C For Loop

Visit a counted range with for, keep the bound exclusive or inclusive on purpose, and prove the loop can stop.

Use for when the count is visible from the start

A for loop places its three moving parts together: initialization runs once, the condition is checked before every cycle, and the update runs after the body. This structure fits numbered lessons, days, retries with a fixed limit, and array indexes. Declaring the index in the for header (for (int day = 1; ...)) is valid C99 and later, which this course uses.

#include <stdio.h>

int main(void) {
  int total_days = 3;

  for (int day = 1; day <= total_days; day++) {
    printf("Practice day %d\n", day);
  }
  return 0;
}

Edit the program, predict its output, then run it.

1. 01
   Initialize once

   int day = 1 creates the starting index before the first condition check.

2. 02
   Test before the body

   day <= total_days decides whether another cycle is allowed.

3. 03
   Run one cycle

   The body uses the current index.

4. 04
   Update toward the stop

   day++ adds one so the condition can become false.

Trace the first, last, and forbidden values

Most loop bugs are boundary bugs. Agree on whether the index is zero-based or one-based and whether the ending value belongs in the range. The starter below prints four lines for three lessons: it starts at 0 and includes the endpoint 3. Repair it by starting at 1 with <= total_lessons, or starting at 0 with lesson < total_lessons.

#include <stdio.h>

int main(void) {
  int total_lessons = 3;

  for (int lesson = 0; lesson <= total_lessons; lesson++) {
    printf("Lesson %d\n", lesson);
  }
  return 0;
}

Edit the program, predict its output, then run it.

Let the inner loop finish before the outer loop advances

With weeks = 2 and days = 3, expect six lines. Give each index a meaningful name and keep the body small. Nested loops that search every pair of a large collection need a better algorithm later; here the job is to trace the total work.

#include <stdio.h>

int main(void) {
  int weeks = 2;
  int days = 3;

  for (int week = 1; week <= weeks; week++) {
    for (int day = 1; day <= days; day++) {
      printf("Week %d, day %d\n", week, day);
    }
  }
  return 0;
}

Edit the program, predict its output, then run it.

Use break and continue sparingly

break leaves the nearest loop. continue skips the rest of this cycle; a for loop still performs its update before testing again. A plain loop condition is still the clearest normal stopping rule.

#include <stdio.h>

int main(void) {
  int attempt;

  for (attempt = 1; attempt <= 5; attempt++) {
    if (attempt == 2) {
      puts("Skip attempt 2.");
      continue;
    }
    printf("Check attempt %d\n", attempt);
    if (attempt == 4) {
      puts("A valid result appeared.");
      break;
    }
  }
  return 0;
}

Edit the program, predict its output, then run it.

Arrays use index < count

When you visit every element of an array with count items, the usual header is for (int index = 0; index < count; index++). The array does not store its own length. Details belong in C Arrays. Prefer C While Loop when the stop depends on changing state rather than a known count.

Independent lab: print a numbered list

Print one labeled line for each of 1 through n, then change n and predict the last printed number and the value of the index after the loop ends.

Lesson review

• A for header names initialization, condition, and update.
• Boundary tests use the first value, last allowed value, cycle count, and the value after the last update.
• An inner loop completes for one outer value before the outer index changes.
• break and continue are exceptional exits, not a substitute for a clear condition.