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

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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] -->

> [Wald][wald-distribution] distribution [expected value][mean].

<!-- 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">

The [expected value][mean] for a [Wald][wald-distribution] random variable with mean `μ` and shape parameter `λ > 0` is

<!-- <equation class="equation" label="eq:wald_expectation" align="center" raw="\mathbb{E}\left[ X \right] = \mu" alt="Expected value for a Wald distribution."> -->

<div class="equation" align="center" data-raw-text="\mathbb{E}\left[ X \right] = \mu" data-equation="eq:wald_expectation">
    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@51534079fef45e990850102147e8945fb023d1d0/lib/node_modules/@stdlib/stats/base/dists/wald/mean/docs/img/equation_wald_expectation.svg" alt="Expected value for a Wald distribution.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

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

</section>

<section class="usage">

## Usage

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

#### mean( mu, lambda )

Returns the [expected value][mean] for a [Wald][wald-distribution] distribution with parameters `mu` (mean) and `lambda` (shape parameter).

```javascript
var y = mean( 2.0, 1.0 );
// returns 2.0

y = mean( 0.0, 1.0 );
// returns NaN

y = mean( -1.0, 4.0 );
// returns NaN
```

If provided `NaN` as any argument, the function returns `NaN`.

```javascript
var y = mean( NaN, 1.0 );
// returns NaN

y = mean( 0.0, NaN );
// returns NaN
```

If provided `mu <= 0` or `lambda <= 0`, the function returns `NaN`.

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

y = mean( 0.0, -1.0 );
// returns NaN

y = mean( -1.0, 0.0 );
// returns NaN
```

</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">

</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 EPS = require( '@stdlib/constants-float64-eps' );
var mean = require( '@stdlib/stats-base-dists-wald-mean' );

var opts = {
    'dtype': 'float64'
};
var mu = uniform( 10, EPS, 10.0, opts );
var lambda = uniform( 10, EPS, 20.0, opts );

logEachMap( 'µ: %0.4f, λ: %0.4f, E(X;µ,λ): %0.4f', mu, lambda, mean );
```

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- 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 -->

<!-- C usage documentation. -->

<section class="usage">

### Usage

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

#### stdlib_base_dists_wald_mean( mu, lambda )

Returns the [expected value][mean] for a [Wald][wald-distribution] distribution with mean `mu` and shape parameter `lambda`.

```c
double out = stdlib_base_dists_wald_mean( 2.0, 1.0 );
// returns 2.0
```

The function accepts the following arguments:

-   **mu**: `[in] double` mean.
-   **lambda**: `[in] double` shape parameter.

```c
double stdlib_base_dists_wald_mean( const double mu, const double lambda );
```

</section>

<!-- /.usage -->

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

<section class="notes">

</section>

<!-- /.notes -->

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

<section class="examples">

### Examples

```c
#include "stdlib/stats/base/dists/wald/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 lambda;
    double mu;
    double y;
    int i;

    for ( i = 0; i < 10; i++ ) {
        mu = random_uniform( 0.1, 5.0 );
        lambda = random_uniform( 0.1, 20.0 );
        y = stdlib_base_dists_wald_mean( mu, lambda );
        printf( "µ: %.4f, λ: %.4f, Mean(X;µ,λ): %.4f\n", mu, lambda, y );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

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<section class="references">

</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].

</section>

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/stats-base-dists-wald-mean/main/LICENSE

[wald-distribution]: https://en.wikipedia.org/wiki/Inverse_Gaussian_distribution

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

</section>

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