npm package: @stdlib/utils-tabulate-by
Description: Generate a frequency table according to an indicator function.
Author: The Stdlib Authors
Latest version: 0.2.3

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# tabulateBy

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> Generate a frequency table according to an indicator function.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-tabulate-by
```

</section>

<section class="usage">

## Usage

```javascript
var tabulateBy = require( '@stdlib/utils-tabulate-by' );
```

#### tabulateBy( collection\[, options,] indicator )

Generates a frequency table according to an `indicator` function, i.e., a function which specifies how to categorize an element in the input `collection`.

```javascript
function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'beep' ];

var out = tabulateBy( arr, indicator );
// returns [ [ 'b', 3, 0.75 ], [ 'f', 1, 0.25 ] ]
```

An `indicator` function is provided two arguments:

-   `value`: collection element.
-   `index`: collection index.

```javascript
function indicator( v, i ) {
    console.log( '%d: %s', i, v );
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = tabulateBy( arr, indicator );
// returns [ [ 'b', 3, 0.75 ], [ 'f', 1, 0.25 ] ]
```

The function accepts the following `options`:

-   `thisArg`: execution context.

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

```javascript
function indicator( v ) {
    this.count += 1;
    return v[ 0 ];
}
var context = {
    'count': 0
};
var opts = {
    'thisArg': context
};
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = tabulateBy( arr, opts, indicator );
// returns [ [ 'b', 3, 0.75 ], [ 'f', 1, 0.25 ] ]

console.log( context.count );
// => 4
```

The returned frequency table is an `array` of `arrays`. Each sub-array corresponds to a unique value in the input `collection` and is structured as follows:

-   `0`: unique value.
-   `1`: value count.
-   `2`: frequency percentage.

</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

-   A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random-base-randu' );
var floor = require( '@stdlib/math-base-special-floor' );
var tabulateBy = require( '@stdlib/utils-tabulate-by' );

var vals;
var arr;
var out;
var i;
var j;

function indicator( value ) {
    return value[ 0 ];
}

vals = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];

// Generate a random collection...
arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
    j = floor( randu()*vals.length );
    arr[ i ] = vals[ j ];
}

// Generate a frequency table:
out = tabulateBy( arr, indicator );
console.log( out );
```

</section>

<!-- /.examples -->

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<section class="references">

</section>

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-count-by`][@stdlib/utils/count-by]</span><span class="delimiter">: </span><span class="description">group values according to an indicator function and return group counts.</span>
-   <span class="package-name">[`@stdlib/utils-group-by`][@stdlib/utils/group-by]</span><span class="delimiter">: </span><span class="description">group values according to an indicator function.</span>
-   <span class="package-name">[`@stdlib/utils-tabulate`][@stdlib/utils/tabulate]</span><span class="delimiter">: </span><span class="description">generate a frequency table.</span>

</section>

<!-- /.related -->

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<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

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---

## License

See [LICENSE][stdlib-license].


## Copyright

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

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[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object

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

[@stdlib/utils/count-by]: https://www.npmjs.com/package/@stdlib/utils-count-by

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[@stdlib/utils/tabulate]: https://www.npmjs.com/package/@stdlib/utils-tabulate

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