Loops & Iteration

Real-world analogy

What is it?

Loops repeat code multiple times. Dart has for loops (counted repetition), for-in loops (iterate collections), while loops (repeat while condition is true), do-while (at least once), and forEach (functional style). break exits early, continue skips an iteration. Functional methods like map/where/reduce often replace manual loops.

Real-world relevance

Loops are everywhere in apps: displaying a list of messages (loop through each one), processing API response data (loop through results), retrying a network request (while not successful), animating UI elements (loop each frame). Without loops, you would need millions of lines of code!

Key points

• for Loop - Counted Repetition — The for loop repeats code a specific number of times. It has three parts: initialization (start), condition (keep going?), and increment (next step). The classic loop for counting, iterating through indices, and precise control.
• for-in Loop - Collection Iteration — for-in loops through every item in a collection without needing an index. It is cleaner and less error-prone than a regular for loop when you just need each item.
• forEach - Functional Style — forEach() is a method on collections that calls a function for each item. It is similar to for-in but uses a callback function. Some developers prefer it for its concise syntax, especially with arrow functions.
• while Loop - Conditional Repetition — A while loop keeps running as long as its condition is true. Use it when you do not know in advance how many times to loop. Be careful -- if the condition never becomes false, you get an infinite loop!
• do-while Loop - At Least Once — do-while is like while, but it runs the code FIRST, then checks the condition. This guarantees the code runs at least once, even if the condition is false from the start.
• break - Exit Early — 'break' immediately exits the current loop, even if the condition is still true. Use break when you find what you are looking for and do not need to keep looping.
• continue - Skip This Round — 'continue' skips the rest of the current iteration and jumps to the next one. Use continue when you want to skip certain items but keep processing the rest.
• map, where, reduce - No Loops Needed — Functional methods like map(), where(), and reduce() can replace many loops with cleaner, more readable code. They describe WHAT you want, not HOW to loop. Dart developers prefer these over manual loops when possible.
• Nested Loops — A loop inside a loop. The inner loop runs completely for each iteration of the outer loop. Use for grids, tables, or comparing every pair of items. Be careful with performance -- nested loops multiply the work!

Code example

void main() {
  // for loop - repeat a specific number of times
  print('=== Countdown ===');
  for (var i = 5; i >= 1; i--) {
    print(i);
  }
  print('Go!');

  // for-in - iterate through a collection
  print('\n=== Shopping List ===');
  var shopping = ['milk', 'bread', 'eggs', 'butter'];
  for (var item in shopping) {
    print('Buy: $item');
  }

  // forEach - functional style
  print('\n=== Uppercase ===');
  shopping.forEach((item) => print(item.toUpperCase()));

  // while loop - repeat until condition is false
  print('\n=== Guessing Game ===');
  var secret = 7;
  var guess = 1;
  while (guess != secret) {
    print('Tried $guess... wrong!');
    guess++;
  }
  print('Got it! The number was $secret');

  // break and continue
  print('\n=== First Even ===');
  var nums = [1, 3, 5, 4, 7, 2];
  for (var n in nums) {
    if (n.isOdd) continue;  // skip odd numbers
    print('First even: $n');
    break;                   // stop after first find
  }

  // Functional alternatives to loops
  print('\n=== Functional ===');
  var scores = [85, 92, 78, 95, 88];
  var passing = scores.where((s) => s >= 80).toList();
  var average = scores.reduce((a, b) => a + b) / scores.length;
  print('Passing: $passing');
  print('Average: ${average.toStringAsFixed(1)}');
}

Line-by-line walkthrough

1. 1. The main function starts
2. 2. A comment for the for loop section
3. 3. Printing a header
4. 4. A for loop counting down from 5 to 1
5. 5. Printing each number in the countdown
6. 6. Closing the for loop
7. 7. Printing 'Go!' after the countdown ends
8. 8.
9. 9. A comment for the for-in section
10. 10. Printing a header with a newline escape for spacing
11. 11. Creating a list of shopping items
12. 12. for-in loops through each item in the list
13. 13. Printing each item with a 'Buy:' prefix
14. 14. Closing the for-in loop
15. 15.
16. 16. A comment for forEach
17. 17. Printing a header
18. 18. forEach calls the arrow function on each item, printing it uppercased
19. 19.
20. 20. A comment for while loop
21. 21. Printing a header
22. 22. Setting the secret number to 7
23. 23. Starting the guess at 1
24. 24. The while loop continues as long as guess does not equal secret
25. 25. Printing the current wrong guess
26. 26. Incrementing guess by 1 for the next try
27. 27. Closing the while loop
28. 28. Printing when the correct number is found
29. 29.
30. 30. A comment for break and continue
31. 31. Printing a header
32. 32. Creating a list with mixed odd and even numbers
33. 33. for-in loops through each number
34. 34. If the number is odd, continue skips to the next iteration
35. 35. If we reach here, the number is even -- print it
36. 36. break stops the loop after finding the first even number
37. 37. Closing the for loop
38. 38.
39. 39. A comment for functional alternatives
40. 40. Printing a header
41. 41. A list of test scores
42. 42. where() keeps only scores 80 and above
43. 43. reduce() sums all scores, then divides by count for average
44. 44. Printing passing scores
45. 45. Printing average formatted to 1 decimal place
46. 46. Closing the main function

Spot the bug

void main() {
  var total = 0;
  for (var i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
      continue;
    }
    total += i;
    break;
  }
  print('Sum of odd numbers: $total');
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• Dart Loops (Dart Official)
• Dart Iteration (Dart Official)
• Dart Control Flow (Dart Official)