# incrmhmean
> Compute a moving [harmonic mean][harmonic-mean] incrementally.
The [harmonic mean][harmonic-mean] of positive real numbers `x_0, x_1, ..., x_{n-1}` is defined as
## Usage
```javascript
var incrmhmean = require( '@stdlib/stats/incr/mhmean' );
```
#### incrmhmean( window )
Returns an accumulator `function` which incrementally computes a moving [harmonic mean][harmonic-mean]. The `window` parameter defines the number of values over which to compute the moving [harmonic mean][harmonic-mean].
```javascript
var accumulator = incrmhmean( 3 );
```
#### accumulator( \[x] )
If provided an input value `x`, the accumulator function returns an updated [harmonic mean][harmonic-mean]. If not provided an input value `x`, the accumulator function returns the current [harmonic-mean][harmonic-mean].
```javascript
var accumulator = incrmhmean( 3 );
var v = accumulator();
// returns null
// Fill the window...
v = accumulator( 2.0 ); // [2.0]
// returns 2.0
v = accumulator( 1.0 ); // [2.0, 1.0]
// returns ~1.33
v = accumulator( 3.0 ); // [2.0, 1.0, 3.0]
// returns ~1.64
// Window begins sliding...
v = accumulator( 7.0 ); // [1.0, 3.0, 7.0]
// returns ~2.03
v = accumulator( 5.0 ); // [3.0, 7.0, 5.0]
// returns ~4.44
v = accumulator();
// returns ~4.44
```
## 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.
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var incrmhmean = require( '@stdlib/stats/incr/mhmean' );
var accumulator;
var v;
var i;
// Initialize an accumulator:
accumulator = incrmhmean( 5 );
// For each simulated datum, update the moving harmonic mean...
for ( i = 0; i < 100; i++ ) {
v = randu() * 100.0;
accumulator( v );
}
console.log( accumulator() );
```
[harmonic-mean]: https://en.wikipedia.org/wiki/Harmonic_mean
