HashSet<E> abstract final#

An unordered hash-table based Set implementation.

The elements of a HashSet must have consistent equality and hashCode implementations. This means that the equals operation must define a stable equivalence relation on the elements (reflexive, symmetric, transitive, and consistent over time), and that the hashCode must be consistent with equality, so that it's the same for objects that are considered equal.

Most simple operations on HashSet are done in (potentially amortized) constant time: add, contains, remove, and length, provided the hash codes of objects are well distributed.

The iteration order of the set is not specified and depends on the hashcodes of the provided elements. However, the order is stable: multiple iterations over the same set produce the same order, as long as the set is not modified.

Note: Do not modify a set (add or remove elements) while an operation is being performed on that set, for example in functions called during a forEach or containsAll call, or while iterating the set.

Do not modify elements in a way which changes their equality (and thus their hash code) while they are in the set. Some specialized kinds of sets may be more permissive with regards to equality, in which case they should document their different behavior and restrictions.

Example:

final letters = HashSet<String>();

To add data to a set, use add or addAll.

letters.add('A');
letters.addAll({'B', 'C', 'D'});

To check if the set is empty, use isEmpty or isNotEmpty. To find the number of elements in the set, use length.

print(letters.isEmpty); // false
print(letters.length); // 4
print(letters); // fx {A, D, C, B}

To check whether the set has an element with a specific value, use contains.

final bExists = letters.contains('B'); // true

The forEach method calls a function with each element of the set.

letters.forEach(print);
// A
// D
// C
// B

To make a copy of the set, use toSet.

final anotherSet = letters.toSet();
print(anotherSet); // fx {A, C, D, B}

To remove an element, use remove.

final removedValue = letters.remove('A'); // true
print(letters); // fx {B, C, D}

To remove multiple elements at the same time, use removeWhere or removeAll.

letters.removeWhere((element) => element.startsWith('B'));
print(letters); // fx {D, C}

To removes all elements in this set that do not meet a condition, use retainWhere.

letters.retainWhere((element) => element.contains('C'));
print(letters); // {C}

To remove all elements and empty the set, use clear.

letters.clear();
print(letters.isEmpty); // true
print(letters); // {}

See also:

• Set is the general interface of collection where each object can occur only once.
• LinkedHashSet objects stored based on insertion order.
• SplayTreeSet iterates the objects in sorted order.

Constructors#

HashSet() factory#

Create a hash set using the provided equals as equality.

The provided equals must define a stable equivalence relation, and hashCode must be consistent with equals.

If equals or hashCode are omitted, the set uses the elements' intrinsic Object.== and Object.hashCode.

If you supply one of equals and hashCode, you should generally also supply the other.

Some equals or hashCode functions might not work for all objects. If isValidKey is supplied, it's used to check a potential element which is not necessarily an instance of E, like the argument to contains which is typed as Object?. If isValidKey returns false, for an object, the equals and hashCode functions are not called, and no key equal to that object is assumed to be in the map. The isValidKey function defaults to just testing if the object is an instance of E, which means that:

HashSet<int>(equals: (int e1, int e2) => (e1 - e2) % 5 == 0,
             hashCode: (int e) => e % 5)

does not need an isValidKey argument because it defaults to only accepting int values which are accepted by both equals and hashCode.

If neither equals, hashCode, nor isValidKey is provided, the default isValidKey instead accepts all values. The default equality and hashcode operations are assumed to work on all objects.

Likewise, if equals is identical, hashCode is identityHashCode and isValidKey is omitted, the resulting set is identity based, and the isValidKey defaults to accepting all keys. Such a map can be created directly using HashSet.identity.

external factory HashSet({
  bool Function(E, E)? equals,
  int Function(E)? hashCode,
  bool Function(dynamic)? isValidKey,
});

HashSet.from() factory#

Create a hash set containing all elements.

Creates a hash set as by HashSet<E>() and adds all given elements to the set. The elements are added in order. If elements contains two entries that are equal, but not identical, then the first one is the one in the resulting set.

