Loops: for, while, do-while & for-in

Why Do We Need Loops?

Imagine you need to print "Hello" 100 times, or process every item in a list of 1,000 products. Writing each line manually would be absurd. Loops let you repeat a block of code as many times as needed -- either a fixed number of times, or until a condition becomes false. Loops are one of the most fundamental tools in every programming language.

The for Loop

The for loop is the most common loop in Dart. It has three parts: initialization (runs once), a condition (checked before each iteration), and an update (runs after each iteration).

void main() {
  // Print numbers 1 through 5
  for (int i = 1; i <= 5; i++) {
    print(i);
  }
  // Output: 1  2  3  4  5

  // Count by twos
  for (int i = 0; i <= 10; i += 2) {
    print(i);
  }
  // Output: 0  2  4  6  8  10

  // Count backwards
  for (int i = 5; i >= 1; i--) {
    print('Countdown: $i');
  }
  // Output: Countdown: 5  Countdown: 4  ... Countdown: 1
}

Looping Through Strings

You can use a for loop to iterate over each character of a string by using the string’s length property and index notation:

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

  for (int i = 0; i < word.length; i++) {
    print('Character $i: ${word[i]}');
  }
  // Output:
  // Character 0: D
  // Character 1: a
  // Character 2: r
  // Character 3: t
}

The while Loop

The while loop repeats a block of code as long as a condition is true. Use it when you don’t know in advance how many iterations you need.

void main() {
  int count = 1;

  while (count <= 5) {
    print('Count: $count');
    count++;  // Don't forget this! Without it, the loop runs forever.
  }
  // Output: Count: 1  Count: 2  Count: 3  Count: 4  Count: 5

  // Practical example: divide until less than 1
  double value = 100.0;
  int divisions = 0;

  while (value >= 1) {
    value /= 2;
    divisions++;
  }
  print('Divided $divisions times. Final value: ${value.toStringAsFixed(4)}');
  // Output: Divided 7 times. Final value: 0.7813
}

The do-while Loop

The do-while loop is similar to while, but with one key difference: it always executes the body at least once before checking the condition. The condition is checked after each iteration.

void main() {
  // This while loop runs 0 times (condition is false immediately)
  int x = 10;
  while (x < 5) {
    print('while: $x');  // Never prints
    x++;
  }

  // This do-while loop runs 1 time (body executes before condition check)
  int y = 10;
  do {
    print('do-while: $y');  // Prints: do-while: 10
    y++;
  } while (y < 5);
}

void main() {
  // Simulate a menu that must show at least once
  List<String> simulatedInputs = ['invalid', 'wrong', 'quit'];
  int attempt = 0;

  do {
    String input = simulatedInputs[attempt];
    print('Attempt ${attempt + 1}: You entered "$input"');
    if (input == 'quit') {
      print('Goodbye!');
      break;
    }
    print('Invalid input, try again.');
    attempt++;
  } while (attempt < simulatedInputs.length);
}

The for-in Loop

The for-in loop is designed for iterating over collections (like Lists, Sets, and other iterables). It is cleaner than a traditional for loop when you don’t need the index.

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

  // for-in -- clean and readable
  for (String fruit in fruits) {
    print(fruit);
  }
  // Output: Apple  Banana  Cherry  Date

  // Compare with traditional for loop
  for (int i = 0; i < fruits.length; i++) {
    print('${i + 1}. ${fruits[i]}');
  }
  // Output: 1. Apple  2. Banana  3. Cherry  4. Date
}

void main() {
  List<int> numbers = [10, 20, 30, 40, 50];

  // Calculate the sum
  int sum = 0;
  for (int number in numbers) {
    sum += number;
  }
  print('Sum: $sum');  // Sum: 150

  // Find the maximum value
  int max = numbers[0];
  for (int number in numbers) {
    if (number > max) {
      max = number;
    }
  }
  print('Max: $max');  // Max: 50
}

The forEach Method

Dart collections also have a forEach method that accepts a function. It does the same thing as for-in but uses a callback:

void main() {
  List<String> colors = ['Red', 'Green', 'Blue'];

  // Using forEach with an anonymous function
  colors.forEach((color) {
    print('Color: $color');
  });

  // Using forEach with arrow syntax (for single expressions)
  colors.forEach((color) => print('I like $color'));

  // With index using asMap()
  colors.asMap().forEach((index, color) {
    print('$index: $color');
  });
  // Output: 0: Red  1: Green  2: Blue
}

break and continue

Sometimes you need to alter the normal flow of a loop. break exits the loop entirely, while continue skips the rest of the current iteration and moves to the next one.

