for-in, break & continue

Now that you know the basic loops, let's explore Dart's cleaner for-in loop for iterating collections, and two essential control keywords — break (exit the loop early) and continue (skip the current iteration).

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🔹 The for-in Loop

The for-in loop is designed specifically for iterating over any iterable — List, Set, Map, String, Range, or custom iterables.

Syntax:

dart
for (var element in iterable) {
  // use element
}

dart
void main() {
  List<String> fruits = ['Apple', 'Banana', 'Cherry', 'Mango'];

  for (var fruit in fruits) {
    print(fruit);
  }
}

Output:

Apple
Banana
Cherry
Mango

Much cleaner than for (int i = 0; i < fruits.length; i++) when you don't need the index!

Iterating a Set

dart
void main() {
  Set<int> primes = {2, 3, 5, 7, 11, 13};

  for (var prime in primes) {
    print('$prime is prime');
  }
}

Iterating a Map

For a Map, for-in gives you each MapEntry:

dart
void main() {
  Map<String, int> scores = {
    'Alice': 92,
    'Bob': 85,
    'Charlie': 78,
  };

  // Iterate entries
  for (var entry in scores.entries) {
    print('${entry.key}: ${entry.value}');
  }

  // Iterate only keys
  for (var name in scores.keys) {
    print(name);
  }

  // Iterate only values
  for (var score in scores.values) {
    print(score);
  }
}

Iterating a String

In Dart, a String is an iterable of characters:

dart
void main() {
  String word = 'Dart';

  for (var char in word.split('')) {
    print(char);
  }
}

Output:

D
a
r
t

---

🔹 for-in vs forEach()

Both let you iterate a collection, but they have important differences:

dart
void main() {
  List<int> numbers = [1, 2, 3, 4, 5];

  // for-in
  for (var n in numbers) {
    print(n);
  }

  // forEach() with lambda
  numbers.forEach((n) => print(n));
}

Aspect	for-in	forEach()
break support	✅ Yes	❌ No
continue support	✅ Yes	❌ No
return exits function	✅ Yes	❌ Only exits lambda
await inside	✅ Works naturally	⚠️ Needs asyncForEach workaround
Readability	Excellent	Good for simple transforms

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🔹 break — Exit the Loop

break immediately stops the loop and resumes execution after the closing }.

dart
void main() {
  List<int> numbers = [4, 7, 2, 9, 1, 6, 3];

  int target = 9;
  int foundAt = -1;

  for (int i = 0; i < numbers.length; i++) {
    if (numbers[i] == target) {
      foundAt = i;
      break; // Stop searching — we found it!
    }
  }

  if (foundAt >= 0) {
    print('Found $target at index $foundAt');
  } else {
    print('$target not found');
  }
}

Output: Found 9 at index 3

Without break, the loop would continue pointlessly even after finding the target.

break in a while Loop

dart
void main() {
  int n = 1;

  while (true) { // intentional "infinite" loop
    if (n * n > 100) {
      break; // exit when n² exceeds 100
    }
    n++;
  }

  print('Largest n where n² <= 100: ${n - 1}');  // 10
}

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🔹 continue — Skip the Current Iteration

continue skips the rest of the current iteration and jumps to the next one.

dart
void main() {
  List<int> numbers = [-3, 7, -1, 5, -8, 2, 4];

  print('Positive numbers only:');
  for (var n in numbers) {
    if (n < 0) {
      continue; // Skip negative numbers
    }
    print(n);
  }
}

Output:

Positive numbers only:
7
5
2
4

continue in a for Loop

dart
void main() {
  // Print all numbers 1–20, skip multiples of 3
  for (int i = 1; i <= 20; i++) {
    if (i % 3 == 0) {
      continue;
    }
    stdout.write('$i ');
  }
  print('');
}

Output: 1 2 4 5 7 8 10 11 13 14 16 17 19 20

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🔹 Labels for Nested Loops

When you have nested loops, break and continue by default only affect the innermost loop. Use labels to control an outer loop.

