Dart Lesson 8 of 102 3 min read

Strings in Dart: Interpolation, Methods and Multi-Line Text

Create and work with strings in Dart: quotes, interpolation, multi-line and raw strings, common methods, and StringBuffer.

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A String is a piece of text. You can write one with single or double quotes; Dart style uses single quotes unless the text itself contains one.

void main() {
  var a = 'Hello';
  var b = "It's easy";
  print('$a. $b.');
}

Hello. It's easy.

String interpolation #

Put $name inside a string to insert a variable. Wrap any other expression in ${ }.

void main() {
  var item = 'coffee';
  var price = 3.5;
  var qty = 2;

  print('$qty x $item = ${price * qty}');
  print('${item.toUpperCase()} has ${item.length} letters');
}

2 x coffee = 7.0
COFFEE has 6 letters

Joining strings #

void main() {
  var first = 'Dart';
  var second = 'Tutorial';

  print(first + ' ' + second);  // works
  print('$first $second');      // preferred
  print('Dart ' 'Tutorial');    // neighbouring literals join automatically
}

Dart Tutorial
Dart Tutorial
Dart Tutorial

Multi-line and raw strings #

Three quotes let a string run over several lines. An r before the quote makes a raw string, where \ and $ are just characters.

void main() {
  var poem = '''
Roses are red,
Dart is blue.''';
  print(poem);

  print('Tab:\tdone');
  print(r'C:\new\table costs $5');
}

Roses are red,
Dart is blue.
Tab:	done
C:\new\table costs $5

Common escapes are \n (new line), \t (tab), \' (quote), \\ (backslash) and \$ (dollar sign).

Methods you will use constantly #

void main() {
  var text = '  Learn Dart Today  ';
  var clean = text.trim();

  print(clean.length);
  print(clean.toLowerCase());
  print(clean.contains('Dart'));
  print(clean.startsWith('Learn'));
  print(clean.indexOf('Dart'));
  print(clean.substring(6, 10));
  print(clean.replaceAll('Today', 'Now'));
  print(clean.split(' '));
  print('7'.padLeft(3, '0'));
  print(clean.isEmpty);
}

16
learn dart today
true
true
6
Dart
Learn Dart Now
[Learn, Dart, Today]
007
false

Strings cannot be changed in place #

A string is immutable. Methods such as toUpperCase return a new string and leave the original alone.

void main() {
  var word = 'dart';
  word.toUpperCase();        // result thrown away
  print(word);
  word = word.toUpperCase(); // keep the result
  print(word);
}

dart
DART

Characters and comparison #

void main() {
  var word = 'Dart';
  print(word[0]);
  print(word[word.length - 1]);
  print(word == 'Dart');             // compares the text
  print('apple'.compareTo('banana')); // negative: apple sorts first
}

D
t
true
-1

Building long strings with StringBuffer #

When you add to a string many times in a loop, a StringBuffer is faster than +.

void main() {
  var buffer = StringBuffer();
  for (var i = 1; i <= 3; i++) {
    buffer.write('Line $i');
    buffer.write('; ');
  }
  print(buffer.toString());
}

Line 1; Line 2; Line 3;

Try it yourself #

Take the string ' flutter AND dart '. Trim it, make it lower case, replace and with &, and print how many characters the result has.