Distinct

suppress duplicate items emitted by an Observable

The Distinct operator filters an Observable by only allowing items through that have not already been emitted.

In some implementations there are variants that allow you to adjust the criteria by which two items are considered “distinct.” In some, there is a variant of the operator that only compares an item against its immediate predecessor for distinctness, thereby filtering only consecutive duplicate items from the sequence.

See Also

Language-Specific Information:

distinct

RxGroovy implements this operator as distinct.

distinct

There is also a variant of this operator that takes a function as a parameter. This function operates on items emitted by the source Observable to generate a “key.” It is these keys, then, and not the items themselves, that distinct will compare to determine whether or not two items are distinct.

distinctUntilChanged

RxGroovy also implements the distinctUntilChanged operator. It only compares emitted items from the source Observable against their immediate predecessors in order to determine whether or not they are distinct.

distinctUntilChanged

As with distinct, there is also a version of distinctUntilChanged that accepts a key selector function and that uses the resulting key to determine whether or not two adjacently-emitted items are distinct.

distinct and distinctUntilChanged do not by default operate on any particular Scheduler.

distinct

RxJava implements this operator as distinct.

Sample Code

Observable.just(1, 2, 1, 1, 2, 3)
          .distinct()
          .subscribe(new Subscriber<Integer>() {
        @Override
        public void onNext(Integer item) {
            System.out.println("Next: " + item);
        }

        @Override
        public void onError(Throwable error) {
            System.err.println("Error: " + error.getMessage());
        }

        @Override
        public void onCompleted() {
            System.out.println("Sequence complete.");
        }
    });
Next: 1
Next: 2
Next: 3
Sequence complete.
distinct

There is also a variant of this operator that takes a function as a parameter. This function operates on items emitted by the source Observable to generate a “key.” It is these keys, then, and not the items themselves, that distinct will compare to determine whether or not two items are distinct.

distinctUntilChanged

RxJava also implements the distinctUntilChanged operator. It only compares emitted items from the source Observable against their immediate predecessors in order to determine whether or not they are distinct.

distinctUntilChanged

As with distinct, there is also a version of distinctUntilChanged that accepts a key selector function and that uses the resulting key to determine whether or not two adjacently-emitted items are distinct.

distinct and distinctUntilChanged do not by default operate on any particular Scheduler.

distinct

In RxJS, the distinct operator has two optional parameters:

  1. a function that accepts an item emitted by the source Observable and returns a key which will be used instead of the item itself when comparing two items for distinctness
  2. a function that accepts two items (or two keys) and compares them for distinctness, returning false if they are distinct (an equality function is the default if you do not supply your own function here)

Sample Code

/* Without key selector */
var source = Rx.Observable.fromArray([
        42, 24, 42, 24
    ])
    .distinct();

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x.toString()); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Next: 42
Next: 24
Completed
/* With key selector */
var source = Rx.Observable.fromArray([
        {value: 42}, {value: 24}, {value: 42}, {value: 24}
    ])
    .distinct(function (x) { return x.value; });

var subscription = source.subscribe(
    function (x) {
        console.log('Next: ' + x.toString());
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    });
Next: { value: 42 }
Next: { value: 24 }
Completed
distinctUntilChanged

RxJS also has a distinctUntilChanged operator. It only compares emitted items from the source Observable against their immediate predecessors in order to determine whether or not they are distinct. It takes the same two optional parameters as the distinct operator.

Sample Code

/* Without key selector */
var source = Rx.Observable.fromArray([
        24, 42, 24, 24
    ])
    .distinctUntilChanged();

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Next: 24
Next: 42
Next: 24
Completed
/* With key selector */
var source = Rx.Observable.fromArray([
        {value: 24}, {value: 42}, {value: 42}, {value: 24}
    ])
    .distinctUntilChanged(function (x) { return x.value; });

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x.toString()); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Next: { value: 24 }
Next: { value: 42 }
Next: { value: 24 }
Completed

distinct and distinctUntilChanged are found in each of the following distributions:

  • rx.js
  • rx.all.js
  • rx.all.compat.js
  • rx.compat.js
  • rx.lite.js
  • rx.lite.compat.js

RxPHP implements this operator as distinct.

Returns an observable sequence that contains only distinct elements according to the keySelector and the comparer. Usage of this operator should be considered carefully due to the maintenance of an internal lookup structure which can grow large.

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/distinct/distinct.php

$source = \Rx\Observable::fromArray([
    42, 24, 42, 24
])->distinct();

$subscription = $source->subscribe($stdoutObserver);

   
Next value: 42
Next value: 24
Complete!
    

RxPHP also has an operator distinctKey.

Variant of distinct that takes a key selector

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/distinct/distinctKey.php

$source = \Rx\Observable::fromArray([
    ['id' => '42'],
    ['id' => '24'],
    ['id' => '42'],
    ['id' => '24']
])
    ->distinctKey(function ($x) {
        return $x['id'];
    })
    ->map(function ($x) {
        return $x['id'];
    });

$subscription = $source->subscribe($stdoutObserver);

   
Next value: 42
Next value: 24
Complete!
    

RxPHP also has an operator distinctUntilChanged.

A variant of distinct that only compares emitted items from the source Observable against their immediate predecessors in order to determine whether or not they are distinct.

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/distinct/distinctUntilChanged.php

$source = \Rx\Observable::fromArray([
    24, 42, 24, 24
])->distinctUntilChanged();

$subscription = $source->subscribe($stdoutObserver);

   
Next value: 24
Next value: 42
Next value: 24
Complete!
    

RxPHP also has an operator distinctUntilKeyChanged.

Variant of distinctUntilChanged that takes a key selector and the comparer.

Sample Code

//from https://github.com/ReactiveX/RxPHP/blob/master/demo/distinct/distinctUntilKeyChanged.php

$source = \Rx\Observable::fromArray([
    ['id' => '24'],
    ['id' => '42'],
    ['id' => '24'],
    ['id' => '24']
])
    ->distinctUntilKeyChanged(function ($x) {
        return $x['id'];
    })
    ->map(function ($x) {
        return $x['id'];
    });

$subscription = $source->subscribe($stdoutObserver);

   
Next value: 24
Next value: 42
Next value: 24
Complete!