Dart predicate

last modified March 8, 2023

In this article we show how to use predicates in Dart.

Predicate

Predicate in general meaning is a statement about something that is either true or false. In programming, predicates are single argument functions that return a boolean value.

Dart Predicate example

The following is a simple predicate example.

bool isPositive(int e) {
  return e > 0;
}

void main() {
  final nums = <int>[0, -1, -2, -4, 5, 3, 6, -8];

  final filtered = nums.where((e) => isPositive(e));
  print(filtered);
}

In the program, the isPositive predicate is used to filter out positive values.

bool isPositive(int e) {
    return e > 0;
}

The isPositive predicate returns true for all values that are bigger than zero.

final filtered = nums.where((e) => isPositive(e));

The predicate is passed to the where function, which returns all elements that satisfy the predicate.

$ dart main.dart
(5, 3, 6)

Dart anonymous predicate

The next example passes an anonymous predicate function to the where function.

void main() {
  final nums = <int>[0, -1, -2, -4, 5, 3, 6, -8];

  final filtered = nums.where((e) => e < 0);
  print(filtered);
}

It is often not necessary to give a function a name. We can just pass an anonymous function.

final filtered = nums.where((e) => e < 0);

With the help of the anonymous function, we filter out all negative values.

$ dart main.dart 
(-1, -2, -4, -8)

Dart predicate with any

The any function checks whether any element of the collection satisfies the given predicate.

void main() {
  final nums = <int>[0, -1, -2, -4, 5, 3, 6, -8];

  bool isAny = nums.any((e) => e > 0);
  
  if (isAny) {
    print("There is at least one positive value");
  } else {
    print("There are no positive values");
  }
}

In the example, we find out if there are any positive values.

$ dart main.dart 
There is at least one positive value

Dart predicate with removeWhere

The removeWhere function removes all elements from a collection that satisfy the given predicate.

void main() {
  final words = <String>['sky', 'blue', 'cup', 'nice', 'top', 'cloud'];

  words.removeWhere((e) => e.length != 3);
  print(words);
}

We have a list of strings. We remove all strings whose length is not 3.

$ dart main.dart 
[sky, cup, top]

Dart predicate multiple conditions

The next example uses a predicate with two conditions.

class Country {
  String name;
  int population;

  Country(this.name, this.population);
  String toString() {
    return "$name $population";
  }
}

void main() {
  final countries = <Country>[
    Country("Iran", 80840713),
    Country("Hungary", 9845000),
    Country("Poland", 38485000),
    Country("India", 1342512000),
    Country("Latvia", 1978000),
    Country("Vietnam", 95261000),
    Country("Sweden", 9967000),
    Country("Iceland", 337600),
    Country("Israel", 8622000)
  ];

  final filtered =
      countries.where((e) => e.name.startsWith("I") && e.population > 10000000);
  print(filtered);
}

We create a list of countries. We find all countries that start with 'I' and have population over one million.

final filtered =
    countries.where((e) => e.name.startsWith("I") && e.population > 10000000);

Two anonymous predicates are combined with the && operator.

$ dart main.dart 
(Iran 80840713, India 1342512000)

In this article, we have covered predicates in Dart.

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