Loops are used to execute a set of statements repeatedly until a particular condition is satisfied. In Dart, we have the following types of loops:

  • for loop
  • while loop
  • do-while loop
  • for-in loop

for loop

The for loop is used to execute a set of statements a specific number of times.

Syntax:

  for (initialization; condition; increment/decrement) {  
    // statements to be executed  
}

Example:

  void main() {  
    for (var i = 1; i <= 10; i++) {  
        print(i);  
    }  
}

Output:

  1
2
3
4
5
6
7
8
9
10

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while loop

The while loop is used to execute a set of statements until a particular condition is satisfied.

Syntax:

  while (condition) {  
    // statements to be executed  
}

Example:

  
void main() {  
    var i = 1;  
    while (i <= 10) {  
        print(i);  
        i++;  
    }  
}

Output:

  
1
2
3
4
5
6
7
8
9
10

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do-while loop

The do-while loop is used to execute a set of statements at least once and then repeatedly executes until a particular condition is satisfied.

Syntax:

  do {  
    // statements to be executed  
} while (condition);

Example:

  void main() {  
    var i = 1;  
    do {  
        print(i);  
        i++;  
    } while (i <= 10);  
}

Output:

  1
2
3
4
5
6
7
8
9
10

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for-in loop

The for-in loop is used to iterate through the elements of lists, sets, or maps.

Syntax:

  for (var item in collection) {  
    // statements to be executed  
}

Example:

  
void main() {  
    var numbers = [1, 2, 3];  
    for (var n in numbers) {  
        print(n);  
    }  
}

Output:

  
1
2
3

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Nested loops

A loop inside another loop is called a nested loop. The inner loop will be executed one time for each iteration of the outer loop.

Example:

  void main() {  
    for (var i = 1; i <= 3; i++) {  
        for (var j = 1; j <= 3; j++) {  
            print("$i $j");  
        }  
    }  
}

Output:

  
1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3

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Infinite loops

A loop that never ends is called an infinite loop. If the condition of the while loop is always true, then the loop will never end.

Example:

  
void main() {  
    while (true) {  
        print("Hello World");  
    }  
}

Output:

  
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World 
.
.
.

Loop Control Statements

Loop control statements change the execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

Dart supports the following control statements:

  • break statement
  • continue statement

break keyword

The break statement is used to terminate the loop immediately.

Example:

  void main() {  
    for (var i = 1; i <= 10; i++) {  
        if (i == 5) {  
            break;  
        }  
        print(i);  
    }  
}

Output:

  1
2
3
4

Try Yourself

break keyword will break the loop immediately if the condition will be ture. Here the output will be only upto 4 because next value of i will terminate the loop.

continue Keyword

The continue statement is used to skip the current iteration of the loop and the control flow will continue with the next iteration.

Example:

  void main() {  
    for (var i = 1; i <= 10; i++) {  
        if (i == 5) {  
            continue;  
        }  
        print(i);  
    }  
}

Output:

  1
2
3
4
6
7
8
9
10

Here, in this code when the value of i = 5, the code left to be executed will be skipped, that’s the reason why we don’t see 5.

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