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# iterFilter

> Create an [iterator][mdn-iterator-protocol] which filters the values of another [iterator][mdn-iterator-protocol] according to a predicate function.

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</section>

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<section class="usage">

## Usage

```javascript
var iterFilter = require( '@stdlib/iter/filter' );
```

#### iterFilter( iterator, predicate\[, thisArg] )

Returns an [iterator][mdn-iterator-protocol] which filters the values of another [`iterator`][mdn-iterator-protocol] according to a `predicate` function.

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

function predicate( v ) {
    return ( v > 2 );
}

var it = iterFilter( array2iterator( [ 1, 3, 2, 4 ] ), predicate );
// returns <Object>

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

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

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.

The `predicate` function is provided two arguments:

-   `value`: iterated value
-   `index`: iteration index (zero-based)

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

function predicate( v, i ) {
    return ( i >= 2 );
}

var it = iterFilter( array2iterator( [ 3, 4, 1, 2 ] ), predicate );
// returns <Object>

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

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

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

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

<!-- eslint-disable no-invalid-this -->

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

function predicate( v ) {
    this.count += 1;
    return ( v > 0 );
}

var ctx = {
    'count': 0
};

var it = iterFilter( array2iterator( [ 1, 2, 3, 4 ] ), predicate, ctx );
// returns <Object>

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

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

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

var count = ctx.count;
// returns 3
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

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## Notes

-   If an environment supports `Symbol.iterator` **and** a provided [iterator][mdn-iterator-protocol] is iterable, the returned [iterator][mdn-iterator-protocol] is iterable.

</section>

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## Examples

<!-- eslint no-undef: "error" -->

```javascript
var randu = require( '@stdlib/random/iter/randu' );
var iterFilter = require( '@stdlib/iter/filter' );

function predicate( v ) {
    return ( v > 0.5 );
}

// Create a seeded iterator for generating pseudorandom numbers:
var rand = randu({
    'seed': 1234,
    'iter': 20
});

// Create an iterator which filters the generated numbers:
var it = iterFilter( rand, predicate );

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

</section>

<!-- /.examples -->

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</section>

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[mdn-iterator-protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterator_protocol

</section>

<!-- /.links -->
