npm package: @stdlib/array-base-group-indices-by
Description: Group element indices according to an indicator function.
Author: The Stdlib Authors
Latest version: 0.2.4

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# groupIndicesBy

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> Group element indices according to an indicator function.

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<section class="intro">

</section>

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<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/array-base-group-indices-by
```

</section>

<section class="usage">

## Usage

```javascript
var groupIndicesBy = require( '@stdlib/array-base-group-indices-by' );
```

#### groupIndicesBy( x, indicator\[, thisArg] )

Groups element indices according to an indicator function.

```javascript
function indicator( v ) {
    return v[ 0 ];
}

var x = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupIndicesBy( x, indicator );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }
```

An `indicator` function is provided the following arguments:

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

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

```javascript
function indicator( v ) {
    this.count += 1;
    return v[ 0 ];
}

var x = [ 'beep', 'boop', 'foo', 'bar' ];

var context = {
    'count': 0
};
var out = groupIndicesBy( x, indicator, context );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }

var cnt = context.count;
// returns 4
```

</section>

<!-- /.usage -->

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

<section class="notes">

## Notes

-   The value returned by an `indicator` function should be a value which can be serialized as an object key. As a counterexample,

    ```javascript
    function indicator( v ) {
        return {};
    }
    var x = [ 'beep', 'boop', 'foo', 'bar' ];

    var out = groupIndicesBy( x, indicator );
    // returns { '[object Object]': [ 0, 1, 2, 3 ] }
    ```

    while each group identifier is unique, all input array elements resolve to the same group because each group identifier serializes to the same string.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var take = require( '@stdlib/array-base-take-indexed' );
var groupIndicesBy = require( '@stdlib/array-base-group-indices-by' );

function indicator( v ) {
    // Use the first letter of each element to define groups:
    return v[ 0 ];
}

// Define an initial array of values:
var values = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];

// Sample from the initial array to generate a random collection:
var indices = discreteUniform( 100, 0, values.length-1, {
    'dtype': 'generic'
});
var x = take( values, indices );
// returns [...]

// Group the values:
var out = groupIndicesBy( x, indicator );
// returns {...}

console.log( out );
```

</section>

<!-- /.examples -->

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* * *

## 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].

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---

## License

See [LICENSE][stdlib-license].


## Copyright

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

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