All the elements should be instances of E. The elements iterable itself may have any element type, so this constructor can be used to down-cast a Set, for example as:

Set<SuperType> superSet = ...;
Set<SubType> subSet =
    HashSet<SubType>.from(superSet.whereType<SubType>());

Example:

final numbers = <num>[10, 20, 30];
final hashSetFrom = HashSet<int>.from(numbers);
print(hashSetFrom); // fx {20, 10, 30}

factory HashSet.from(Iterable<dynamic> elements) {
  HashSet<E> result = HashSet<E>();
  for (final e in elements) {
    result.add(e as E);
  }
  return result;
}

HashSet.identity() factory#

Creates an unordered identity-based set.

Effectively shorthand for:

HashSet<E>(equals: identical, hashCode: identityHashCode)

external factory HashSet.identity();

HashSet.of() factory#

Create a hash set containing all elements.

Creates a hash set as by HashSet<E>() and adds all given elements to the set. The elements are added in order. If elements contains two entries that are equal, but not identical, then the first one is the one in the resulting set. Example:

final baseSet = <int>{1, 2, 3};
final hashSetOf = HashSet<num>.of(baseSet);
print(hashSetOf); // fx {3, 1, 2}

factory HashSet.of(Iterable<E> elements) => HashSet<E>()..addAll(elements);

Properties#

first no setter inherited#

The first element.

Throws a StateError if this is empty. Otherwise returns the first element in the iteration order, equivalent to this.elementAt(0).

Inherited from Iterable.

E get first {
  Iterator<E> it = iterator;
  if (!it.moveNext()) {
    throw IterableElementError.noElement();
  }
  return it.current;
}

firstOrNull extension no setter#

The first element of this iterator, or null if the iterable is empty.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

T? get firstOrNull {
  var iterator = this.iterator;
  if (iterator.moveNext()) return iterator.current;
  return null;
}

hashCode no setter inherited#

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

external int get hashCode;

indexed extension no setter#

Pairs of elements of the indices and elements of this iterable.

The elements are (0, this.first) through (this.length - 1, this.last), in index/iteration order.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

@pragma('vm:prefer-inline')
Iterable<(int, T)> get indexed => IndexedIterable<T>(this, 0);

isEmpty no setter inherited#

Whether this collection has no elements.

May be computed by checking if iterator.moveNext() returns false.

Example:

final emptyList = <int>[];
print(emptyList.isEmpty); // true;
print(emptyList.iterator.moveNext()); // false

Inherited from Iterable.

bool get isEmpty => !iterator.moveNext();

isNotEmpty no setter inherited#

Whether this collection has at least one element.

May be computed by checking if iterator.moveNext() returns true.

Example:

final numbers = <int>{1, 2, 3};
print(numbers.isNotEmpty); // true;
print(numbers.iterator.moveNext()); // true

Inherited from Iterable.

bool get isNotEmpty => !isEmpty;

iterator no setter override#

Provides an iterator that iterates over the elements of this set.

The order of iteration is unspecified, but is consistent between changes to the set.

Iterator<E> get iterator;

last no setter inherited#

The last element.

Throws a StateError if this is empty. Otherwise may iterate through the elements and returns the last one seen. Some iterables may have more efficient ways to find the last element (for example a list can directly access the last element, without iterating through the previous ones).

Inherited from Iterable.

E get last {
  Iterator<E> it = iterator;
  if (!it.moveNext()) {
    throw IterableElementError.noElement();
  }
  E result;
  do {
    result = it.current;
  } while (it.moveNext());
  return result;
}

lastOrNull extension no setter#

The last element of this iterable, or null if the iterable is empty.

This computation may not be efficient. The last value is potentially found by iterating the entire iterable and temporarily storing every value. The process only iterates the iterable once. If iterating more than once is not a problem, it may be more efficient for some iterables to do:

var lastOrNull = iterable.isEmpty ? null : iterable.last;

Available on Iterable<E>, provided by the IterableExtensions<T> extension

T? get lastOrNull {
  if (this is EfficientLengthIterable) {
    if (isEmpty) return null;
    return last;
  }
  var iterator = this.iterator;
  if (!iterator.moveNext()) return null;
  T result;
  do {
    result = iterator.current;
  } while (iterator.moveNext());
  return result;
}

length no setter inherited#

The number of elements in this Iterable.

Counting all elements may involve iterating through all elements and can therefore be slow. Some iterables have a more efficient way to find the number of elements. These must override the default implementation of length.

