Dart Enum comparison operator| Enum compareByIndex example| Flutter By Example

Dart Enum comparison operator| Enum compareByIndex example| Flutter By Example

Comparison operators (<,>,>=,<=,==) are useful to compare the values.

Enum contains constant values that represent the index starting from 0,1 onwards.

enum WEEK { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }

Enum Week contains constants such as MONDAY and index is 0, the second value is TUESDAY and index is 1.

By default, two enum objects are equal using the == operator.

void main() {
  WEEK sat = WEEK.SATURDAY;
  WEEK mon = WEEK.MONDAY;
  print(sat == mon); //true
}

Let’s see a comparison operator such as < or >.

enum WEEK { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }
void main() {
  WEEK sat = WEEK.SATURDAY;
  WEEK mon = WEEK.MONDAY;

  print(sat < mon); //throws an error
}

Output:

**Error: The operator '<' isn't defined for the class 'WEEK'.
 'WEEK' is from 'package:dartpad_sample/main.dart' ('lib/main.dart').
  print(sat < mon); //throws an error**

Dart Comparision Operator Examples

Enum provides compareByIndex that compares the values by their index.

int compareByIndex<T extends Enum>(
T value1,
T value2
)

It returns an int value.

Here is a dart enum compareByIndex example

enum WEEK { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }

void main() {
  print(Enum.compareByIndex(WEEK.SUNDAY, WEEK.MONDAY)); // 6
  print(Enum.compareByIndex(WEEK.MONDAY, WEEK.MONDAY)); // 0
  print(Enum.compareByIndex(WEEK.MONDAY, WEEK.SUNDAY)); //-6
}

You can also add an extension method to Enum constants in Dart.

When you are defining an extension method as an operator, the keyword operator must be added before an operator.

Otherwise, throws an error Error: Operator declarations must be preceded by the keyword ‘operator’.

Here is a Dart Enum Comparision operator example

enum WEEK { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }

extension EnumOperators on WEEK {
  bool operator >(WEEK current) {
    return index > current.index;
  }

  bool operator >=(WEEK current) {
    return index >= current.index;
  }

  bool operator <(WEEK current) {
    return index < current.index;
  }

  bool operator <=(WEEK current) {
    return index <= current.index;
  }
}

void main() {
  print(WEEK.SUNDAY < WEEK.MONDAY); // false
  print(WEEK.MONDAY > WEEK.MONDAY); // false
  print(WEEK.MONDAY <= WEEK.SUNDAY); // true
}

Conclusion

By default, the double equal(==) operator works with Enum objects in dart.Method Dart Enum.compareByIndex returns an index that compares with the operator.

Enum Comparision operators work by adding an extension operator.