dart
void main() {
  // Without label: break only exits inner loop
  for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
      if (j == 1) break; // only breaks inner loop
      print('($i, $j)');
    }
  }
}

Output:

(0, 0)
(1, 0)
(2, 0)

Labeled break — Exit Outer Loop

dart
void main() {
  outerLoop: // label
  for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
      if (i == 1 && j == 1) {
        print('Breaking out of both loops at ($i, $j)');
        break outerLoop; // breaks the OUTER loop
      }
      print('($i, $j)');
    }
  }
  print('Done.');
}

Output:

(0, 0)
(0, 1)
(0, 2)
(1, 0)
Breaking out of both loops at (1, 1)
Done.

Labeled continue — Skip to Next Outer Iteration

dart
void main() {
  outerLoop:
  for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
      if (j == 2) {
        continue outerLoop; // skip to next i
      }
      print('($i, $j)');
    }
  }
}

Output:

(1, 1)
(2, 1)
(3, 1)

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🔹 Real-World Examples

Example 1: Find First Match

dart
String? findFirstAdult(List<Map<String, dynamic>> users) {
  for (var user in users) {
    if (user['age'] >= 18) {
      return user['name']; // return also exits the loop
    }
  }
  return null;
}

void main() {
  List<Map<String, dynamic>> users = [
    {'name': 'Tom', 'age': 15},
    {'name': 'Lisa', 'age': 22},
    {'name': 'Mark', 'age': 19},
  ];

  String? firstAdult = findFirstAdult(users);
  print('First adult: $firstAdult');  // First adult: Lisa
}

Example 2: Skip Negative Numbers, Sum the Rest

dart
void main() {
  List<int> data = [10, -3, 25, -8, 4, -1, 17, 6];
  int sum = 0;
  int skipped = 0;

  for (var n in data) {
    if (n < 0) {
      skipped++;
      continue;
    }
    sum += n;
  }

  print('Sum of positives: $sum');      // Sum of positives: 62
  print('Negative values skipped: $skipped');  // Negative values skipped: 3
}

Example 3: Process Until Condition Met

dart
void main() {
  List<String> tasks = [
    'init_database',
    'load_config',
    'connect_server',
    'ERROR_TIMEOUT',  // Simulate an error task
    'start_app',
    'send_report',
  ];

  print('Processing tasks:');
  for (var task in tasks) {
    if (task.startsWith('ERROR')) {
      print('❌ Error encountered: $task — stopping.');
      break;
    }
    print('✅ Completed: $task');
  }
}

Output:

Processing tasks:
✅ Completed: init_database
✅ Completed: load_config
✅ Completed: connect_server
❌ Error encountered: ERROR_TIMEOUT — stopping.

Example 4: Collect Valid Emails Only

dart
bool isValidEmail(String email) {
  return email.contains('@') && email.contains('.');
}

void main() {
  List<String> rawEmails = [
    'alice@example.com',
    'invalid-email',
    'bob@test.org',
    'noatsign',
    'charlie@domain.net',
  ];

  List<String> validEmails = [];

  for (var email in rawEmails) {
    if (!isValidEmail(email)) {
      print('Skipping invalid: $email');
      continue;
    }
    validEmails.add(email);
  }

  print('\nValid emails: $validEmails');
}

Output:

Skipping invalid: invalid-email
Skipping invalid: noatsign

Valid emails: [alice@example.com, bob@test.org, charlie@domain.net]

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Summary

Feature	Keyword	Purpose
For-in loop	for (var x in iterable)	Cleanly iterate any iterable
Exit a loop	break	Stop the loop immediately
Skip iteration	continue	Skip rest of current iteration, go to next
Exit outer loop	break labelName	Break out of a specific outer loop
Skip outer iteration	continue labelName	Continue on a specific outer loop

With for-in, break, and continue in your toolkit, you can write expressive, efficient loops. Up next: functions — the building blocks of reusable code!