Inherited from Iterable.

int get length {
  assert(this is! EfficientLengthIterable);
  int count = 0;
  Iterator<Object?> it = iterator;
  while (it.moveNext()) {
    count++;
  }
  return count;
}

nonNulls extension no setter#

The non-null elements of this iterable.

The same elements as this iterable, except that null values are omitted.

Available on Iterable<E>, provided by the NullableIterableExtensions<T extends Object> extension

Iterable<T> get nonNulls => NonNullsIterable<T>(this);

runtimeType no setter inherited#

A representation of the runtime type of the object.

Inherited from Object.

external Type get runtimeType;

single no setter inherited#

Checks that this iterable has only one element, and returns that element.

Throws a StateError if this is empty or has more than one element. This operation will not iterate past the second element.

Inherited from Iterable.

E get single {
  Iterator<E> it = iterator;
  if (!it.moveNext()) throw IterableElementError.noElement();
  E result = it.current;
  if (it.moveNext()) throw IterableElementError.tooMany();
  return result;
}

singleOrNull extension no setter#

The single element of this iterator, or null.

If the iterator has precisely one element, this is that element. Otherwise, if the iterator has zero elements, or it has two or more, the value is null.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

T? get singleOrNull {
  var iterator = this.iterator;
  if (iterator.moveNext()) {
    var result = iterator.current;
    if (!iterator.moveNext()) return result;
  }
  return null;
}

wait extension no setter#

Waits for futures in parallel.

Waits for all the futures in this iterable. Returns a list of the resulting values, in the same order as the futures which created them, if all futures are successful.

Similar to Future.wait, but reports errors using a ParallelWaitError, which allows the caller to handle errors and dispose successful results if necessary.

The returned future is completed when all the futures have completed. If any of the futures do not complete, nor does the returned future.

If any future completes with an error, the returned future completes with a ParallelWaitError. The ParallelWaitError.values is a list of the values for successful futures and null for futures with errors. The ParallelWaitError.errors is a list of the same length, with null values for the successful futures and an AsyncError with the error for futures which completed with an error.

Available on Iterable<E>, provided by the FutureIterable<T> extension

Future<List<T>> get wait {
  var results = [for (var f in this) _FutureResult<T>(f)];
  if (results.isEmpty) return Future<List<T>>.value(<T>[]);

  @pragma('vm:awaiter-link')
  final c = Completer<List<T>>.sync();

  _FutureResult._waitAll(results, (errors) {
    if (errors == 0) {
      c.complete([for (var r in results) r.value]);
    } else {
      var errorList = [for (var r in results) r.errorOrNull];
      c.completeError(
        ParallelWaitError<List<T?>, List<AsyncError?>>(
          [for (var r in results) r.valueOrNull],
          errorList,
          errorCount: errors,
          defaultError: errorList.firstWhere(_notNull),
        ),
      );
    }
  });
  return c.future;
}

Methods#

add() inherited#

Adds value to the set.

Returns true if value (or an equal value) was not yet in the set. Otherwise returns false and the set is not changed.

Example:

final dateTimes = <DateTime>{};
final time1 = DateTime.fromMillisecondsSinceEpoch(0);
final time2 = DateTime.fromMillisecondsSinceEpoch(0);
// time1 and time2 are equal, but not identical.
assert(time1 == time2);
assert(!identical(time1, time2));
final time1Added = dateTimes.add(time1);
print(time1Added); // true
// A value equal to time2 exists already in the set, and the call to
// add doesn't change the set.
final time2Added = dateTimes.add(time2);
print(time2Added); // false

print(dateTimes); // {1970-01-01 02:00:00.000}
assert(dateTimes.length == 1);
assert(identical(time1, dateTimes.first));
print(dateTimes.length);

Inherited from Set.

bool add(E value);

addAll() inherited#

Adds all elements to this set.

Equivalent to adding each element in elements using add, but some collections may be able to optimize it.

final characters = <String>{'A', 'B'};
characters.addAll({'A', 'B', 'C'});
print(characters); // {A, B, C}

Inherited from Set.

void addAll(Iterable<E> elements);

any() inherited#

Checks whether any element of this iterable satisfies test.

Checks every element in iteration order, and returns true if any of them make test return true, otherwise returns false. Returns false if the iterable is empty.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.any((element) => element >= 5); // true;
result = numbers.any((element) => element >= 10); // false;

Inherited from Iterable.

bool any(bool test(E element)) {
  for (E element in this) {
    if (test(element)) return true;
  }
  return false;
}

asNameMap() extension#

Creates a map from the names of enum values to the values.

