Lære Break, Continue, and Labeled Loops | Loops and Iteration Patterns

In Dart, you often need to control how loops behave beyond simple iteration. The break and continue statements let you fine-tune a loop's flow, and labels allow you to manage even complex nested loops. By using these tools, you can exit a loop early, skip particular iterations, or break out of multiple levels of nested loops efficiently.

main.dart


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void main() {
  List<int> numbers = [3, 5, 7, -2, 9, 4];
  int? firstNegative;

  for (var number in numbers) {
    if (number < 0) {
      firstNegative = number;
      break;
    }
  }

  if (firstNegative != null) {
    print('First negative number found: $firstNegative');
  } else {
    print('No negative numbers found.');
  }
}

The break statement immediately stops the closest enclosing loop. In the example above, as soon as a negative number is found in the list, break exits the for loop, and no further numbers are checked. This is useful when you are searching for a specific value and want to avoid unnecessary iterations once your goal is met.

main.dart


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void main() {
  List<int> numbers = [1, 2, 3, 4, 5, 6];
  print('Even numbers in the list:');

  for (var number in numbers) {
    if (number % 2 != 0) {
      continue;
    }
    print(number);
  }
}

The continue statement skips the rest of the loop body for the current iteration and proceeds with the next one. In the previous example, whenever an odd number is encountered, continue causes the loop to skip printing that number. This lets you filter which elements are processed, making your code cleaner and more focused on relevant data.

main.dart


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void main() {
  List<List<int>> matrix = [
    [2, 4, 6],
    [8, 10, 12],
    [14, 15, 16]
  ];

  int? foundNumber;

  outerLoop:
  for (var row in matrix) {
    for (var value in row) {
      if (value % 5 == 0) {
        foundNumber = value;
        break outerLoop;
      }
    }
  }

  if (foundNumber != null) {
    print('First number divisible by 5 found: $foundNumber');
  } else {
    print('No number divisible by 5 found.');
  }
}

Labeled loops give you control over which loop is affected by a break or continue statement, especially in nested situations. By assigning a label to a loop, you can use break labelName; to exit not just the innermost loop, but any outer loop with that label. In the matrix search example, the label outerLoop allows the code to break out of both the inner and outer loops as soon as a number divisible by 5 is found. Use labeled loops when you need to escape multiple levels of iteration based on a single condition, keeping your code readable and efficient.

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main.dart


              1234567891011121314151617
            
void main() {
  List<int> numbers = [3, 5, 7, -2, 9, 4];
  int? firstNegative;

  for (var number in numbers) {
    if (number < 0) {
      firstNegative = number;
      break;
    }
  }

  if (firstNegative != null) {
    print('First negative number found: $firstNegative');
  } else {
    print('No negative numbers found.');
  }
}

main.dart


              1234567891011
            
void main() {
  List<int> numbers = [1, 2, 3, 4, 5, 6];
  print('Even numbers in the list:');

  for (var number in numbers) {
    if (number % 2 != 0) {
      continue;
    }
    print(number);
  }
}

main.dart


              12345678910111213141516171819202122232425
            
void main() {
  List<List<int>> matrix = [
    [2, 4, 6],
    [8, 10, 12],
    [14, 15, 16]
  ];

  int? foundNumber;

  outerLoop:
  for (var row in matrix) {
    for (var value in row) {
      if (value % 5 == 0) {
        foundNumber = value;
        break outerLoop;
      }
    }
  }

  if (foundNumber != null) {
    print('First number divisible by 5 found: $foundNumber');
  } else {
    print('No number divisible by 5 found.');
  }
}

Var alt klart?

Tak for dine kommentarer!

Sektion 2. Kapitel 4