# mskmax
> Calculate the maximum value of a strided array according to a mask.
## Usage
```javascript
var mskmax = require( '@stdlib/stats/base/mskmax' );
```
#### mskmax( N, x, strideX, mask, strideMask )
Computes the maximum value of a strided array `x` according to a `mask`.
```javascript
var x = [ 1.0, -2.0, 4.0, 2.0 ];
var mask = [ 0, 0, 1, 0 ];
var v = mskmax( 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 = mskmax( 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.
```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 = mskmax( N, x1, 2, mask1, 2 );
// returns 4.0
```
#### mskmax.ndarray( N, x, strideX, offsetX, mask, strideMask, offsetMask )
Computes the maximum value of a strided array according to a `mask` and using alternative indexing semantics.
```javascript
var x = [ 1.0, -2.0, 4.0, 2.0 ];
var mask = [ 0, 0, 1, 0 ];
var v = mskmax.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 = mskmax.ndarray( N, x, 2, 1, mask, 2, 1 );
// returns 4.0
```
## Notes
- If `N <= 0`, both functions return `NaN`.
- Depending on the environment, the typed versions ([`dmskmax`][@stdlib/stats/base/dmskmax], [`smskmax`][@stdlib/stats/base/smskmax], etc.) are likely to be significantly more performant.
## Examples
```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 mskmax = require( '@stdlib/stats/base/mskmax' );
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;
}
x[ i ] = round( (randu()*100.0) - 50.0 );
}
console.log( x );
console.log( mask );
var v = mskmax( x.length, x, 1, mask, 1 );
console.log( v );
```
[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/dmskmax]: https://www.npmjs.com/package/@stdlib/stats/tree/main/base/dmskmax
[@stdlib/stats/base/smskmax]: https://www.npmjs.com/package/@stdlib/stats/tree/main/base/smskmax