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https://github.com/stdlib-js/random-sample

Sample elements from an array-like object.
https://github.com/stdlib-js/random-sample

choice choose discrete distribution draw javascript math mathematics node node-js nodejs rand random replacement sampling statistics stats stdlib urn with-replacement

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Sample elements from an array-like object.

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README 

        



  

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# Sample



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



> Sample elements from an array-like object.



## Installation



```bash

npm install @stdlib/random-sample

```



Alternatively,



-   To load the package in a website via a `script` tag without installation and bundlers, use the [ES Module][es-module] available on the [`esm`][esm-url] branch (see [README][esm-readme]).

-   If you are using Deno, visit the [`deno`][deno-url] branch (see [README][deno-readme] for usage intructions).

-   For use in Observable, or in browser/node environments, use the [Universal Module Definition (UMD)][umd] build available on the [`umd`][umd-url] branch (see [README][umd-readme]).



The [branches.md][branches-url] file summarizes the available branches and displays a diagram illustrating their relationships.



To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.



## Usage



```javascript

var sample = require( '@stdlib/random-sample' );

```



#### sample( x\[, options] )



Samples elements from an `array`-like object. By default, elements are drawn with replacement from `x` to create an output `array` having the same length as `x`.



```javascript

var out = sample( [ 'a', 'b', 'c' ] );

// e.g., returns [ 'a', 'a', 'b' ]



out = sample( [ 3, 6, 9 ] );

// e.g., returns [ 3, 9, 6 ]



var bool = ( out.length === 3 );

// returns true

```



The function accepts the following `options`:



-   **size**: sample size. Default: `N = x.length`.

-   **probs**: a probability `array`. Default: `[1/N,...,1/N]`.

-   **replace**: `boolean` indicating whether to sample from `x` with replacement. Default: `true`.



By default, the function returns an `array` having the same length as `x`. To generate a sample of a different size, set the `size` option.



```javascript

var out = sample( [ 3, 6, 9 ], {

    'size': 10

});

// e.g., returns [ 6, 3, 9, 9, 9, 6, 9, 6, 9, 3 ]



out = sample( [ 0, 1 ], {

    'size': 20

});

// e.g., returns [ 0, 0, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0 ]

```



To draw a sample _without_ replacement, set the `replace` option to `false`. In this case, the `size` option cannot be an integer larger than the number of elements in `x`.



```javascript

var out = sample( [ 1, 2, 3, 4, 5, 6 ], {

    'replace': false,

    'size': 3

});

// e.g., returns [ 6, 1, 5 ]



out = sample( [ 0, 1 ], {

    'replace': false

});

// e.g., returns [ 0, 1 ]

```



By default, the probability of sampling an element is the same for all elements. To assign elements different probabilities, set the `probs` option.



```javascript

var x = [ 1, 2, 3, 4, 5, 6 ];

var out = sample( x, {

    'probs': [ 0.1, 0.1, 0.1, 0.1, 0.1, 0.5 ]

});

// e.g., returns [ 5, 6, 6, 5, 6, 4 ]



x = [ 1, 2, 3, 4, 5, 6 ];

out = sample( x, {

    'probs': [ 0.1, 0.1, 0.1, 0.1, 0.1, 0.5 ],

    'size': 3,

    'replace': false

});

// e.g., returns [ 6, 4, 1 ]

```



The `probs` option **must** be a numeric array consisting of nonnegative values which sum to one. When sampling _without_ replacement, note that the `probs` option denotes the initial element probabilities which are then updated after each draw.



#### sample.factory( \[pool, ]\[options] )



Returns a `function` to sample elements from an `array`-like object. 



```javascript

var mysample = sample.factory();



var out = mysample( [ 0, 1, 2, 3, 4 ] );

// e.g., returns [ 4, 3, 4, 4 ]

```



If provided an array-like object `pool`, the returned function will always sample from the supplied object.



```javascript

var mysample = sample.factory( [ 1, 2, 3, 4, 5, 6 ] );



var out = mysample();

// e.g., returns [ 2, 4, 1, 6, 5, 1 ]



out = mysample();

// e.g., returns [ 5, 2, 3, 6, 1, 4 ]

```



The function accepts the following `options`:



-   **seed**: pseudorandom number generator seed.

-   **size**: sample size.

-   **mutate**: `boolean` indicating whether to mutate the `pool` when sampling without replacement. Default: `false`.

-   **replace**: `boolean` indicating whether to sample with replacement. Default: `true`.



