Dart while

Summary: in this tutorial, you’ll learn how to use the Dart while statement to execute statements repeatedly as long as a condition is true.

Introduction to the Dart while statement

The while statement evaluates a Boolean expression and executes statements repeatedly as long as the result of the expression is true.

The following illustrates the syntax of the while statement:

while (expression)
{
    // statement
}Code language: Dart (dart)

In this syntax:

The expression is a Boolean expression that evaluates to true or false.

The while statement evaluates the expression. If the result is true, it’ll execute statements inside the curly braces.

The while statement evaluates the expression before each iteration. And it’ll execute statements again and again as long as the result of the expression is true.

If the result of expression is false, the while statement exits and passes the control to the statement after it.

Therefore, you need to change some variables inside the body of the while statement to make the expression false at some point. Otherwise, you’ll have an indefinite loop. An indefinite loop executes the statements repeatedly until the program crashes.

Since the while statement checks the expression at the beginning of each iteration, it is often called a pretest loop.

The following flowchart illustrates how the Dart while statement works.

Dart while statement examples

Let’s take some examples of using the while statement.

1) Simple Dart while statement example

The following example uses the while loop statement to display five integers from 1 to 5:

void main() {
  int current = 0;

  while (current < 5) {
    current++;
    print(current);
  }
}
Code language: Dart (dart)

Output:

1
2
3
4
5Code language: Dart (dart)

How it works.

First, declare a current variable and initialize its value to zero.

Second, enter the while loop because the following expression is true:

current < 5Code language: Dart (dart)

Third, increase the current by one and display it; The while statement repeats this step as long as until the current is 5.

2) Using the Dart while statement to display even numbers between 0 and 10

The following program uses a while loop to display all even numbers from 0 to 10:

void main() {
  int number = 0;

  while (number < 10) {
    if (number % 2 == 0) {
      print(number);
    }
    number++;
  }
}Code language: Dart (dart)

Output:

0
2
4
6
8

How it works.

First, declare a variable number and initialize its value to zero:

int number = 0;Code language: Dart (dart)

Second, execute the loop as long as the number is less than 10. Inside the loop, we use the if statement to display the number if it is even.

while (number < 10) {
    if (number % 2 == 0) {
      print(number);
    }
    number++;
}Code language: Dart (dart)

Note that an even number is a number that has a remainder of 0 upon division by 2.

Summary

• Use the while statement to execute statements repeatedly as long as a condition is true.