npm package: @stdlib/stats-base-dists-halfnormal-mean
Description: Half-normal distribution expected value.
Author: The Stdlib Authors
Latest version: 0.1.1

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<details>
  <summary>
    About stdlib...
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# Mean

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

> [Half-normal][half-normal-distribution] distribution [expected value][mean].

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<section class="intro">

The [expected value][mean] for a [half-normal][half-normal-distribution] random variable with scale parameter `σ > 0` is

<!-- <equation class="equation" label="eq:halfnormal_expectation" align="center" raw="\mathbb{E}\left[ X \right] = \sigma \sqrt{\frac{2}{\pi}}" alt="Expected value for a half-normal distribution."> -->

<div class="equation" align="center" data-raw-text="\mathbb{E}\left[ X \right] = \sigma \sqrt{\frac{2}{\pi}}" data-equation="eq:halfnormal_expectation">
    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@51534079fef45e990850102147e8945fb023d1d0/lib/node_modules/@stdlib/stats/base/dists/halfnormal/mean/docs/img/equation_halfnormal_expectation.svg" alt="Expected value for a half-normal distribution.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/stats-base-dists-halfnormal-mean
```

</section>

<section class="usage">

## Usage

```javascript
var mean = require( '@stdlib/stats-base-dists-halfnormal-mean' );
```

#### mean( sigma )

Returns the [expected value][mean] for a [half-normal][half-normal-distribution] distribution with scale parameter `sigma`.

```javascript
var y = mean( 1.0 );
// returns ~0.798

y = mean( 4.0 );
// returns ~3.192

y = mean( 0.5 );
// returns ~0.399
```

If provided `NaN`, the function returns `NaN`.

```javascript
var y = mean( NaN );
// returns NaN
```

If provided `sigma <= 0`, the function returns `NaN`.

```javascript
var y = mean( 0.0 );
// returns NaN

y = mean( -1.0 );
// returns NaN
```

</section>

<!-- /.usage -->

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<section class="notes">

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var uniform = require( '@stdlib/random-array-uniform' );
var logEachMap = require( '@stdlib/console-log-each-map' );
var mean = require( '@stdlib/stats-base-dists-halfnormal-mean' );

var opts = {
    'dtype': 'float64'
};
var sigma = uniform( 10, 0.1, 20.0, opts );

logEachMap( 'σ: %lf, E(X;σ): %lf', sigma, mean );
```

</section>

<!-- /.examples -->

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<section class="references">

</section>

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<section class="related">

</section>

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<!-- Section for C API documentation. -->

* * *

<section class="c">

## C APIs

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<section class="intro">

</section>

<!-- /.intro -->

<!-- C API usage documentation. -->

<section class="usage">

### Usage

```c
#include "stdlib/stats/base/dists/halfnormal/mean.h"
```

#### stdlib_base_dists_halfnormal_mean( sigma )

Returns the expected value for a [half-normal][half-normal-distribution] distribution with scale parameter `sigma`.

-   **sigma**: `[in] double` scale parameter.

```c
double stdlib_base_dists_halfnormal_mean( const double sigma );
```

</section>

<!-- /.usage -->

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<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

```c
#include "stdlib/stats/base/dists/halfnormal/mean.h"
#include <stdlib.h>
#include <stdio.h>

static double random_uniform( const double min, const double max ) {
    double v = (double)rand() / ( (double)RAND_MAX + 1.0 );
    return min + ( v*(max-min) );
}

int main( void ) {
    double sigma;
    double y;
    int i;

    for ( i = 0; i < 10; i++ ) {
        sigma = random_uniform( 0.1, 20.0 );
        y = stdlib_base_dists_halfnormal_mean( sigma );
        printf( "σ: %lf, E(X;σ): %lf\n", sigma, y );
    }
}
```

</section>

<!-- /.examples -->

</section>

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<section class="related">

</section>

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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

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

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

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[half-normal-distribution]: https://en.wikipedia.org/wiki/Half-normal_distribution

[mean]: https://en.wikipedia.org/wiki/Mean

</section>

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