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# nanmskmax

> Calculate the maximum value of a strided array according to a mask, ignoring `NaN` values.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var nanmskmax = require( '@stdlib/stats/base/nanmskmax' );
```

#### nanmskmax( N, x, strideX, mask, strideMask )

Computes the maximum value of a strided array `x` according to a `mask`, ignoring `NaN` values.

```javascript
var x = [ 1.0, -2.0, 4.0, 2.0, NaN ];
var mask = [ 0, 0, 1, 0, 0 ];

var v = nanmskmax( x.length, x, 1, mask, 1 );
// returns 2.0
```

The function has the following parameters:

-   **N**: number of indexed elements.
-   **x**: input [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideX**: index increment for `x`.
-   **mask**: mask [`Array`][mdn-array] or [`typed array`][mdn-typed-array]. If a `mask` array element is `0`, the corresponding element in `x` is considered valid and **included** in computation. If a `mask` array element is `1`, the corresponding element in `x` is considered invalid/missing and **excluded** from computation.
-   **strideMask**: index increment for `mask`.

The `N` and `stride` parameters determine which elements are accessed at runtime. For example, to compute the maximum value of every other element in `x`,

```javascript
var floor = require( '@stdlib/math/base/special/floor' );

var x = [ 1.0, 2.0, -7.0, -2.0, 4.0, 3.0, 5.0, 6.0 ];
var mask = [ 0, 0, 0, 0, 0, 0, 1, 1 ];
var N = floor( x.length / 2 );

var v = nanmskmax( N, x, 2, mask, 2 );
// returns 4.0
```

Note that indexing is relative to the first index. To introduce offsets, use [`typed array`][mdn-typed-array] views.

<!-- eslint-disable stdlib/capitalized-comments -->

```javascript
var Float64Array = require( '@stdlib/array/float64' );
var Uint8Array = require( '@stdlib/array/uint8' );
var floor = require( '@stdlib/math/base/special/floor' );

var x0 = new Float64Array( [ 2.0, 1.0, -2.0, -2.0, 3.0, 4.0, 5.0, 6.0 ] );
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

var mask0 = new Uint8Array( [ 0, 0, 0, 0, 0, 0, 1, 1 ] );
var mask1 = new Uint8Array( mask0.buffer, mask0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

var N = floor( x0.length / 2 );

var v = nanmskmax( N, x1, 2, mask1, 2 );
// returns 4.0
```

#### nanmskmax.ndarray( N, x, strideX, offsetX, mask, strideMask, offsetMask )

Computes the maximum value of a strided array according to a `mask`, ignoring `NaN` values and using alternative indexing semantics.

```javascript
var x = [ 1.0, -2.0, 4.0, 2.0, NaN ];
var mask = [ 0, 0, 1, 0, 0 ];

var v = nanmskmax.ndarray( x.length, x, 1, 0, mask, 1, 0 );
// returns 2.0
```

The function has the following additional parameters:

-   **offsetX**: starting index for `x`.
-   **offsetMask**: starting index for `mask`.

While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying `buffer`, the `offset` parameter supports indexing semantics based on a starting index. For example, to calculate the maximum value for every other value in `x` starting from the second value

```javascript
var floor = require( '@stdlib/math/base/special/floor' );

var x = [ 2.0, 1.0, -2.0, -2.0, 3.0, 4.0, 5.0, 6.0 ];
var mask = [ 0, 0, 0, 0, 0, 0, 1, 1 ];
var N = floor( x.length / 2 );

var v = nanmskmax.ndarray( N, x, 2, 1, mask, 2, 1 );
// returns 4.0
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions return `NaN`.
-   Depending on the environment, the typed versions ([`dnanmskmax`][@stdlib/stats/base/dnanmskmax], [`snanmskmax`][@stdlib/stats/base/snanmskmax], etc.) are likely to be significantly more performant.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random/base/randu' );
var round = require( '@stdlib/math/base/special/round' );
var Float64Array = require( '@stdlib/array/float64' );
var Uint8Array = require( '@stdlib/array/uint8' );
var nanmskmax = require( '@stdlib/stats/base/nanmskmax' );

var mask;
var x;
var i;

x = new Float64Array( 10 );
mask = new Uint8Array( x.length );
for ( i = 0; i < x.length; i++ ) {
    if ( randu() < 0.2 ) {
        mask[ i ] = 1;
    } else {
        mask[ i ] = 0;
    }
    if ( randu() < 0.1 ) {
        x[ i ] = NaN;
    } else {
        x[ i ] = round( (randu()*100.0) - 50.0 );
    }
}
console.log( x );
console.log( mask );

var v = nanmskmax( x.length, x, 1, mask, 1 );
console.log( v );
```

</section>

<!-- /.examples -->

<section class="links">

[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

[@stdlib/stats/base/dnanmskmax]: https://github.com/stdlib-js/stats/tree/main/base/dnanmskmax

[@stdlib/stats/base/snanmskmax]: https://github.com/stdlib-js/stats/tree/main/base/snanmskmax

</section>

<!-- /.links -->
