countIf

Count the number of elements in an ndarray which pass a test implemented by a predicate function.

Installation

npm install @stdlib/ndarray-base-count-if

Usage

var countIf = require( '@stdlib/ndarray-base-count-if' );

countIf( arrays, predicate[, thisArg] )

Counts the number of elements in an ndarray which pass a test implemented by a predicate function.

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

function clbk( value ) {
    return value > 0.0;
}

// Create a data buffer:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 0.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );

// Define the shape of the input array:
var shape = [ 3, 1, 2 ];

// Define the array strides:
var sx = [ 4, 4, 1 ];

// Define the index offset:
var ox = 0;

// Create the input ndarray-like object:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': shape,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};

// Perform operation:
var out = countIf( [ x ], clbk );
// returns 5

The function accepts the following arguments:

• arrays: array-like object containing an input ndarray.
• predicate: predicate function.
• thisArg: predicate function execution context (optional).

The provided ndarray should be an object with the following properties:

• dtype: data type.
• data: data buffer.
• shape: dimensions.
• strides: stride lengths.
• offset: index offset.
• order: specifies whether an ndarray is row-major (C-style) or column major (Fortran-style).

The predicate function is provided the following arguments:

• value: current array element.
• indices: current array element indices.
• arr: the input ndarray.

To set the predicate function execution context, provide a thisArg.

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

function clbk( value ) {
    this.count += 1;
    return value > 0.0;
}

// Create a data buffer:
var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 0.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );

// Define the shape of the input array:
var shape = [ 3, 1, 2 ];

// Define the array strides:
var sx = [ 4, 4, 1 ];

// Define the index offset:
var ox = 0;

// Create the input ndarray-like object:
var x = {
    'dtype': 'float64',
    'data': xbuf,
    'shape': shape,
    'strides': sx,
    'offset': ox,
    'order': 'row-major'
};

// Create a context object:
var ctx = {
    'count': 0
};

// Perform operation:
var out = countIf( [ x ], clbk, ctx );
// returns 5

var count = ctx.count;
// returns 6

Notes

• For very high-dimensional ndarrays which are non-contiguous, one should consider copying the underlying data to contiguous memory before performing the operation in order to achieve better performance.
• If provided an empty ndarray, the function returns 0.

Examples

var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray-base-to-array' );
var countIf = require( '@stdlib/ndarray-base-count-if' );

function clbk( value ) {
    return value > 0;
}

var x = {
    'dtype': 'generic',
    'data': discreteUniform( 10, -2, 10, {
        'dtype': 'generic'
    }),
    'shape': [ 5, 2 ],
    'strides': [ 2, 1 ],
    'offset': 0,
    'order': 'row-major'
};
console.log( ndarray2array( x.data, x.shape, x.strides, x.offset, x.order ) );

var out = countIf( [ x ], clbk );
console.log( out );

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.