To seed the pseudorandom number generator, set the `seed` option.



```javascript

var mysample = sample.factory({

    'seed': 430

});



var out = mysample( [ 1, 2, 3, 4, 5, 6 ] );

// e.g., returns [ 1, 1, 1, 5, 4, 4 ]



mysample = sample.factory( [ 1, 2, 3, 4, 5, 6 ], {

    'seed': 430

});



out = mysample();

// e.g., returns [ 1, 1, 1, 5, 4, 4 ]

```



To specify a sample size and/or override the default sample size, set the `size` option.



```javascript

var mysample = sample.factory({

    'size': 4

});



var out = mysample( [ 0, 1 ] );

// e.g., returns [ 0, 0, 0, 1 ]



// Override the size option...

out = mysample( [ 0, 1 ], {

    'size': 1

});

// e.g., returns [ 1 ]

```



By default, the returned function draws samples _with_ replacement. To override the default `replace` strategy, set the `replace` option.



```javascript

var mysample = sample.factory({

    'replace': false

});



var out = mysample( [ 1, 2, 3 ] );

// e.g., returns [ 3, 1, 2 ]

```



If a population from which to sample is provided, the underlying `pool` remains constant for each function invocation. To mutate the `pool` by permanently removing observations when sampling _without_ replacement, set the `mutate` option.



```javascript

var mysample = sample.factory( [ 1, 2, 3, 4, 5, 6 ], {

    'mutate': true,

    'replace': false,

    'size': 3,

    'seed': 342

});



var out = mysample();

// e.g., returns [ 6, 5, 3 ]



// Override the mutate option...

out = mysample({

    'mutate': false

});

// e.g., returns [ 1, 2, 4 ]



out = mysample();

// e.g., returns [ 1, 2, 4 ]

```



The returned function returns `null` after all population units are exhausted.



```javascript

var mysample = sample.factory( [ 1, 2, 3, 4, 5, 6 ], {

    'mutate': true,

    'replace': false

});



var out = mysample();

// e.g., returns [ 3, 2, 1, 6, 5, 4 ]



out = mysample();

// returns null

```



## Examples



```javascript

var sample = require( '@stdlib/random-sample' );



// By default, sample uniformly with replacement:

var x = [ 'a', 'b', 'c', 'd' ];

var out = sample( x, {

    'size': 10

});

// e.g., returns [ 'd', 'c', 'b', 'b', 'b', 'd', 'c', 'c', 'b', 'd' ]



// Sample with replacement with custom probabilities:

x = [ 'a', 'b', 'c', 'd' ];

out = sample( x, {

    'probs': [ 0.1, 0.1, 0.2, 0.6 ],

    'size': 10

});

// e.g., returns [ 'b', 'a', 'c', 'd', 'd', 'd', 'd', 'c', 'd', 'd' ]



// Sample without replacement:

x = [ 'a', 'b', 'c', 'd' ];

out = sample( x, {

    'size': 3,

    'replace': false

});

// e.g., returns [ 'd', 'c', 'a' ]



// Sample without replacement when (initial) probabilities are nonuniform:

x = [ 1, 2, 3, 4, 5, 6 ];

out = sample( x, {

    'probs': [ 0.1, 0.1, 0.1, 0.1, 0.1, 0.5 ],

    'size': 3,

    'replace': false

});

// e.g., returns [ 2, 3, 6 ]

```



### References



-   Knuth, Donald E. 1997. _The Art of Computer Programming, Volume 2 (3rd Ed.): Seminumerical Algorithms_. Boston, MA, USA: Addison-Wesley Longman Publishing Co., Inc.

-   Vose, Michael D. 1991. "A linear algorithm for generating random numbers with a given distribution." _IEEE Transactions on Software Engineering_ 17 (9): 972–75. doi:[10.1109/32.92917][@vose:1991].



* * *



## 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 © 2016-2025. The Stdlib [Authors][stdlib-authors].



[npm-image]: http://img.shields.io/npm/v/@stdlib/random-sample.svg

[npm-url]: https://npmjs.org/package/@stdlib/random-sample



[test-image]: https://github.com/stdlib-js/random-sample/actions/workflows/test.yml/badge.svg?branch=main

[test-url]: https://github.com/stdlib-js/random-sample/actions/workflows/test.yml?query=branch:main



[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/random-sample/main.svg

[coverage-url]: https://codecov.io/github/stdlib-js/random-sample?branch=main



[chat-image]: https://img.shields.io/gitter/room/stdlib-js/stdlib.svg

[chat-url]: https://app.gitter.im/#/room/#stdlib-js_stdlib:gitter.im



[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/random-sample/tree/deno

[deno-readme]: https://github.com/stdlib-js/random-sample/blob/deno/README.md

[umd-url]: https://github.com/stdlib-js/random-sample/tree/umd

[umd-readme]: https://github.com/stdlib-js/random-sample/blob/umd/README.md

[esm-url]: https://github.com/stdlib-js/random-sample/tree/esm

[esm-readme]: https://github.com/stdlib-js/random-sample/blob/esm/README.md

[branches-url]: https://github.com/stdlib-js/random-sample/blob/main/branches.md



[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/random-sample/main/LICENSE



[@vose:1991]: https://doi.org/10.1109/32.92917