The collection that this method is called on is expected to have enums with distinct names, like the values list of an enum class. Only one value for each name can occur in the created map, so if two or more enum values have the same name (either being the same value, or being values of different enum type), at most one of them will be represented in the returned map.

Available on Iterable<E>, provided by the EnumByName<T extends Enum> extension

Map<String, T> asNameMap() => <String, T>{
  for (var value in this) value._name: value,
};

byName() extension#

Finds the enum value in this list with name name.

Goes through this collection looking for an enum with name name, as reported by EnumName.name. Returns the first value with the given name. Such a value must be found.

Available on Iterable<E>, provided by the EnumByName<T extends Enum> extension

T byName(String name) {
  for (var value in this) {
    if (value._name == name) return value;
  }
  throw ArgumentError.value(name, "name", "No enum value with that name");
}

cast() inherited#

Provides a view of this set as a set of R instances.

If this set contains only instances of R, all read operations will work correctly. If any operation tries to access an element that is not an instance of R, the access will throw instead.

Elements added to the set (e.g., by using add or addAll) must be instances of R to be valid arguments to the adding function, and they must be instances of E as well to be accepted by this set as well.

Methods which accept one or more Object? as argument, like contains, remove and removeAll, will pass the argument directly to the this set's method without any checks. That means that you can do setOfStrings.cast<int>().remove("a") successfully, even if it looks like it shouldn't have any effect.

Inherited from Set.

Set<R> cast<R>();

clear() inherited#

Removes all elements from the set.

final characters = <String>{'A', 'B', 'C'};
characters.clear(); // {}

Inherited from Set.

void clear();

contains() inherited#

Whether value is in the set.

final characters = <String>{'A', 'B', 'C'};
final containsB = characters.contains('B'); // true
final containsD = characters.contains('D'); // false

Inherited from Set.

bool contains(Object? value);

containsAll() inherited#

Whether this set contains all the elements of other.

final characters = <String>{'A', 'B', 'C'};
final containsAB = characters.containsAll({'A', 'B'});
print(containsAB); // true
final containsAD = characters.containsAll({'A', 'D'});
print(containsAD); // false

Inherited from Set.

bool containsAll(Iterable<Object?> other);

difference() inherited#

Creates a new set with the elements of this that are not in other.

That is, the returned set contains all the elements of this Set that are not elements of other according to other.contains.

final characters1 = <String>{'A', 'B', 'C'};
final characters2 = <String>{'A', 'E', 'F'};
final differenceSet1 = characters1.difference(characters2);
print(differenceSet1); // {B, C}
final differenceSet2 = characters2.difference(characters1);
print(differenceSet2); // {E, F}

Inherited from Set.

Set<E> difference(Set<Object?> other);

elementAt() inherited#

Returns the indexth element.

The index must be non-negative and less than length. Index zero represents the first element (so iterable.elementAt(0) is equivalent to iterable.first).

May iterate through the elements in iteration order, ignoring the first index elements and then returning the next. Some iterables may have a more efficient way to find the element.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
final elementAt = numbers.elementAt(4); // 6

Inherited from Iterable.

E elementAt(int index) {
  RangeError.checkNotNegative(index, "index");
  var iterator = this.iterator;
  var skipCount = index;
  while (iterator.moveNext()) {
    if (skipCount == 0) return iterator.current;
    skipCount--;
  }
  throw IndexError.withLength(
    index,
    index - skipCount,
    indexable: this,
    name: "index",
  );
}

elementAtOrNull() extension#

The element at position index of this iterable, or null.

The index is zero based, and must be non-negative.

Returns the result of elementAt(index) if the iterable has at least index + 1 elements, and null otherwise.

Available on Iterable<E>, provided by the IterableExtensions<T> extension

T? elementAtOrNull(int index) {
  RangeError.checkNotNegative(index, "index");
  if (this is EfficientLengthIterable) {
    if (index >= length) return null;
    return elementAt(index);
  }
  var iterator = this.iterator;
  do {
    if (!iterator.moveNext()) return null;
  } while (--index >= 0);
  return iterator.current;
}

every() inherited#

Checks whether every element of this iterable satisfies test.

