Dart break

Summary: in this tutorial, you’ll learn how to use the Dart break statement to terminate a loop prematurely.

Introduction to the Dart break statement

The break statement prematurely terminates a loop including while, do while, and for loops. The following shows the syntax of the break statement:

break;Code language: Dart (dart)

In practice, you’ll use the break statement with an if statement to terminate a loop based on a condition. For example, the following checks a condition and only terminates the loop if the condition is true:

while (expression)
{
    if (condition)
    {
        break;
    }
}Code language: Dart (dart)

Note that you have statements before and/or after the if statement. The following flowchart shows how the break statement works:

Similarly, you can use the break statement inside a do while loop as follows:

do
{
    if (condition)
    {
        break;
    }
} while (expression);Code language: Dart (dart)

And the following flowchart illustrates how the break statement works inside a do while loop:

The following shows how to use the break statement inside a for loop:

for (initializer; condition; iterator)
{
    if (condition)
    {
        break;
    }
}Code language: Dart (dart)

This flowchart illustrates how the break statement works in the for loop:

When you use the break statement inside a nested loop, it only terminates the enclosing loop, not all the loops.

Dart break statement examples

Let’s take some examples of using the Dart break statement.

1) Using Dart break statement inside a for loop examples

The following example uses the break statement to terminate a for loop once it found the first occurrence of the letter s in a string:

void main() {
  String message = "Dart is awesome!";
  for (int i = 0; i < message.length; i++) {
    if (message[i] == 's') {
      print("The letter s is found at index $i");
      break;
    }
  }
}Code language: Dart (dart)

Output:

The letter s is found at index 6Code language: plaintext (plaintext)

This example uses a for loop to iterate over the letters of the string “Dart is awesome”. It compares the current letter with the letter “s” in each iteration. And it terminates the loop immediately if the current letter is the letter “s”.

Without the break statement, the for loop will iterate over all characters in the string message even though it already found the letter s.

If the message string doesn’t have the letter s, the condition in the if statement will evaluate to false and the break statement will never be reached. As the result, the for loop will iterate over all characters in the message string.

2) Using the Dart break statement inside a while loop example

The following example uses a while loop to find the first occurrence of the letter a in a string:

void main() {
  String message = 'Dart is awesome!';
  int i = 0;
  while (i < message.length) {
    print(message[i]);
    if (message[i] == 'a') {
      print("The letter a was found at the index $i");
      break;
    }
    i++;
  }
}Code language: Dart (dart)

3) Using the Dart break statement inside a do while loop

The following example uses the break statement inside a do while statement to terminate the loop if it finds the letter 'a' in a string:

void main() {
  String message = 'Dart is awesome!';
  int i = 0;
  do {
    print(message[i]);
    if (message[i] == 'a') {
      print("The letter a was found at the index $i");
      break;
    }
    i++;
  } while (i < message.length);
}Code language: Dart (dart)

Output:

D
a
The letter a was found at the index 1Code language: plaintext (plaintext)

Summary

• Use the break statement to prematurely terminate a loop including while, do while, and for.
• The break statement terminates only the enclosing loop when you use it in a nested loop.