@stdlib/stats/incr/msum/README.md

Standard library statistical functions.

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# incrmsum

> Compute a moving sum incrementally.

<section class="intro">

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

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

<div class="equation" align="center" data-raw-text="s = \sum_{i=0}^{W-1} x_i" data-equation="eq:moving_sum">
    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@49d8cabda84033d55d7b8069f19ee3dd8b8d1496/lib/node_modules/@stdlib/stats/incr/msum/docs/img/equation_moving_sum.svg" alt="Equation for the moving sum.">
    <br>
</div>

<!-- </equation> -->

</section>

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<section class="usage">

## Usage

```javascript
var incrmsum = require( '@stdlib/stats/incr/msum' );
```

#### incrmsum( window )

Returns an accumulator `function` which incrementally computes a moving sum. The `window` parameter defines the number of values over which to compute the moving sum.

```javascript
var accumulator = incrmsum( 3 );
```

#### accumulator( \[x] )

If provided an input value `x`, the accumulator function returns an updated sum. If not provided an input value `x`, the accumulator function returns the current sum.

```javascript
var accumulator = incrmsum( 3 );

var sum = accumulator();
// returns null

// Fill the window...
sum = accumulator( 2.0 ); // [2.0]
// returns 2.0

sum = accumulator( 1.0 ); // [2.0, 1.0]
// returns 3.0

sum = accumulator( 3.0 ); // [2.0, 1.0, 3.0]
// returns 6.0

// Window begins sliding...
sum = accumulator( -7.0 ); // [1.0, 3.0, -7.0]
// returns -3.0

sum = accumulator( -5.0 ); // [3.0, -7.0, -5.0]
// returns -9.0

sum = accumulator();
// returns -9.0
```

</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 **at least** `W-1` 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.
-   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 provided values.

</section>

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<section class="examples">

## Examples

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

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

var accumulator;
var v;
var i;

// Initialize an accumulator:
accumulator = incrmsum( 5 );

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

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/stats/incr/mmean`][@stdlib/stats/incr/mmean]</span><span class="delimiter">: </span><span class="description">compute a moving arithmetic mean incrementally.</span>
-   <span class="package-name">[`@stdlib/stats/incr/msummary`][@stdlib/stats/incr/msummary]</span><span class="delimiter">: </span><span class="description">compute a moving statistical summary incrementally.</span>
-   <span class="package-name">[`@stdlib/stats/incr/sum`][@stdlib/stats/incr/sum]</span><span class="delimiter">: </span><span class="description">compute a sum incrementally.</span>

</section>

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<section class="links">

<!-- <related-links> -->

[@stdlib/stats/incr/mmean]: https://github.com/stdlib-js/stats/tree/main/incr/mmean

[@stdlib/stats/incr/msummary]: https://github.com/stdlib-js/stats/tree/main/incr/msummary

[@stdlib/stats/incr/sum]: https://github.com/stdlib-js/stats/tree/main/incr/sum

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</section>

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