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# incrsumprod

> Compute a sum of products incrementally.

<section class="intro">

The sum of products is defined as

<!-- <equation class="equation" label="eq:sum_product" align="center" raw="s = \sum_{i=0}^{n-1} x_i y_i" alt="Equation for the sum of products."> -->

<div class="equation" align="center" data-raw-text="s = \sum_{i=0}^{n-1} x_i y_i" data-equation="eq:sum_product">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/27e2a43c70db648bb5bbc3fd0cdee050c25adc0b/lib/node_modules/@stdlib/stats/incr/sumprod/docs/img/equation_sum_product.svg" alt="Equation for the sum of products.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var incrsumprod = require( '@stdlib/stats/incr/sumprod' );
```

#### incrsumprod()

Returns an accumulator `function` which incrementally computes a sum of products.

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

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

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

```javascript
var accumulator = incrsumprod();

var sum = accumulator( 2.0, 3.0 );
// returns 6.0

sum = accumulator( 1.0, -1.0 );
// returns 5.0

sum = accumulator( 3.0, 4.0 );
// returns 17.0

sum = accumulator();
// returns 17.0
```

</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 **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.
-   For long running accumulations or accumulations of large numbers, care should be taken to prevent overflow.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

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

var accumulator;
var v1;
var v2;
var i;

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

// For each simulated datum, update the sum-product...
for ( i = 0; i < 100; i++ ) {
    v1 = randu() * 100.0;
    v2 = randu() * 100.0;
    accumulator( v1, v2 );
}
console.log( accumulator() );
```

</section>

<!-- /.examples -->

<section class="links">

</section>

<!-- /.links -->
