Pattern matching
- Dart course
- Beginner
- Lesson 39/116
- 5 min
Learn Dart 3 switch expressions and pattern matching.
Pattern matching
The switch expression, introduced in Dart 3, is the expression (value-producing) form of switch. The result can be assigned to a variable: var name = switch (day) { "Mon" => "Monday", _ => "Unknown" };.
Two key differences: (1) instead of case and a colon, the => arrow is used; (2) break is not needed because each arm ends automatically. The _ underscore is the wildcard pattern meaning "everything else"; it replaces default.
The wildcard pattern _
In a switch expression the _ underscore is the wildcard pattern meaning "all other values". It replaces default and is usually written last. In a switch expression the _ arm is required; otherwise some values would produce no result and a compile error occurs.
Example: var t = switch (n) { 1 => "one", 2 => "two", _ => "other" };. Here if n is neither 1 nor 2 the _ arm runs and returns "other".
Practice Day type with switch expression
Declare a String variable day with the value "Fri". Declare a variable desc and assign it to a switch (day) expression: use arms "Fri" => "Almost weekend" and _ => "Midweek". Then print desc with print(desc);. The output on a single line must be Almost weekend.
Starter code
// Declare day, assign desc with a switch expression and print itSample solution
String day = "Fri";
var desc = switch (day) {
"Fri" => "Almost weekend",
_ => "Midweek",
};
print(desc);Expected output
Almost weekend
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which sign separates arms in a switch expression?
var x = switch (n) {
1 => "one",
_ => "other",
};- case ... :
- => Correct answer
- :
- ->
Explanation: Each arm in a switch expression produces a value with the => arrow.
2 Match each part to its job
| Term | Meaning |
|---|---|
switch (x) { |
Value to check |
=> |
Result arrow |
_ |
Wildcard (other) |
var y = switch |
Assign result to variable |
3 Write the switch expression arm
String day = "Fri";
var desc = switch (day) {
____
_ => "Midweek",
};
print(desc);String day = "Fri";
var desc = switch (day) {
"Fri" => "Almost weekend",
_ => "Midweek",
};
print(desc);Explanation: The => arrow produces a value; the line ends with a comma.