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# incrwmean

> Compute a [weighted arithmetic mean][weighted-arithmetic-mean] incrementally.

<section class="intro">

The [weighted arithmetic mean][weighted-arithmetic-mean] is defined as

<!-- <equation class="equation" label="eq:weighted_arithmetic_mean" align="center" raw="\bar{x} = \frac{\sum_{i=0}^{n-1} w_{i} x_{i}}{\sum_{i=0}^{n-1} w_{i}}" alt="Equation for the weighted arithmetic mean."> -->

<div class="equation" align="center" data-raw-text="\bar{x} = \frac{\sum_{i=0}^{n-1} w_{i} x_{i}}{\sum_{i=0}^{n-1} w_{i}}" data-equation="eq:weighted_arithmetic_mean">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/adbea9806383f70c982e3191475c874efba1296b/lib/node_modules/@stdlib/stats/incr/wmean/docs/img/equation_weighted_arithmetic_mean.svg" alt="Equation for the weighted arithmetic mean.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var incrwmean = require( '@stdlib/stats/incr/wmean' );
```

#### incrwmean()

Returns an accumulator `function` which incrementally computes a [weighted arithmetic mean][weighted-arithmetic-mean].

```javascript
var accumulator = incrwmean();
```

#### accumulator( \[x, w] )

If provided an input value `x` and a weight `w`, the accumulator function returns an updated weighted mean. If not provided any input values, the accumulator function returns the current mean.

```javascript
var accumulator = incrwmean();

var mu = accumulator( 2.0, 1.0 );
// returns 2.0

mu = accumulator( 2.0, 0.5 );
// returns 2.0

mu = accumulator( 3.0, 1.5 );
// returns 2.5

mu = accumulator();
// returns 2.5
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   Input values are **not** type checked. If provided `NaN` or a value which, when used in computations, results in `NaN`, the accumulated value is `NaN` for **all** future invocations. If non-numeric inputs are possible, you are advised to type check and handle accordingly **before** passing the value to the accumulator function.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random/base/randu' );
var incrwmean = require( '@stdlib/stats/incr/wmean' );

var accumulator;
var v;
var w;
var i;

// Initialize an accumulator:
accumulator = incrwmean();

// For each simulated datum, update the weighted mean...
for ( i = 0; i < 100; i++ ) {
    v = randu() * 100.0;
    w = randu() * 100.0;
    accumulator( v, w );
}
console.log( accumulator() );
```

</section>

<!-- /.examples -->

<section class="links">

[weighted-arithmetic-mean]: https://en.wikipedia.org/wiki/Weighted_arithmetic_mean

</section>

<!-- /.links -->
