npm package: @stdlib/ndarray-filter
Description: Return a shallow copy of an ndarray containing only those elements which pass a test implemented by a predicate function.
Author: The Stdlib Authors
Latest version: 0.1.1

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# filter

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Return a shallow copy of an [ndarray][@stdlib/ndarray/ctor] containing only those elements which pass a test implemented by a predicate function.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/ndarray-filter
```

</section>

<section class="usage">

## Usage

```javascript
var filter = require( '@stdlib/ndarray-filter' );
```

#### filter( x\[, options], predicate\[, thisArg] )

Returns a shallow copy of an [ndarray][@stdlib/ndarray/ctor] containing only those elements which pass a test implemented by a `predicate` function.

<!-- eslint-disable max-len -->

```javascript
var Float64Array = require( '@stdlib/array-float64' );
var ndarray = require( '@stdlib/ndarray-ctor' );

function predicate( z ) {
    return z > 6.0;
}

var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;

var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns <ndarray>

var y = filter( x, predicate );
// returns <ndarray>[ 8.0, 9.0, 10.0 ]
```

The function accepts the following arguments:

-   **x**: input [ndarray][@stdlib/ndarray/ctor].
-   **options**: function options _(optional)_.
-   **predicate**: predicate function.
-   **thisArg**: predicate function execution context _(optional)_.

The function accepts the following options:

-   **dtype**: output ndarray [data type][@stdlib/ndarray/dtypes]. If not specified, the output ndarray [data type][@stdlib/ndarray/dtypes] is inferred from the input [ndarray][@stdlib/ndarray/ctor].
-   **order**: index iteration order. By default, the function iterates over elements according to the [layout order][@stdlib/ndarray/orders] of the provided [ndarray][@stdlib/ndarray/ctor]. Accordingly, for row-major input [ndarrays][@stdlib/ndarray/ctor], the last dimension indices increment fastest. For column-major input [ndarrays][@stdlib/ndarray/ctor], the first dimension indices increment fastest. To override the inferred order and ensure that indices increment in a specific manner, regardless of the input [ndarray][@stdlib/ndarray/ctor]'s layout order, explicitly set the iteration order. Note, however, that iterating according to an order which does not match that of the input [ndarray][@stdlib/ndarray/ctor] may, in some circumstances, result in performance degradation due to cache misses. Must be either `'row-major'` or `'column-major'`.

By default, the output ndarray [data type][@stdlib/ndarray/dtypes] is inferred from the input [ndarray][@stdlib/ndarray/ctor]. To return an ndarray with a different [data type][@stdlib/ndarray/dtypes], specify the `dtype` option.

<!-- eslint-disable max-len -->

```javascript
var Float64Array = require( '@stdlib/array-float64' );
var ndarray = require( '@stdlib/ndarray-ctor' );
var dtype = require( '@stdlib/ndarray-dtype' );

function predicate( z ) {
    return z > 6.0;
}

var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;

var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns <ndarray>

var opts = {
    'dtype': 'float32'
};
var y = filter( x, opts, predicate );
// returns <ndarray>[ 8.0, 9.0, 10.0 ]

var dt = dtype( y );
// returns 'float32'
```

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

<!-- eslint-disable no-invalid-this, max-len -->

```javascript
var Float64Array = require( '@stdlib/array-float64' );
var ndarray = require( '@stdlib/ndarray-ctor' );

function predicate( z ) {
    this.count += 1;
    return z > 6.0;
}

var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;

var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns <ndarray>

var ctx = {
    'count': 0
};
var y = filter( x, predicate, ctx );
// returns <ndarray>[ 8.0, 9.0, 10.0 ]

var count = ctx.count;
// returns 6
```

The `predicate` function is provided the following arguments:

-   **value**: current array element.
-   **indices**: current array element indices.
-   **arr**: the input [ndarray][@stdlib/ndarray/ctor].

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   The function does **not** perform explicit casting (e.g., from a real-valued floating-point number to a complex floating-point number). Any such casting should be performed **prior to** calling this function.
-   The function **always** returns a one-dimensional [ndarray][@stdlib/ndarray/ctor].

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var naryFunction = require( '@stdlib/utils-nary-function' );
var array = require( '@stdlib/ndarray-array' );
var isPositive = require( '@stdlib/assert-is-positive-number' ).isPrimitive;
var filter = require( '@stdlib/ndarray-filter' );

var buffer = discreteUniform( 10, -100, 100, {
    'dtype': 'generic'
});
var x = array( buffer, {
    'shape': [ 5, 2 ],
    'dtype': 'generic'
});
console.log( ndarray2array( x ) );

var y = filter( x, naryFunction( isPositive, 1 ) );
console.log( ndarray2array( y ) );
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/ndarray-filter-map`][@stdlib/ndarray/filter-map]</span><span class="delimiter">: </span><span class="description">filter and map elements in an input ndarray to elements in a new output ndarray according to a callback function.</span>
-   <span class="package-name">[`@stdlib/ndarray-map`][@stdlib/ndarray/map]</span><span class="delimiter">: </span><span class="description">apply a callback to elements in an input ndarray and assign results to elements in a new output ndarray.</span>
-   <span class="package-name">[`@stdlib/ndarray-reject`][@stdlib/ndarray/reject]</span><span class="delimiter">: </span><span class="description">return a shallow copy of an ndarray containing only those elements which fail a test implemented by a predicate function.</span>
-   <span class="package-name">[`@stdlib/ndarray-slice`][@stdlib/ndarray/slice]</span><span class="delimiter">: </span><span class="description">return a read-only view of an input ndarray.</span>

</section>

<!-- /.related -->


<section class="main-repo" >

* * *

## Notice

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].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

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[@stdlib/ndarray/ctor]: https://www.npmjs.com/package/@stdlib/ndarray-ctor

[@stdlib/ndarray/dtypes]: https://www.npmjs.com/package/@stdlib/ndarray-dtypes

[@stdlib/ndarray/orders]: https://www.npmjs.com/package/@stdlib/ndarray-orders

<!-- <related-links> -->

[@stdlib/ndarray/filter-map]: https://www.npmjs.com/package/@stdlib/ndarray-filter-map

[@stdlib/ndarray/map]: https://www.npmjs.com/package/@stdlib/ndarray-map

[@stdlib/ndarray/reject]: https://www.npmjs.com/package/@stdlib/ndarray-reject

[@stdlib/ndarray/slice]: https://www.npmjs.com/package/@stdlib/ndarray-slice

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