Checks every element in iteration order, and returns false if any of them make test return false, otherwise returns true. Returns true if the iterable is empty.

Example:

final planetsByMass = <double, String>{0.06: 'Mercury', 0.81: 'Venus',
  0.11: 'Mars'};
// Checks whether all keys are smaller than 1.
final every = planetsByMass.keys.every((key) => key < 1.0); // true

Inherited from Iterable.

bool every(bool test(E element)) {
  for (E element in this) {
    if (!test(element)) return false;
  }
  return true;
}

expand() inherited#

Expands each element of this Iterable into zero or more elements.

The resulting Iterable runs through the elements returned by toElements for each element of this, in iteration order.

The returned Iterable is lazy, and calls toElements for each element of this iterable every time the returned iterable is iterated.

Example:

Iterable<int> count(int n) sync* {
  for (var i = 1; i <= n; i++) {
    yield i;
   }
 }

var numbers = [1, 3, 0, 2];
print(numbers.expand(count)); // (1, 1, 2, 3, 1, 2)

Equivalent to:

Iterable<T> expand<T>(Iterable<T> toElements(E e)) sync* {
  for (var value in this) {
    yield* toElements(value);
  }
}

Inherited from Iterable.

Iterable<T> expand<T>(Iterable<T> toElements(E element)) =>
    ExpandIterable<E, T>(this, toElements);

firstWhere() inherited#

The first element that satisfies the given predicate test.

Iterates through elements and returns the first to satisfy test.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.firstWhere((element) => element < 5); // 1
result = numbers.firstWhere((element) => element > 5); // 6
result =
    numbers.firstWhere((element) => element > 10, orElse: () => -1); // -1

If no element satisfies test, the result of invoking the orElse function is returned. If orElse is omitted, it defaults to throwing a StateError. Stops iterating on the first matching element.

Inherited from Iterable.

E firstWhere(bool test(E element), {E orElse()?}) {
  for (E element in this) {
    if (test(element)) return element;
  }
  if (orElse != null) return orElse();
  throw IterableElementError.noElement();
}

fold() inherited#

Reduces a collection to a single value by iteratively combining each element of the collection with an existing value

Uses initialValue as the initial value, then iterates through the elements and updates the value with each element using the combine function, as if by:

var value = initialValue;
for (E element in this) {
  value = combine(value, element);
}
return value;

Example of calculating the sum of an iterable:

final numbers = <double>[10, 2, 5, 0.5];
const initialValue = 100.0;
final result = numbers.fold<double>(
    initialValue, (previousValue, element) => previousValue + element);
print(result); // 117.5

Inherited from Iterable.

T fold<T>(T initialValue, T combine(T previousValue, E element)) {
  var value = initialValue;
  for (E element in this) value = combine(value, element);
  return value;
}

followedBy() inherited#

Creates the lazy concatenation of this iterable and other.

The returned iterable will provide the same elements as this iterable, and, after that, the elements of other, in the same order as in the original iterables.

Example:

var planets = <String>['Earth', 'Jupiter'];
var updated = planets.followedBy(['Mars', 'Venus']);
print(updated); // (Earth, Jupiter, Mars, Venus)

Inherited from Iterable.

Iterable<E> followedBy(Iterable<E> other) {
  var self = this; &#47;&#47; TODO(lrn): Remove when we can promote `this`.
  if (self is EfficientLengthIterable<E>) {
    return FollowedByIterable<E>.firstEfficient(self, other);
  }
  return FollowedByIterable<E>(this, other);
}

forEach() inherited#

Invokes action on each element of this iterable in iteration order.

Example:

final numbers = <int>[1, 2, 6, 7];
numbers.forEach(print);
// 1
// 2
// 6
// 7

Inherited from Iterable.

void forEach(void action(E element)) {
  for (E element in this) action(element);
}

intersection() inherited#

Creates a new set which is the intersection between this set and other.

That is, the returned set contains all the elements of this Set that are also elements of other according to other.contains.

final characters1 = <String>{'A', 'B', 'C'};
final characters2 = <String>{'A', 'E', 'F'};
final intersectionSet = characters1.intersection(characters2);
print(intersectionSet); // {A}

Inherited from Set.

Set<E> intersection(Set<Object?> other);

join() inherited#

Converts each element to a String and concatenates the strings.

