Dart Lesson 9 of 102 2 min read

Booleans in Dart: true, false and Logical Operators

Understand the bool type in Dart, comparison results, and how to combine conditions with the and, or and not operators.

A bool has only two possible values: true and false. Booleans are how a program makes decisions.

Where booleans come from #

Most booleans are the result of a comparison.

void main() {
  var age = 20;

  bool isAdult = age >= 18;
  bool isTeen = age >= 13 && age <= 19;

  print(isAdult);
  print(isTeen);
}

true
false

Combining conditions #

void main() {
  var hasTicket = true;
  var hasId = false;
  var isVip = true;

  print(hasTicket && hasId);
  print(hasTicket || hasId);
  print(!hasId);
  print((hasTicket && hasId) || isVip);
}

false
true
true
true

Short-circuit evaluation #

Dart stops as soon as it knows the answer. With &&, if the left side is false the right side never runs. With ||, if the left side is true the right side never runs. This lets you guard a risky check.

void main() {
  var items = <int>[];

  // items.first would throw on an empty list, but it is never reached.
  if (items.isNotEmpty && items.first > 10) {
    print('Big first item');
  } else {
    print('Empty, or first item is small');
  }
}

Empty, or first item is small

Only bool works in a condition #

Some languages treat 0, '' or null as false. Dart does not. A condition must be a real bool.

void main() {
  var name = '';
  var count = 0;

  // if (name) { }   // error: a String is not a bool
  // if (count) { }  // error: an int is not a bool

  if (name.isEmpty) print('No name given');
  if (count == 0) print('Nothing counted');
}

No name given
Nothing counted

Naming booleans #

Start the name with is, has, can or should so the code reads like a sentence: isLoggedIn, hasError, canEdit.

Try it yourself #

A cinema lets you in if you are 16 or older, or if you are with an adult. Create age and withAdult variables and print whether entry is allowed. Test it with three different combinations.