Dart bool

Summary: in this tutorial, you’ll learn how to use the Dart bool type that holds boolean values including true and false.

Introduction to the Dart bool type

Dart uses the bool type to represent Boolean values. The bool type has two constant values true and false.

The following example declares a variable with the bool type and initialize its value to true:

void main() {
  bool isActive = true;
  print(isActive);
}Code language: Dart (dart)

In practice, you often use the boolean values in the control statements such as if else, while, do while, and for.

Logical operators

Dart provides you with logical operators for working with boolean values as illustrated in the following table:

Operator	Name	Meaning
&&	Logical AND	Accept two boolean values and return true if both of them are true or false otherwise
||	Logical OR	Accept two boolean values and return false if both values are false or true otherwise
!	Logical NOT	Accept a boolean value and negate it.

Logical AND operator (&&)

The following table shows the result of the logical AND operator:

x	y	x && y
true	true	true
true	false	false
false	true	false
false	false	false

For example, the following program illustrates how to use the logical AND operator:

void main() {
  bool isActive = true;
  bool isPasswordCorrect = true;
  bool isAuthenticated = isPasswordCorrect && isActive;

  print(isAuthenticated);
}Code language: Dart (dart)

Output:

trueCode language: Dart (dart)

In this example, both isActive and isPasswordCorrect are true. Therefore, the result of the logical AND of these variables is true.

Logical OR operator (||)

The following table illustrates the result of the logical OR operator:

x	y	x || y
true	true	true
true	false	true
false	true	true
false	false	false

The table shows that x || y returns false only if both values are false. Otherwise, it returns true.

For example, the following program shows how to use the logical OR operator:

void main() {
  bool isRequired = false;
  bool isValid = false;
  bool result = isRequired || isValid;

  print(result);
}Code language: Dart (dart)

In this example, both isRequired and isValid are false. Therefore, the result of the expression isRequired || isValid is false.

Short-circuit evaluation

Both logical AND and OR operators are short-circuited.

If the first value is false, the logical AND operator will be false regardless of the value of the second value. Therefore, the logical AND operator will not evaluate the second value if the first one is false. For example:

void main() {
  bool isRequired = false;
  bool isValid = true;
  bool result = isRequired && isValid;

  print(result);
}Code language: Dart (dart)

In this example, the logical AND operator doesn’t evaluate the isValid variable because the result is always false due to the value of the isRequired.

In VS Code, you’ll see the following warning message for the isValid variable in the expression:

Dead code.
Try removing the code, or fixing the code before it so that it can be reachedCode language: Dart (dart)

It means that the isValid will never be reached.

Similarly, if the first value is true, the logical OR operator will return true whether the value of the second value is true or false. Therefore, the logical OR operator will not evaluate the second value if the first one is true.

The following program illustrates how the logical OR operator is short-circuited:

void main() {
  bool isRequired = true;
  bool isValid = false;
  bool result = isRequired || isValid;

  print(result);
}Code language: Dart (dart)

In this example, the isRequiredis true. Therefore, the result is always true. The logical OR operator will not evaluate the isValid in the expression isRequired || isValid.

Logical NOT operator (!)

The logical NOT operator negates the value of a boolean value:

x	!x
true	false
false	true

The following program uses the logical OR operator to negate a value of the variable isRequired:

void main() {
  bool isRequired = true;
  print(!isRequired);
}Code language: Dart (dart)

Output:

falseCode language: Dart (dart)

Summary

• The bool type to represent boolean values including true and false.
• The logical AND operator (&&) returns true only if both values are true.
• The logical OR operator (||) returns false only if both values are false.
• Both logical AND and OR operators are short-circuited.
• The logical NOT operator (!) negates a boolean value.