# incrmsumprod
> Compute a moving sum of products incrementally.
For a window of size `W`, the moving sum of products is defined as
## Usage
```javascript
var incrmsumprod = require( '@stdlib/stats/incr/msumprod' );
```
#### incrmsumprod( window )
Returns an accumulator `function` which incrementally computes a moving sum of products. The `window` parameter defines the number of values over which to compute the moving sum of products.
```javascript
var accumulator = incrmsumprod( 3 );
```
#### accumulator( \[x, y] )
If provided input values `x` and `y`, the accumulator function returns an updated sum. If not provided input values `x` and `y`, the accumulator function returns the current sum.
```javascript
var accumulator = incrmsumprod( 3 );
var sum = accumulator();
// returns null
// Fill the window...
sum = accumulator( 2.0, 3.0 ); // [(2.0,3.0)]
// returns 6.0
sum = accumulator( 1.0, -1.0 ); // [(2.0,3.0), (1.0,-1.0)]
// returns 5.0
sum = accumulator( -3.0, 5.0 ); // [(2.0,3.0), (1.0,-1.0), (-3.0,5.0)]
// returns -10.0
// Window begins sliding...
sum = accumulator( -7.0, -1.0 ); // [(1.0,-1.0), (-3.0,5.0), (-7.0,-1.0)]
// returns -9.0
sum = accumulator( 5.0, 4.0 ); // [(-3.0,5.0), (-7.0,-1.0), (5.0,4.0)]
// returns 12.0
sum = accumulator();
// returns 12.0
```
## 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` (x,y) pairs 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.
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var incrmsumprod = require( '@stdlib/stats/incr/msumprod' );
var accumulator;
var v1;
var v2;
var i;
// Initialize an accumulator:
accumulator = incrmsumprod( 5 );
// For each simulated datum, update the moving sum-product...
for ( i = 0; i < 100; i++ ) {
v1 = randu() * 100.0;
v2 = randu() * 100.0;
accumulator( v1, v2 );
}
console.log( accumulator() );
```
