nanmaxBy

Calculate the maximum value of a strided array via a callback function, ignoring NaN values.

Usage

var nanmaxBy = require( '@stdlib/stats/base/nanmax-by' );

nanmaxBy( N, x, stride, clbk[, thisArg] )

Calculates the maximum value of strided array x via a callback function, ignoring NaN values.

function accessor( v ) {
    return v * 2.0;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, NaN, 0.0, -1.0, -3.0, NaN ];

var v = nanmaxBy( x.length, x, 1, accessor );
// returns 8.0

The function has the following parameters:

N: number of indexed elements.
x: input Array, 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.

function accessor( v ) {
    this.count += 1;
    return v * 2.0;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, NaN, 0.0, -1.0, -3.0, NaN ];

var context = {
    'count': 0
};

var v = nanmaxBy( x.length, x, 1, accessor, context );
// returns 8.0

var cnt = context.count;
// returns 10

The N and stride parameters determine which elements in x are accessed at runtime. For example, to access every other element

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, NaN, NaN ];
var N = floor( x.length / 2 );

var v = nanmaxBy( N, x, 2, accessor );
// returns 8.0

Note that indexing is relative to the first index. To introduce an offset, use typed array views.

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 = nanmaxBy( N, x1, 2, accessor );
// returns -4.0

nanmaxBy.ndarray( N, x, stride, offset, clbk[, thisArg] )

Calculates the maximum value of strided array x via a callback function, ignoring NaN values and using alternative indexing semantics.

function accessor( v ) {
    return v * 2.0;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, NaN, 0.0, -1.0, -3.0, NaN ];

var v = nanmaxBy.ndarray( x.length, x, 1, 0, accessor );
// returns 8.0

The function has the following additional parameters:

offset: starting index.

While 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

function accessor( v ) {
    return v * 2.0;
}

var x = [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ];

var v = nanmaxBy.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 returns NaN, the value is ignored.
If a provided callback function does not return any value (or equivalently, explicitly returns undefined), the value is ignored.
When possible, prefer using dnanmax, snanmax, and/or nanmax, as, depending on the environment, these interfaces are likely to be significantly more performant.

Examples

var discreteUniform = require( '@stdlib/random/base/discrete-uniform' );
var randu = require( '@stdlib/random/base/randu' );
var filledarrayBy = require( '@stdlib/array/filled-by' );
var nanmaxBy = require( '@stdlib/stats/base/nanmax-by' );

function fill() {
    if ( randu() < 0.2 ) {
        return NaN;
    }
    return discreteUniform( -50, 50 );
}

function accessor( v ) {
    return v * 2.0;
}

var x = filledarrayBy( 10, 'float64', fill );
console.log( x );

var v = nanmaxBy( x.length, x, 1, accessor );
console.log( v );