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# itermsumabs2

> Create an [iterator][mdn-iterator-protocol] which iteratively computes a moving sum of squared absolute values.

<section class="intro">

For a window of size `W`, the moving sum of squared absolute values is defined as

<!-- <equation class="equation" label="eq:moving_sum_squared_absolute_values" align="center" raw="s = \sum_{i=0}^{W-1} x_i^2" alt="Equation for the moving sum of squared absolute values."> -->

<div class="equation" align="center" data-raw-text="s = \sum_{i=0}^{W-1} x_i^2" data-equation="eq:moving_sum_squared_absolute_values">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/601912bba7cacd1e1ffed3583e9b86e6383b4202/lib/node_modules/@stdlib/stats/iter/msumabs2/docs/img/equation_moving_sum_squared_absolute_values.svg" alt="Equation for the moving sum of squared absolute values.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="usage">

## Usage

```javascript
var itermsumabs2 = require( '@stdlib/stats/iter/msumabs2' );
```

#### itermsumabs2( iterator, W )

Returns an [iterator][mdn-iterator-protocol] which iteratively computes a moving sum of squared absolute values. The `W` parameter defines the number of iterated values over which to compute the moving sum.

```javascript
var array2iterator = require( '@stdlib/array/to-iterator' );

var arr = array2iterator( [ 2.0, 1.0, 3.0, -7.0, -5.0 ] );
var it = itermsumabs2( arr, 3 );

// Fill the window...
var s = it.next().value; // [2.0]
// returns 4.0

s = it.next().value; // [2.0, 1.0]
// returns 5.0

s = it.next().value; // [2.0, 1.0, 3.0]
// returns 14.0

// Window begins sliding...
s = it.next().value; // [1.0, 3.0, -7.0]
// returns 59.0

s = it.next().value; // [3.0, -7.0, -5.0]
// returns 83.0
```

</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

-   If an iterated value is non-numeric (including `NaN`), the function returns `NaN` for **at least** `W-1` future invocations. If non-numeric iterated values are possible, you are advised to provide an [`iterator`][mdn-iterator-protocol] which type checks and handles non-numeric values accordingly.
-   As `W` values are needed to fill the window buffer, the first `W-1` returned values are calculated from smaller sample sizes. Until the window is full, each returned value is calculated from all previously iterated values.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var runif = require( '@stdlib/random/iter/uniform' );
var itermsumabs2 = require( '@stdlib/stats/iter/msumabs2' );

// Create an iterator for generating uniformly distributed pseudorandom numbers:
var rand = runif( -10.0, 10.0, {
    'seed': 1234,
    'iter': 100
});

// Create an iterator for iteratively computing a moving sum of squared absolute values:
var it = itermsumabs2( rand, 3 );

// Perform manual iteration...
var v;
while ( true ) {
    v = it.next();
    if ( typeof v.value === 'number' ) {
        console.log( 'sumabs2: %d', v.value );
    }
    if ( v.done ) {
        break;
    }
}
```

</section>

<!-- /.examples -->

<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="references">

</section>

<!-- /.references -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="links">

[mdn-iterator-protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterator_protocol

</section>

<!-- /.links -->
