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findIndex

Return the index of the first element along an ndarray dimension which passes a test implemented by a predicate function.

Installation

npm install @stdlib/blas-ext-find-index

Alternatively,

• To load the package in a website via a script tag without installation and bundlers, use the ES Module available on the esm branch (see README).
• If you are using Deno, visit the deno branch (see README for usage intructions).
• For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the umd branch (see README).

The branches.md file summarizes the available branches and displays a diagram illustrating their relationships.

To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.

Usage

var findIndex = require( '@stdlib/blas-ext-find-index' );

findIndex( x[, options], clbk[, thisArg] )

Returns the index of the first element along an ndarray dimension which passes a test implemented by a predicate function.

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

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

// Create an input ndarray:
var x = array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
// returns <ndarray>

// Perform operation:
var out = findIndex( x, isEven );
// returns <ndarray>

var idx = out.get();
// returns 1

The function has the following parameters:

• x: input ndarray. Must have at least one dimension.
• options: function options (optional).
• clbk: callback function.
• thisArg: callback execution context (optional).

The invoked callback is provided three arguments:

• value: current array element.
• idx: current array element index.
• array: input ndarray.

To set the callback execution context, provide a thisArg.

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

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

var x = array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );

var ctx = {
    'count': 0
};
var out = findIndex( x, isEven, ctx );
// returns <ndarray>

var idx = out.get();
// returns 1

var count = ctx.count;
// returns 2

The function accepts the following options:

• dtype: output ndarray data type. Must be an integer index or generic data type.
• dim: dimension over which to perform operation. If provided a negative integer, the dimension along which to perform the operation is determined by counting backward from the last dimension (where -1 refers to the last dimension). Default: -1.
• keepdims: boolean indicating whether the reduced dimensions should be included in the returned ndarray as singleton dimensions. Default: false.

If no element along an ndarray dimension passes a test implemented by the predicate function, the corresponding element in the returned ndarray is -1.

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

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

// Create an input ndarray:
var x = array( [ 1.0, 3.0, 5.0, 7.0 ] );
// returns <ndarray>

// Perform operation:
var out = findIndex( x, isEven );
// returns <ndarray>

var idx = out.get();
// returns -1

By default, the function performs the operation over elements in the last dimension. To perform the operation over a different dimension, provide a dim option.

var ndarray2array = require( '@stdlib/ndarray-to-array' );
var array = require( '@stdlib/ndarray-array' );

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

var x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );

var opts = {
    'dim': 0
};

var out = findIndex( x, opts, isEven );
// returns <ndarray>

var idx = ndarray2array( out );
// returns [ -1, 0 ]

By default, the function excludes reduced dimensions from the output ndarray. To include the reduced dimensions as singleton dimensions, set the keepdims option to true.

var array = require( '@stdlib/ndarray-array' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );

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

var x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );

var opts = {
    'dim': 0,
    'keepdims': true
};

var out = findIndex( x, opts, isEven );
// returns <ndarray>

var idx = ndarray2array( out );
// returns [ [ -1, 0 ] ]

By default, the function returns an ndarray having a data type determined by the function's output data type policy. To override the default behavior, set the dtype option.

var ndarray2array = require( '@stdlib/ndarray-to-array' );
var dtype = require( '@stdlib/ndarray-dtype' );
var array = require( '@stdlib/ndarray-array' );

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

var x = array( [ 1.0, 2.0, 3.0, 4.0 ] );

var opts = {
    'dtype': 'generic'
};

var idx = findIndex( x, opts, isEven );
// returns <ndarray>

var dt = dtype( idx );
// returns 'generic'

findIndex.assign( x, out[, options], clbk[, thisArg] )

Returns the index of the first element along an ndarray dimension which passes a test implemented by a predicate function and assigns results to a provided output ndarray.

var array = require( '@stdlib/ndarray-array' );
var zeros = require( '@stdlib/ndarray-zeros' );

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

var x = array( [ 1.0, 2.0, 3.0, 4.0 ] );
var y = zeros( [], {
    'dtype': 'int32'
});

var out = findIndex.assign( x, y, isEven );
// returns <ndarray>

var idx = out.get();
// returns 1

var bool = ( out === y );
// returns true

The method has the following parameters:

• x: input ndarray. Must have at least one dimension.
• out: output ndarray.
• options: function options (optional).
• clbk: callback function.
• thisArg: callback execution context (optional).

The method accepts the following options:

• dim: dimension over which to perform operation. If provided a negative integer, the dimension along which to perform the operation is determined by counting backward from the last dimension (where -1 refers to the last dimension). Default: -1.

Notes

• A provided callback function should return a boolean.
• Setting the keepdims option to true can be useful when wanting to ensure that the output ndarray is broadcast-compatible with ndarrays having the same shape as the input ndarray.
• The output data type policy only applies to the main function and specifies that, by default, the function must return an ndarray having an integer index or "generic" data type. For the assign method, the output ndarray is allowed to have any supported output data type.

Examples

var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var ndarray = require( '@stdlib/ndarray-ctor' );
var findIndex = require( '@stdlib/blas-ext-find-index' );

// Define a callback function:
function isEven( v ) {
    return v % 2.0 === 0.0;
}

// Generate an array of random numbers:
var xbuf = discreteUniform( 10, 0, 20, {
    'dtype': 'generic'
});

// Wrap in an ndarray:
var x = new ndarray( 'generic', xbuf, [ 5, 2 ], [ 2, 1 ], 0, 'row-major' );
console.log( ndarray2array( x ) );

var opts = {
    'dim': 0
};

// Perform operation:
var idx = findIndex( x, opts, isEven );

// Print the results:
console.log( ndarray2array( idx ) );

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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

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License

See LICENSE.

Copyright

Copyright © 2016-2025. The Stdlib Authors.