C Loops: `For`

Introduction

A for loop is a way to repeat a block of code a specific number of times. It is commonly used when you know in advance how many times you want to loop.

Syntax

for (initialisation; condition; update) {
  // code to repeat
}

initialisation: runs once at the start (e.g., int i = 0)
condition: checked before each loop; if true, the loop runs
update: runs after each loop iteration (e.g., i++)

Example: Print numbers 0 to 4

#include <stdio.h>

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

Why use `for` loops?

Easy to write loops that run a set number of times
Keeps loop control in one place (initialisation, condition, update)
Useful for iterating over arrays

Example: Sum elements of an array

#include <stdio.h>

int main(void) {
  int nums[] = {1, 2, 3, 4, 5};
  int sum = 0;
  int length = sizeof(nums) / sizeof(nums[0]);

  for (int i = 0; i < length; i++) {
    sum += nums[i];
  }
  printf("Sum = %d\n", sum);
  return 0;
}

`continue` and `break`

As we saw with while loops, we can also have more control over how the for loop runs using break and continue:

break immediately stops the loop and moves to the next line after the loop.
continue skips the rest of the current loop iteration and starts the next iteration right away.

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; i++) {
        if (i == 5) continue; // Skip printing 5
        if (i == 8) break;    // Stop the loop completely when i is 8
        printf("%d ", i);
    }
    printf("\n");
    return 0;
}

Common mistakes

Forgetting to update the loop variable (can cause infinite loops)
Using the wrong condition so loop runs one too many or too few times, known as off-by-one errors - if you are like then you will make this error all the time
Changing the loop variable inside the loop body (can make code confusing)

Things to remember

The loop variable is often declared inside the for statement
The loop runs as long as the condition is true
You can use break to exit early, or continue to skip to the next iteration

Summary

For loops are best when you know how many times to repeat
Syntax: for (init; condition; update)
Great for arrays and counting

C Loops: For