C Loops

Loops repeat a block of code as long as a condition stays true.

The while Loop

Example

#include <stdio.h>

int main(void) {
  int i = 1;
  while (i <= 3) {
    printf("Round %d\n", i);
    i++;
  }
  return 0;
}

Output

Round 1
Round 2
Round 3

If the condition never becomes false, the loop runs forever. Make sure something inside the loop moves it towards the end.

The do...while Loop

do...while runs the body once before it checks the condition:

Example

#include <stdio.h>

int main(void) {
  int pin;
  do {
    printf("Enter PIN: ");
    scanf("%d", &pin);
  } while (pin != 1234);
  printf("Welcome!\n");
  return 0;
}

Output

Enter PIN: 1111
Enter PIN: 1234
Welcome!

This is useful for menus and input checks, where you must ask at least once.

The for Loop

A for loop keeps the start, condition and step together:

Example

text

for (start; condition; step) {
  // code to repeat
}

Example

#include <stdio.h>

int main(void) {
  for (int i = 1; i <= 5; i++) {
    printf("%d x 9 = %d\n", i, i * 9);
  }
  return 0;
}

Output

Which Loop Should I Use?

Loop	Best when
`for`	You know how many times to repeat
`while`	You repeat until something changes
`do...while`	The body must run at least once

break and continue

break stops the loop immediately
continue skips to the next round

Example

#include <stdio.h>

int main(void) {
  for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
      continue;     // skip even numbers
    }
    if (i > 7) {
      break;        // stop after 7
    }
    printf("%d ", i);
  }
  printf("\n");
  return 0;
}

Output

1 3 5 7

Nested Loops

A loop inside another loop. The inner loop finishes all its rounds for each round of the outer loop:

Example

#include <stdio.h>

int main(void) {
  for (int row = 1; row <= 3; row++) {
    for (int star = 1; star <= row; star++) {
      printf("*");
    }
    printf("\n");
  }
  return 0;
}

Output

*
**
***

Exercise

How many times does printf run in: for (int i = 2; i <= 10; i += 2) printf("x");

45910