# @stdlib/iter-unique-by

0.2.1 • Public • Published

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# iterUniqueBy

Create an iterator which returns unique values according to a predicate function.

## Installation

`npm install @stdlib/iter-unique-by`

## Usage

`var iterUniqueBy = require( '@stdlib/iter-unique-by' );`

#### iterUniqueBy( iterator, predicate[, thisArg] )

Returns an iterator which returns unique values according to a `predicate` function.

```var array2iterator = require( '@stdlib/array-to-iterator' );

function predicate( a, b ) {
return ( a !== b );
}

var src = array2iterator( [ 2, 1, 1, 2, 4, 3, 4, 3 ] );

var it = iterUniqueBy( src, predicate );
// returns <Object>

var v = it.next().value;
// returns 2

v = it.next().value;
// returns 1

v = it.next().value;
// returns 4

v = it.next().value;
// returns 3

var bool = it.next().done;
// returns true```

The returned iterator protocol-compliant object has the following properties:

• next: function which returns an iterator protocol-compliant object containing the next iterated value (if one exists) assigned to a `value` property and a `done` property having a `boolean` value indicating whether the iterator is finished.
• return: function which closes an iterator and returns a single (optional) argument in an iterator protocol-compliant object.

A `predicate` function is provided two arguments:

• a: a previously identified unique value
• b: the value whose uniqueness is being determined

To set the execution context of the `predicate` function, provide a `thisArg`.

```var array2iterator = require( '@stdlib/array-to-iterator' );

function predicate( a, b ) {
this.count += 1;
return ( a.v !== b.v );
}

var values = [
{ 'v': 2 },
{ 'v': 1 },
{ 'v': 1 },
{ 'v': 2 },
{ 'v': 4 },
{ 'v': 3 },
{ 'v': 4 },
{ 'v': 3 }
];

var src = array2iterator( values );

var ctx = {
'count': 0
};

var it = iterUniqueBy( src, predicate, ctx );
// returns <Object>

var v = it.next().value;
// returns { 'v': 2 }

v = it.next().value;
// returns { 'v': 1 }

v = it.next().value;
// returns { 'v': 4 }

v = it.next().value;
// returns { 'v': 3 }

var bool = it.next().done;
// returns true

bool = ( ctx.count > 0 );
// returns true```

## Notes

• A returned iterator internally buffers unique values and, thus, has `O(N)` memory requirements.
• A `predicate` function is invoked for each iterated value against each value in an internal buffer consisting of previously identified unique values. Thus, as the number of unique values grows, so, too, does the number of `predicate` function invocations per iterated value.
• An iterated value is considered "unique" if the `predicate` function returns truthy values for all comparisons of the iterated value with each value in the internal buffer.
• If an environment supports `Symbol.iterator` and a provided iterator is iterable, the returned iterator is iterable.

## Examples

```var discreteUniform = require( '@stdlib/random-iter-discrete-uniform' );
var iterUniqueBy = require( '@stdlib/iter-unique-by' );

function predicate( a, b ) {
return ( a !== b );
}

// Create a seeded iterator which can generate 1000 pseudorandom numbers:
var rand = discreteUniform( 1, 10, {
'seed': 1234,
'iter': 1000
});

// Create an iterator which returns unique values:
var it = iterUniqueBy( rand, predicate );

// Perform manual iteration...
var v;
while ( true ) {
v = it.next();
if ( v.done ) {
break;
}
console.log( v.value );
}```

## Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

## Package Sidebar

### Install

`npm i @stdlib/iter-unique-by`

### Repository

github.com/stdlib-js/iter-unique-by

stdlib.io

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0.2.1

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