Iterates through elements of this iterable, converts each one to a String by calling Object.toString, and then concatenates the strings, with the separator string interleaved between the elements.

Example:

final planetsByMass = <double, String>{0.06: 'Mercury', 0.81: 'Venus',
  0.11: 'Mars'};
final joinedNames = planetsByMass.values.join('-'); // Mercury-Venus-Mars

Inherited from Iterable.

String join([String separator = ""]) {
  Iterator<E> iterator = this.iterator;
  if (!iterator.moveNext()) return "";
  var first = iterator.current.toString();
  if (!iterator.moveNext()) return first;
  var buffer = StringBuffer(first);
  &#47;&#47; TODO(51681): Drop null check when de-supporting pre-2.12 code.
  if (separator == null || separator.isEmpty) {
    do {
      buffer.write(iterator.current.toString());
    } while (iterator.moveNext());
  } else {
    do {
      buffer
        ..write(separator)
        ..write(iterator.current.toString());
    } while (iterator.moveNext());
  }
  return buffer.toString();
}

lastWhere() inherited#

The last element that satisfies the given predicate test.

An iterable that can access its elements directly may check its elements in any order (for example a list starts by checking the last element and then moves towards the start of the list). The default implementation iterates elements in iteration order, checks test(element) for each, and finally returns that last one that matched.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.lastWhere((element) => element < 5); // 3
result = numbers.lastWhere((element) => element > 5); // 7
result = numbers.lastWhere((element) => element > 10,
    orElse: () => -1); // -1

If no element satisfies test, the result of invoking the orElse function is returned. If orElse is omitted, it defaults to throwing a StateError.

Inherited from Iterable.

E lastWhere(bool test(E element), {E orElse()?}) {
  var iterator = this.iterator;
  // Potential result during first loop.
  E result;
  do {
    if (!iterator.moveNext()) {
      if (orElse != null) return orElse();
      throw IterableElementError.noElement();
    }
    result = iterator.current;
  } while (!test(result));
  // Now `result` is actual result, unless a later one is found.
  while (iterator.moveNext()) {
    var current = iterator.current;
    if (test(current)) result = current;
  }
  return result;
}

lookup() inherited#

If an object equal to object is in the set, return it.

Checks whether object is in the set, like contains, and if so, returns the object in the set, otherwise returns null.

If the equality relation used by the set is not identity, then the returned object may not be identical to object. Some set implementations may not be able to implement this method. If the contains method is computed, rather than being based on an actual object instance, then there may not be a specific object instance representing the set element.

final characters = <String>{'A', 'B', 'C'};
final containsB = characters.lookup('B');
print(containsB); // B
final containsD = characters.lookup('D');
print(containsD); // null

Inherited from Set.

E? lookup(Object? object);

map() inherited#

The current elements of this iterable modified by toElement.

Returns a new lazy Iterable with elements that are created by calling toElement on each element of this Iterable in iteration order.

The returned iterable is lazy, so it won't iterate the elements of this iterable until it is itself iterated, and then it will apply toElement to create one element at a time. The converted elements are not cached. Iterating multiple times over the returned Iterable will invoke the supplied toElement function once per element for on each iteration.

Methods on the returned iterable are allowed to omit calling toElement on any element where the result isn't needed. For example, elementAt may call toElement only once.

Equivalent to:

Iterable<T> map<T>(T toElement(E e)) sync* {
  for (var value in this) {
    yield toElement(value);
  }
}

Example:

var products = jsonDecode('''
[
  {"name": "Screwdriver", "price": 42.00},
  {"name": "Wingnut", "price": 0.50}
]
''');
var values = products.map((product) => product['price'] as double);
var totalPrice = values.fold(0.0, (a, b) => a + b); // 42.5.

Inherited from Iterable.

Iterable<T> map<T>(T toElement(E e)) => MappedIterable<E, T>(this, toElement);

noSuchMethod() inherited#

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

reduce() inherited#

Reduces a collection to a single value by iteratively combining elements of the collection using the provided function.

The iterable must have at least one element. If it has only one element, that element is returned.

