# maxBy
> Calculate the maximum value of a strided array via a callback function.
## Usage
```javascript
var maxBy = require( '@stdlib/stats/base/max-by' );
```
#### maxBy( N, x, stride, clbk\[, thisArg] )
Calculates the maximum value of strided array `x` via a callback function.
```javascript
function accessor( v ) {
return v * 2.0;
}
var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var v = maxBy( x.length, x, 1, accessor );
// returns 8.0
```
The function has the following parameters:
- **N**: number of indexed elements.
- **x**: input [`Array`][mdn-array], [`typed array`][mdn-typed-array], or an array-like object (excluding strings and functions).
- **stride**: index increment.
- **clbk**: callback function.
- **thisArg**: execution context (_optional_).
The invoked callback is provided four arguments:
- **value**: array element.
- **aidx**: array index.
- **sidx**: strided index (`offset + aidx*stride`).
- **array**: input array/collection.
To set the callback execution context, provide a `thisArg`.
```javascript
function accessor( v ) {
this.count += 1;
return v * 2.0;
}
var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var context = {
'count': 0
};
var v = maxBy( x.length, x, 1, accessor, context );
// returns 8.0
var cnt = context.count;
// returns 8
```
The `N` and `stride` parameters determine which elements in `x` are accessed at runtime. For example, to access every other element
```javascript
var floor = require( '@stdlib/math/base/special/floor' );
function accessor( v ) {
return v * 2.0;
}
var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var N = floor( x.length / 2 );
var v = maxBy( N, x, 2, accessor );
// returns 8.0
```
Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var floor = require( '@stdlib/math/base/special/floor' );
function accessor( v ) {
return v * 2.0;
}
// Initial array...
var x0 = new Float64Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );
// Create an offset view...
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var N = floor( x0.length/2 );
// Access every other element...
var v = maxBy( N, x1, 2, accessor );
// returns -4.0
```
#### maxBy.ndarray( N, x, stride, offset, clbk\[, thisArg] )
Calculates the maximum value of strided array `x` via a callback function and using alternative indexing semantics.
```javascript
function accessor( v ) {
return v * 2.0;
}
var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var v = maxBy.ndarray( x.length, x, 1, 0, accessor );
// returns 8.0
```
The function has the following additional parameters:
- **offset**: starting index.
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 access only the last three elements of `x`
```javascript
function accessor( v ) {
return v * 2.0;
}
var x = [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ];
var v = maxBy.ndarray( 3, x, 1, x.length-3, accessor );
// returns 10.0
```
## Notes
- If `N <= 0`, both functions return `NaN`.
- A provided callback function should return a numeric value.
- If a provided callback function does not return any value (or equivalently, explicitly returns `undefined`), the value is **ignored**.
- When possible, prefer using [`dmax`][@stdlib/stats/base/dmax], [`smax`][@stdlib/stats/base/smax], and/or [`max`][@stdlib/stats/base/max], as, depending on the environment, these interfaces are likely to be significantly more performant.
## Examples
```javascript
var round = require( '@stdlib/math/base/special/round' );
var randu = require( '@stdlib/random/base/randu' );
var Float64Array = require( '@stdlib/array/float64' );
var gfillBy = require( '@stdlib/blas/ext/base/gfill-by' );
var maxBy = require( '@stdlib/stats/base/max-by' );
function fill() {
return round( ( randu()*100.0 ) - 50.0 );
}
function accessor( v ) {
return v * 2.0;
}
var x = new Float64Array( 10 );
gfillBy( x.length, x, 1, fill );
console.log( x );
var v = maxBy( x.length, x, 1, accessor );
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/dmax]: https://www.npmjs.com/package/@stdlib/stats/tree/main/base/dmax
[@stdlib/stats/base/max]: https://www.npmjs.com/package/@stdlib/stats/tree/main/base/max
[@stdlib/stats/base/smax]: https://www.npmjs.com/package/@stdlib/stats/tree/main/base/smax