void main() {
  // Find the first number divisible by 7
  for (int i = 1; i <= 100; i++) {
    if (i % 7 == 0) {
      print('First number divisible by 7: $i');
      break;  // Stop the loop immediately
    }
  }
  // Output: First number divisible by 7: 7

  // Search for an item in a list
  List<String> names = ['Alice', 'Bob', 'Charlie', 'Diana'];
  String searchFor = 'Charlie';
  bool found = false;

  for (String name in names) {
    if (name == searchFor) {
      found = true;
      break;  // No need to check remaining items
    }
  }
  print('$searchFor found: $found');  // Charlie found: true
}

void main() {
  // Print only odd numbers from 1 to 10
  for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
      continue;  // Skip even numbers
    }
    print(i);
  }
  // Output: 1  3  5  7  9

  // Process only valid scores
  List<int> scores = [85, -1, 92, 0, 78, -5, 95];

  for (int score in scores) {
    if (score <= 0) {
      continue;  // Skip invalid scores
    }
    print('Valid score: $score');
  }
  // Output: Valid score: 85  Valid score: 92  Valid score: 78  Valid score: 95
}

Nested Loops

You can place one loop inside another. The inner loop completes all its iterations for each single iteration of the outer loop.

void main() {
  // Multiplication table (1-5)
  for (int i = 1; i <= 5; i++) {
    String row = '';
    for (int j = 1; j <= 5; j++) {
      row += '${(i * j).toString().padLeft(3)}';
    }
    print(row);
  }
  // Output:
  //   1  2  3  4  5
  //   2  4  6  8 10
  //   3  6  9 12 15
  //   4  8 12 16 20
  //   5 10 15 20 25
}

void main() {
  // Labels let you break out of a specific loop
  outerLoop:
  for (int i = 0; i < 5; i++) {
    for (int j = 0; j < 5; j++) {
      if (i + j == 6) {
        print('Breaking at i=$i, j=$j');
        break outerLoop;  // Breaks the OUTER loop
      }
    }
  }
  // Output: Breaking at i=2, j=4
  // Without the label, break would only exit the inner loop
}

Common Loop Patterns

void main() {
  List<double> prices = [9.99, 24.50, 3.75, 15.00, 42.99];

  // Sum accumulator
  double total = 0;
  for (double price in prices) {
    total += price;
  }
  print('Total: \$${total.toStringAsFixed(2)}');  // Total: $96.23

  // Count accumulator
  int expensiveCount = 0;
  for (double price in prices) {
    if (price > 20) {
      expensiveCount++;
    }
  }
  print('Expensive items: $expensiveCount');  // Expensive items: 2

  // String builder
  List<String> words = ['Dart', 'is', 'awesome'];
  String sentence = '';
  for (int i = 0; i < words.length; i++) {
    sentence += words[i];
    if (i < words.length - 1) sentence += ' ';
  }
  print(sentence);  // Dart is awesome
}

void main() {
  List<int> numbers = [12, 5, 8, 130, 44, 3, 99, 67];

  // Find all numbers greater than 50
  List<int> large = [];
  for (int n in numbers) {
    if (n > 50) {
      large.add(n);
    }
  }
  print('Numbers > 50: $large');  // Numbers > 50: [130, 99, 67]

  // Find the first even number
  for (int n in numbers) {
    if (n % 2 == 0) {
      print('First even: $n');  // First even: 12
      break;
    }
  }
}

Choosing the Right Loop

Practical Examples

void main() {
  // Classic programming challenge
  for (int i = 1; i <= 20; i++) {
    if (i % 3 == 0 && i % 5 == 0) {
      print('FizzBuzz');
    } else if (i % 3 == 0) {
      print('Fizz');
    } else if (i % 5 == 0) {
      print('Buzz');
    } else {
      print(i);
    }
  }
}

void main() {
  int secretNumber = 7;
  List<int> guesses = [3, 9, 5, 7, 10];
  int attempts = 0;

  for (int guess in guesses) {
    attempts++;
    if (guess == secretNumber) {
      print('Correct! Found $secretNumber in $attempts attempts.');
      break;
    } else if (guess < secretNumber) {
      print('$guess is too low.');
    } else {
      print('$guess is too high.');
    }
  }
  // Output:
  // 3 is too low.
  // 9 is too high.
  // 5 is too low.
  // Correct! Found 7 in 4 attempts.
}

void main() {
  String original = 'Hello, Dart!';
  String reversed = '';

  for (int i = original.length - 1; i >= 0; i--) {
    reversed += original[i];
  }

  print('Original: $original');   // Original: Hello, Dart!
  print('Reversed: $reversed');   // Reversed: !traD ,olleH
}