Otherwise this method starts with the first element from the iterator, and then combines it with the remaining elements in iteration order, as if by:

E value = iterable.first;
iterable.skip(1).forEach((element) {
  value = combine(value, element);
});
return value;

Example of calculating the sum of an iterable:

final numbers = <double>[10, 2, 5, 0.5];
final result = numbers.reduce((value, element) => value + element);
print(result); // 17.5

Consider using fold if the iterable can be empty.

Inherited from Iterable.

E reduce(E combine(E value, E element)) {
  Iterator<E> iterator = this.iterator;
  if (!iterator.moveNext()) {
    throw IterableElementError.noElement();
  }
  E value = iterator.current;
  while (iterator.moveNext()) {
    value = combine(value, iterator.current);
  }
  return value;
}

remove() inherited#

Removes value from the set.

Returns true if value was in the set, and false if not. The method has no effect if value was not in the set.

final characters = <String>{'A', 'B', 'C'};
final didRemoveB = characters.remove('B'); // true
final didRemoveD = characters.remove('D'); // false
print(characters); // {A, C}

Inherited from Set.

bool remove(Object? value);

removeAll() inherited#

Removes each element of elements from this set.

final characters = <String>{'A', 'B', 'C'};
characters.removeAll({'A', 'B', 'X'});
print(characters); // {C}

Inherited from Set.

void removeAll(Iterable<Object?> elements);

removeWhere() inherited#

Removes all elements of this set that satisfy test.

final characters = <String>{'A', 'B', 'C'};
characters.removeWhere((element) => element.startsWith('B'));
print(characters); // {A, C}

Inherited from Set.

void removeWhere(bool test(E element));

retainAll() inherited#

Removes all elements of this set that are not elements in elements.

Checks for each element of elements whether there is an element in this set that is equal to it (according to this.contains), and if so, the equal element in this set is retained, and elements that are not equal to any element in elements are removed.

final characters = <String>{'A', 'B', 'C'};
characters.retainAll({'A', 'B', 'X'});
print(characters); // {A, B}

Inherited from Set.

void retainAll(Iterable<Object?> elements);

retainWhere() inherited#

Removes all elements of this set that fail to satisfy test.

final characters = <String>{'A', 'B', 'C'};
characters.retainWhere(
    (element) => element.startsWith('B') || element.startsWith('C'));
print(characters); // {B, C}

Inherited from Set.

void retainWhere(bool test(E element));

singleWhere() inherited#

The single element that satisfies test.

Checks elements to see if test(element) returns true. If exactly one element satisfies test, that element is returned. If more than one matching element is found, throws StateError. If no matching element is found, returns the result of orElse. If orElse is omitted, it defaults to throwing a StateError.

Example:

final numbers = <int>[2, 2, 10];
var result = numbers.singleWhere((element) => element > 5); // 10

When no matching element is found, the result of calling orElse is returned instead.

result = numbers.singleWhere((element) => element == 1,
    orElse: () => -1); // -1

There must not be more than one matching element.

result = numbers.singleWhere((element) => element == 2); // Throws Error.

Inherited from Iterable.

E singleWhere(bool test(E element), {E orElse()?}) {
  var iterator = this.iterator;
  E result;
  do {
    if (!iterator.moveNext()) {
      if (orElse != null) return orElse();
      throw IterableElementError.noElement();
    }
    result = iterator.current;
  } while (!test(result));
  while (iterator.moveNext()) {
    if (test(iterator.current)) throw IterableElementError.tooMany();
  }
  return result;
}

skip() inherited#

Creates an Iterable that provides all but the first count elements.

When the returned iterable is iterated, it starts iterating over this, first skipping past the initial count elements. If this has fewer than count elements, then the resulting Iterable is empty. After that, the remaining elements are iterated in the same order as in this iterable.

Some iterables may be able to find later elements without first iterating through earlier elements, for example when iterating a List. Such iterables are allowed to ignore the initial skipped elements.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
final result = numbers.skip(4); // (6, 7)
final skipAll = numbers.skip(100); // () - no elements.

The count must not be negative.

Inherited from Iterable.

Iterable<E> skip(int count) => SkipIterable<E>(this, count);

skipWhile() inherited#

Creates an Iterable that skips leading elements while test is satisfied.

The filtering happens lazily. Every new Iterator of the returned iterable iterates over all elements of this.

