gfindLastIndex

Return the index of the last element which passes a test implemented by a predicate function.

Installation

npm install @stdlib/blas-ext-base-gfind-last-index

Usage

var gfindLastIndex = require( '@stdlib/blas-ext-base-gfind-last-index' );

gfindLastIndex( N, x, strideX, clbk[, thisArg] )

Returns the index of the last element which passes a test implemented by a predicate function.

function isEven( v ) {
    return v % 2.0 === 0.0;
}

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

var idx = gfindLastIndex( x.length, x, 1, isEven );
// returns 3

If no element passes a test implemented by a predicate function, the function returns -1.

function isEven( v ) {
    return v % 2.0 === 0.0;
}

var x = [ 1.0, 3.0, 5.0, 7.0, 9.0 ];

var idx = gfindLastIndex( x.length, x, 1, isEven );
// returns -1

The function has the following parameters:

• N: number of indexed elements.
• x: input array.
• strideX: stride length.
• clbk: callback function.
• thisArg: execution context (optional).

The callback function is provided the following arguments:

• value: current array element.
• aidx: array index.
• sidx: strided index (offset + aidx*stride).
• array: the input array.

To set the callback execution context, provide a thisArg.

function isEven( v ) {
    this.count += 1;
    return v % 2.0 === 0.0;
}

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

var ctx = {
    'count': 0
};

var idx = gfindLastIndex( x.length, x, 1, isEven, ctx );
// returns 3

var count = ctx.count;
// returns 2

The N and stride parameters determine which elements in the strided array are accessed at runtime. For example, to test every other element:

function isEven( v ) {
    return v % 2.0 === 0.0;
}

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

var idx = gfindLastIndex( 3, x, 2, isEven );
// returns 1

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

var Float64Array = require( '@stdlib/array-float64' );

function isEven( v ) {
    return v % 2.0 === 0.0;
}

// 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 idx = gfindLastIndex( 3, x1, 2, isEven );
// returns 2

gfindLastIndex.ndarray( N, x, strideX, offsetX, clbk[, thisArg] )

Returns the index of the last element which passes a test implemented by a predicate function using alternative indexing semantics.

function isEven( v ) {
    return v % 2.0 === 0.0;
}

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

var idx = gfindLastIndex.ndarray( x.length, x, 1, 0, isEven );
// returns 3

The function has the following additional parameters:

• offsetX: 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:

function isEven( v ) {
    return v % 2.0 === 0.0;
}

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

var idx = gfindLastIndex.ndarray( 3, x, 1, x.length-3, isEven );
// returns 1

Notes

• If N <= 0, both functions return -1.
• Both functions support array-like objects having getter and setter accessors for array element access (e.g., @stdlib/array-base/accessor).

Examples

var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var gfindLastIndex = require( '@stdlib/blas-ext-base-gfind-last-index' );

function isEven( v ) {
    return v % 2.0 === 0.0;
}

var x = discreteUniform( 10, -100, 100, {
    'dtype': 'generic'
});
console.log( x );

var idx = gfindLastIndex.ndarray( x.length, x, 1, 0, isEven );
console.log( idx );

Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.