. What does this code do?

Q6. What's the syntax detail you must not forget in `do-while`?

while — repeat while the condition is true

Example: how many doublings to pass 100?

The evaluation order (pre-test)

for vs while — which should I use?

Same task, two forms

When to use which

do-while — guaranteed to run at least once

Typical use: prompt and re-prompt for valid input

while vs do-while — zero-iteration case

Infinite-loop traps — forgetting to update

Try it yourself — while loop

Related Lessons

Quiz

Q1. What does while(1) do?

Q2. Difference between do-while and while?

Q3. How many times does this print?

Q4. Same condition but as do-while?

Q5. What does this code do?

Q6. What's the syntax detail you must not forget in do-while?

Q1. What does `while(1)` do?

Q2. Difference between `do-while` and `while`?

Q4. Same condition but as `do-while`?

Q6. What's the syntax detail you must not forget in `do-while`?