The returned iterable provides elements by iterating this iterable, but skipping over all initial elements where test(element) returns true. If all elements satisfy test the resulting iterable is empty, otherwise it iterates the remaining elements in their original order, starting with the first element for which test(element) returns false.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.skipWhile((x) => x < 5); // (5, 6, 7)
result = numbers.skipWhile((x) => x != 3); // (3, 5, 6, 7)
result = numbers.skipWhile((x) => x != 4); // ()
result = numbers.skipWhile((x) => x.isOdd); // (2, 3, 5, 6, 7)

Inherited from Iterable.

Iterable<E> skipWhile(bool test(E value)) => SkipWhileIterable<E>(this, test);

take() inherited#

Creates a lazy iterable of the count first elements of this iterable.

The returned Iterable may contain fewer than count elements, if this contains fewer than count elements.

The elements can be computed by stepping through iterator until count elements have been seen.

The count must not be negative.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
final result = numbers.take(4); // (1, 2, 3, 5)
final takeAll = numbers.take(100); // (1, 2, 3, 5, 6, 7)

Inherited from Iterable.

Iterable<E> take(int count) => TakeIterable<E>(this, count);

takeWhile() inherited#

Creates a lazy iterable of the leading elements satisfying test.

The filtering happens lazily. Every new iterator of the returned iterable starts iterating over the elements of this.

The elements can be computed by stepping through iterator until an element is found where test(element) is false. At that point, the returned iterable stops (its moveNext() returns false).

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.takeWhile((x) => x < 5); // (1, 2, 3)
result = numbers.takeWhile((x) => x != 3); // (1, 2)
result = numbers.takeWhile((x) => x != 4); // (1, 2, 3, 5, 6, 7)
result = numbers.takeWhile((x) => x.isOdd); // (1)

Inherited from Iterable.

Iterable<E> takeWhile(bool test(E value)) => TakeWhileIterable<E>(this, test);

toList() inherited#

Creates a List containing the elements of this Iterable.

The elements are in iteration order. The list is fixed-length if growable is false.

Example:

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Mars'};
final keysList = planets.keys.toList(growable: false); // [1, 2, 3]
final valuesList =
    planets.values.toList(growable: false); // [Mercury, Venus, Mars]

Inherited from Iterable.

List<E> toList({bool growable = true}) =>
    List<E>.of(this, growable: growable);

toSet() inherited#

Creates a Set with the same elements and behavior as this Set.

The returned set behaves the same as this set with regard to adding and removing elements. It initially contains the same elements. If this set specifies an ordering of the elements, the returned set will have the same order.

Inherited from Set.

Set<E> toSet();

toString() inherited#

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Object.

external String toString();

union() inherited#

Creates a new set which contains all the elements of this set and other.

That is, the returned set contains all the elements of this Set and all the elements of other.

final characters1 = <String>{'A', 'B', 'C'};
final characters2 = <String>{'A', 'E', 'F'};
final unionSet1 = characters1.union(characters2);
print(unionSet1); // {A, B, C, E, F}
final unionSet2 = characters2.union(characters1);
print(unionSet2); // {A, E, F, B, C}

Inherited from Set.

Set<E> union(Set<E> other);

where() inherited#

Creates a new lazy Iterable with all elements that satisfy the predicate test.

The matching elements have the same order in the returned iterable as they have in iterator.

This method returns a view of the mapped elements. As long as the returned Iterable is not iterated over, the supplied function test will not be invoked. Iterating will not cache results, and thus iterating multiple times over the returned Iterable may invoke the supplied function test multiple times on the same element.

Example:

final numbers = <int>[1, 2, 3, 5, 6, 7];
var result = numbers.where((x) => x < 5); // (1, 2, 3)
result = numbers.where((x) => x > 5); // (6, 7)
result = numbers.where((x) => x.isEven); // (2, 6)

Inherited from Iterable.

Iterable<E> where(bool test(E element)) => WhereIterable<E>(this, test);

whereType() inherited#

Creates a new lazy Iterable with all elements that have type T.

The matching elements have the same order in the returned iterable as they have in iterator.

This method returns a view of the mapped elements. Iterating will not cache results, and thus iterating multiple times over the returned Iterable may yield different results, if the underlying elements change between iterations.

Inherited from Iterable.

Iterable<T> whereType<T>() => WhereTypeIterable<T>(this);

Operators#

operator ==() inherited#

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

external bool operator ==(Object other);