Dart runes
last modified January 28, 2024
In this article we show how to work with runes in Dart.
A string is a sequence of UTF-16 code units. Strings represent some text in a program. A character may be represented by multiple code points. Each code point can have one or two code units. A code point is a numerical offset in a character set. Each code point is a number whose meaning is given by the Unicode standard.
A grapheme is the smallest unit of a writing system of any given language. An individual grapheme may or may not carry meaning by itself, and may or may not correspond to a single phoneme of the spoken language. The term character has been used to represent a single character in the original ASCII table. This table, however, can represent a limited set of characters. Outside of the ASCII table it is better to use the term grapheme instead of the term character.
A rune is an integer representing a Unicode code point. The runes
property of a string returns its runes. The term was borrowed from Go. The
runes property of a string returns the Unicode code points of a
string. To express a Unicode code point, the \uXXXX syntax is used,
where XXXX is a 4-digit hexadecimal value. If a Unicode code point requires more
than 4 digits, we place the value in curly brackets.
The bytes are the actual information stored for the string contents. Each code point can require one or more bytes of storage depending on the Unicode standard being used (UTF-8, UTF-16, etc.).
The characters package contains functions for more advanced
manipulation of Unicode graphemes.
Dart runes simple example
The following simple example works with runes.
void main() {
final msg = 'an old falcon';
print(msg.codeUnits);
for (final rune in msg.runes) {
print(rune);
}
}
The codePoints attribute returns a list of code points, while the
runes property provides an iterable over the code points/runes.
$ dart main.dart [97, 110, 32, 111, 108, 100, 32, 102, 97, 108, 99, 111, 110] 97 110 32 111 108 100 32 102 97 108 99 111 110
Dart String.fromCharCode
The String.fromCharCode turns a code point into a grapheme.
void main() {
final msg = "one 🐘 and three 🐋";
for (final rune in msg.runes) {
stdout.write("${String.fromCharCode(rune)} ");
}
}
In the example, we go through the string runes. We use the
String.fromCharCode member function to transform each rune into a
grapheme.
$ dart main.dart o n e 🐘 a n d t h r e e 🐋
Dart emojis
The following example displays four emojis.
void main() {
final c1 = '\u{1F9F6}';
final c2 = '\u{1FA86}';
final c3 = '\u26C4';
final c4 = '\u{1F37A}';
print(c1);
print(c2);
print(c3);
print(c4);
print(c3.codeUnits);
print(c4.codeUnits);
}
The emojis are expressed using the special syntax with hexadecimal values.
$ dart emojis.dart 🧶 🪆 ⛄ 🍺 [9924] [55356, 57210]
Note that the fourth emoji uses two code points.
Dart runes length
With the length property, we can determine the length of a string
or the corresponding runes attribute.
void main() {
final msg = 'one 🐘 and three 🐋';
print(msg.length);
print(msg.runes.length);
}
We have a string consisting of ASCII characters and two emojis.
print(msg.length); print(msg.runes.length);
We access the length property of the string object and its
runes attribute.
$ dart main.dart 19 17
The runes.length gives the correct answer; there are 17 graphemes
in the string.
In the next example, we count the graphemes of different writing systems.
void main() {
final msg1 = "falcon";
final msg2 = "вишня";
final msg3 = "🐺🦊🦝";
final msg4 = "नमस्ते";
print(msg1.length);
print(msg2.runes.length);
print(msg3.runes.length);
print(msg4.runes.length);
}
In the program, we count the number of graphemes in ASCII, Cyrillic, and Sanskrit and emojis.
$ dart main.dart 6 5 3 6
The example gives correct results for all except for the Sanskrit. For more complex examples, we need to use the characters package.
Dart characters
The characters package supports Unicode (extended) grapheme
clusters.
$ dart pub add characters
We add the package to the project.
import 'package:characters/characters.dart';
void main() {
final m1 = "🐺🦊🦝";
final m2 = "вишня";
final m3 = "नमस्ते";
print(m1.characters.length);
print(m2.characters.length);
print(m3.characters.length);
print(m3.characters.first);
print(m3.characters.last);
for (final e in m3.characters) {
print(e);
}
}
In the program, we count the graphemes of emojis and Cyrillic and Sanskrit text.
import 'package:characters/characters.dart';
The package is imported.
print(m1.characters.length); print(m2.characters.length); print(m3.characters.length);
The package gives us the characters attribute.
print(m3.characters.first); print(m3.characters.last);
We get the first and last grapheme of the Sanskrit text.
for (final e in m3.characters) {
print(e);
}
We print all its graphemes.
$ dart main.dart 3 5 4 न ते न म स् ते
Now we get correct results.
Source
Dart Runes - language reference
In this article we covered Dart runes.
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