@stdlib/utils/map-reduce/README.md

Standard utilities.

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# mapReduce

> Perform a single-pass map-reduce operation against each element in an array and return the accumulated result.

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## Usage

```javascript
var mapReduce = require( '@stdlib/utils/map-reduce' );
```

#### mapReduce( arr, initial, mapper, reducer\[, thisArg ] )

Performs a map-reduce operation against each element in an array and returns the accumulated result.

```javascript
function square( value ) {
    return value * value;
}

function sum( accumulator, value ) {
    return accumulator + value;
}

var arr = [ 1, 2, 3, 4 ];

var out = mapReduce( arr, 0, square, sum );
// returns 30
```

The function accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.

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

function square( value ) {
    return value * value;
}

function sum( accumulator, value ) {
    return accumulator + value;
}

var opts = {
    'dtype': 'generic'
};
var arr = array( [ [ 1, 2, 3 ], [ 4, 5, 6 ] ], opts );

var out = mapReduce( arr, 0, square, sum );
// returns 91
```

The mapping function is provided the following arguments:

-   **value**: array element.
-   **index**: element index.
-   **arr**: input array.

The reducing function is provided the following arguments:

-   **accumulator**: accumulated value.
-   **value**: result of applying the mapping function to the current array element.
-   **index**: element index.
-   **arr**: input array.

To set the `this` context when invoking the reducing function, provide a `thisArg`.

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

```javascript
function square( value ) {
    return value * value;
}

function sum( accumulator, value ) {
    this.count += 1;
    return accumulator + value;
}

var arr = [ 1, 2, 3, 4 ];

var ctx = {
    'count': 0
};

var out = mapReduce( arr, 0, square, sum, ctx );
// returns 30

var mean = out / ctx.count;
// returns 7.5
```

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## Notes

-   The function supports array-like objects exposing getters and setters for array element access (e.g., [`Complex64Array`][@stdlib/array/complex64], [`Complex128Array`][@stdlib/array/complex128], etc).

    ```javascript
    var Complex64Array = require( '@stdlib/array/complex64' );
    var Complex64 = require( '@stdlib/complex/float32/ctor' );
    var cceil = require( '@stdlib/math/base/special/cceil' );
    var realf = require( '@stdlib/complex/float32/real' );
    var imagf = require( '@stdlib/complex/float32/imag' );

    function sum( acc, z ) {
        var re1 = realf( acc );
        var im1 = imagf( acc );
        var re2 = realf( z );
        var im2 = imagf( z );
        return new Complex64( re1+re2, im1+im2 );
    }

    var x = new Complex64Array( [ 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5 ] );

    var v = mapReduce( x, new Complex64( 0.0, 0.0 ), cceil, sum );
    // returns <Complex64>

    var re = realf( v );
    // returns 20.0

    var im = imagf( v );
    // returns 24.0
    ```

-   For [`ndarray`][@stdlib/ndarray/ctor]-like objects, the function performs a single-pass map-reduce operation over the entire input [`ndarray`][@stdlib/ndarray/ctor] (i.e., higher-order [`ndarray`][@stdlib/ndarray/ctor] dimensions are flattened to a single-dimension).

-   When applying a function to [`ndarray`][@stdlib/ndarray/ctor]-like objects, performance will be best for [`ndarray`][@stdlib/ndarray/ctor]-like objects which are single-segment contiguous.

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## Examples

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

```javascript
var filledarrayBy = require( '@stdlib/array/filled-by' );
var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
var naryFunction = require( '@stdlib/utils/nary-function' );
var add = require( '@stdlib/math/base/ops/add' );
var abs = require( '@stdlib/math/base/special/abs' );
var array = require( '@stdlib/ndarray/array' );
var mapReduce = require( '@stdlib/utils/map-reduce' );

function fill( i ) {
    var rand = discreteUniform( -10*(i+1), 10*(i+1) );
    return filledarrayBy( 10, 'generic', rand );
}

// Create a two-dimensional ndarray (i.e., a matrix):
var x = array( filledarrayBy( 10, 'generic', fill ), {
    'dtype': 'generic',
    'flatten': true
});

// Create an explicit unary function:
var f1 = naryFunction( abs, 1 );

// Create an explicit binary function:
var f2 = naryFunction( add, 2 );

// Compute the sum of absolute values:
var out = mapReduce( x, 0, f1, f2 );

console.log( 'x:' );
console.log( x.data );

console.log( 'sum: %d', out );
```

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* * *

## See Also

-   <span class="package-name">[`@stdlib/utils/map`][@stdlib/utils/map]</span><span class="delimiter">: </span><span class="description">apply a function to each element in an array and assign the result to an element in an output array.</span>
-   <span class="package-name">[`@stdlib/utils/map-reduce-right`][@stdlib/utils/map-reduce-right]</span><span class="delimiter">: </span><span class="description">perform a single-pass map-reduce operation against each element in an array while iterating from right to left and return the accumulated result.</span>
-   <span class="package-name">[`@stdlib/utils/reduce`][@stdlib/utils/reduce]</span><span class="delimiter">: </span><span class="description">apply a function against an accumulator and each element in an array and return the accumulated result.</span>

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

[@stdlib/array/complex64]: https://www.npmjs.com/package/@stdlib/array-complex64

[@stdlib/array/complex128]: https://www.npmjs.com/package/@stdlib/array-complex128

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

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

[@stdlib/utils/map-reduce-right]: https://github.com/stdlib-js/utils/tree/main/map-reduce-right

[@stdlib/utils/reduce]: https://github.com/stdlib-js/utils/tree/main/reduce

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</section>

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