npm package: @stdlib/ndarray-filter-map
Description: Filter and map elements in an input ndarray to elements in a new output ndarray according to a callback function.
Author: The Stdlib Authors
Latest version: 0.1.1

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<details>
  <summary>
    About stdlib...
  </summary>
  <p>We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.</p>
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</details>

# filterMap

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

> Filter and map elements in an input [ndarray][@stdlib/ndarray/ctor] to elements in a new output [ndarray][@stdlib/ndarray/ctor] according to a callback function.

<section class="intro">

</section>

<!-- /.intro -->

<section class="installation">

## Installation

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

</section>

<section class="usage">

## Usage

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

#### filterMap( x\[, options], fcn\[, thisArg] )

Filters and maps elements in an input [ndarray][@stdlib/ndarray/ctor] to elements in a new output [ndarray][@stdlib/ndarray/ctor] according to a callback function.

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

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

function fcn( z ) {
    if ( z > 5.0 ) {
        return z * 10.0;
    }
}

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 = filterMap( x, fcn );
// returns <ndarray>[ 80.0, 90.0, 100.0 ]
```

The function accepts the following arguments:

-   **x**: input [ndarray][@stdlib/ndarray/ctor].
-   **options**: function options _(optional)_.
-   **fcn**: callback function.
-   **thisArg**: callback 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 fcn( z ) {
    if ( z > 5.0 ) {
        return z * 10.0;
    }
}

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 = filterMap( x, opts, fcn );
// returns <ndarray>[ 80.0, 90.0, 100.0 ]

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

To set the callback 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 fcn( z ) {
    this.count += 1;
    if ( z > 5.0 ) {
        return z * 10.0;
    }
}

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 = filterMap( x, fcn, ctx );
// returns <ndarray>[ 80.0, 90.0, 100.0 ]

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

The callback 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 by a provided callback function.

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

    ```javascript
    var Float64Array = require( '@stdlib/array-float64' );
    var ndarray = require( '@stdlib/ndarray-ctor' );
    var Complex128 = require( '@stdlib/complex-float64-ctor' );

    function fcn( z ) {
        if ( z > 5.0 ) {
            return new Complex128( z, 0.0 );
        }
    }

    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': 'complex128'
    };
    var y = filterMap( x, opts, fcn );
    // returns <ndarray>
    ```

-   If a provided callback function returns `undefined`, the function skips the respective [ndarray][@stdlib/ndarray/ctor] element. If the callback function returns a value other than `undefined`, the function stores the callback's return value in the output [ndarray][@stdlib/ndarray/ctor].

-   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 array = require( '@stdlib/ndarray-array' );
var filterMap = require( '@stdlib/ndarray-filter-map' );

function fcn( v ) {
    if ( v > 0 ) {
        return v * 100;
    }
}

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

var y = filterMap( x, fcn );
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`][@stdlib/ndarray/filter]</span><span class="delimiter">: </span><span class="description">return a shallow copy of an ndarray containing only those elements which pass a test implemented by a predicate 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>

<!-- /.stdlib -->

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<section class="links">

[npm-image]: http://img.shields.io/npm/v/@stdlib/ndarray-filter-map.svg
[npm-url]: https://npmjs.org/package/@stdlib/ndarray-filter-map

[test-image]: https://github.com/stdlib-js/ndarray-filter-map/actions/workflows/test.yml/badge.svg?branch=v0.1.1
[test-url]: https://github.com/stdlib-js/ndarray-filter-map/actions/workflows/test.yml?query=branch:v0.1.1

[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/ndarray-filter-map/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/ndarray-filter-map?branch=main

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[stdlib]: https://github.com/stdlib-js/stdlib

[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors

[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/ndarray-filter-map/tree/deno
[deno-readme]: https://github.com/stdlib-js/ndarray-filter-map/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/ndarray-filter-map/tree/umd
[umd-readme]: https://github.com/stdlib-js/ndarray-filter-map/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/ndarray-filter-map/tree/esm
[esm-readme]: https://github.com/stdlib-js/ndarray-filter-map/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/ndarray-filter-map/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/ndarray-filter-map/main/LICENSE

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

[@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

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

</section>

<!-- /.links -->