Create an iterator which returns unique values according to a predicate function.
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> Create an [iterator][mdn-iterator-protocol] which returns unique values according to a predicate function.
``bash`
npm install @stdlib/iter-unique-by
`javascript`
var iterUniqueBy = require( '@stdlib/iter-unique-by' );
#### iterUniqueBy( iterator, predicate\[, thisArg] )
Returns an [iterator][mdn-iterator-protocol] which returns unique values according to a predicate function.
`javascript
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
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][mdn-iterator-protocol] protocol-compliant object has the following properties:
- next: function which returns an [iterator][mdn-iterator-protocol] 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][mdn-iterator-protocol] is finished.
- return: function which closes an [iterator][mdn-iterator-protocol] and returns a single (optional) argument in an [iterator][mdn-iterator-protocol] 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.
`javascript
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
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
`
- A returned [iterator][mdn-iterator-protocol] internally buffers unique values and, thus, has O(N) memory requirements.predicate
- A 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.predicate
- An iterated value is considered "unique" if the function returns truthy values for all comparisons of the iterated value with each value in the internal buffer.Symbol.iterator
- If an environment supports and a provided [iterator][mdn-iterator-protocol] is iterable, the returned [iterator][mdn-iterator-protocol] is iterable.
`javascript
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 );
}
`
*
- [@stdlib/iter-unique][@stdlib/iter/unique]: create an iterator which returns unique values.
- [@stdlib/iter-unique-by-hash][@stdlib/iter/unique-by-hash]: create an iterator which returns unique values according to a hash function.
*
This package is part of [stdlib][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][stdlib], see the main project [repository][stdlib].
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---
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[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules
[deno-url]: https://github.com/stdlib-js/iter-unique-by/tree/deno
[deno-readme]: https://github.com/stdlib-js/iter-unique-by/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/iter-unique-by/tree/umd
[umd-readme]: https://github.com/stdlib-js/iter-unique-by/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/iter-unique-by/tree/esm
[esm-readme]: https://github.com/stdlib-js/iter-unique-by/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/iter-unique-by/blob/main/branches.md
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[mdn-iterator-protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterator_protocol
[@stdlib/iter/unique]: https://www.npmjs.com/package/@stdlib/iter-unique
[@stdlib/iter/unique-by-hash]: https://www.npmjs.com/package/@stdlib/iter-unique-by-hash