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# filterMap

> 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="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>[ <Complex128>[ 8.0, 0.0 ], <Complex128>[ 9.0, 0.0 ], <Complex128>[ 10.0, 0.0 ] ]
    ```

-   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="links">

[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/ndarray/tree/main/ctor

[@stdlib/ndarray/dtypes]: https://github.com/stdlib-js/ndarray/tree/main/dtypes

[@stdlib/ndarray/orders]: https://github.com/stdlib-js/ndarray/tree/main/orders

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

[@stdlib/ndarray/filter]: https://github.com/stdlib-js/ndarray/tree/main/filter

[@stdlib/ndarray/map]: https://github.com/stdlib-js/ndarray/tree/main/map

[@stdlib/ndarray/reject]: https://github.com/stdlib-js/ndarray/tree/main/reject

[@stdlib/ndarray/slice]: https://github.com/stdlib-js/ndarray/tree/main/slice

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

</section>

<!-- /.links -->
