Dart Lesson 12 of 102 2 min read

Type Conversion in Dart: Converting Between Types

Convert between String, int, double, bool and collections in Dart using parse, tryParse, toString, toInt and friends.

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Dart never converts types behind your back. '5' + 3 is an error, not 8 or '53'. When you need a different type, you ask for it.

String to number #

void main() {
  var a = int.parse('25');
  var b = double.parse('9.75');

  print(a + 5);
  print(b + 0.25);
}

30
10.0

parse throws a FormatException when the text is not a valid number. For input you do not control, use tryParse, which returns null on failure.

void main() {
  var input = '12abc';
  var number = int.tryParse(input);

  if (number == null) {
    print('"$input" is not a whole number');
  } else {
    print(number * 2);
  }

  // Provide a fallback in one line.
  print(int.tryParse('oops') ?? 0);
}

"12abc" is not a whole number
0

Number to string #

void main() {
  var n = 42;
  var price = 1234.5678;

  print(n.toString() + '!');
  print('$n!');                      // interpolation converts for you
  print(price.toStringAsFixed(2));
  print(255.toRadixString(16));      // hexadecimal
  print(5.toString().padLeft(2, '0'));
}

42!
42!
1234.57
ff
05

int and double #

void main() {
  print(7.toDouble());
  print(7.9.toInt());   // drops the fraction
  print(7.9.round());   // nearest
  print(7.1.ceil());
  print(7.9.floor());
}

String to bool #

There is no built-in conversion that guesses. Compare the text yourself, or use bool.tryParse, which accepts 'true' and 'false'.

void main() {
  var answer = 'Yes';
  var agreed = answer.toLowerCase() == 'yes';
  print(agreed);

  print(bool.tryParse('true'));
  print(bool.tryParse('maybe'));
}

true
true
null

Between collections #

void main() {
  var list = [3, 1, 3, 2, 1];

  var unique = list.toSet();
  print(unique);
  print(unique.toList());

  var words = ['a', 'b', 'c'];
  print(words.join('-'));
  print('a-b-c'.split('-'));

  var texts = ['1', '2', '3'];
  var numbers = texts.map(int.parse).toList();
  print(numbers);
}

{3, 1, 2}
[3, 1, 2]
a-b-c
[a, b, c]
[1, 2, 3]

Casting with as #

as does not convert anything. It tells Dart “trust me, this value already is that type”, and it throws if you are wrong.

void main() {
  num n = 5;
  int i = n as int; // fine: n really holds an int
  print(i + 1);
}

Try it yourself #

Given var raw = ['10', '20', 'x', '30'];, build a list of only the valid numbers using int.tryParse, then print their sum. You should get 60.