filter

Return a shallow copy of an ndarray containing only those elements which pass a test implemented by a predicate function.

Installation

npm install @stdlib/ndarray-filter

Usage

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

filter( x[, options], predicate[, thisArg] )

Returns a shallow copy of an ndarray containing only those elements which pass a test implemented by a predicate function.

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

function predicate( z ) {
    return z > 6.0;
}

var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;

var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns <ndarray>

var y = filter( x, predicate );
// returns <ndarray>[ 8.0, 9.0, 10.0 ]

The function accepts the following arguments:

• x: input ndarray.
• options: function options (optional).
• predicate: predicate function.
• thisArg: predicate function execution context (optional).

The function accepts the following options:

• dtype: output ndarray data type. If not specified, the output ndarray data type is inferred from the input ndarray.
• order: index iteration order. By default, the function iterates over elements according to the layout order of the provided ndarray. Accordingly, for row-major input ndarrays, the last dimension indices increment fastest. For column-major input ndarrays, the first dimension indices increment fastest. To override the inferred order and ensure that indices increment in a specific manner, regardless of the input ndarray's layout order, explicitly set the iteration order. Note, however, that iterating according to an order which does not match that of the input ndarray may, in some circumstances, result in performance degradation due to cache misses. Must be either 'row-major' or 'column-major'.

By default, the output ndarray data type is inferred from the input ndarray. To return an ndarray with a different data type, specify the dtype option.

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

function predicate( z ) {
    return z > 6.0;
}

var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;

var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns <ndarray>

var opts = {
    'dtype': 'float32'
};
var y = filter( x, opts, predicate );
// returns <ndarray>[ 8.0, 9.0, 10.0 ]

var dt = dtype( y );
// returns 'float32'

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

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

function predicate( z ) {
    this.count += 1;
    return z > 6.0;
}

var buffer = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
var shape = [ 2, 3 ];
var strides = [ 6, 1 ];
var offset = 1;

var x = ndarray( 'float64', buffer, shape, strides, offset, 'row-major' );
// returns <ndarray>

var ctx = {
    'count': 0
};
var y = filter( x, predicate, ctx );
// returns <ndarray>[ 8.0, 9.0, 10.0 ]

var count = ctx.count;
// returns 6

The predicate function is provided the following arguments:

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

Notes

• The function does not perform explicit casting (e.g., from a real-valued floating-point number to a complex floating-point number). Any such casting should be performed prior to calling this function.
• The function always returns a one-dimensional ndarray.

Examples

var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );
var naryFunction = require( '@stdlib/utils-nary-function' );
var array = require( '@stdlib/ndarray-array' );
var isPositive = require( '@stdlib/assert-is-positive-number' ).isPrimitive;
var filter = require( '@stdlib/ndarray-filter' );

var buffer = discreteUniform( 10, -100, 100, {
    'dtype': 'generic'
});
var x = array( buffer, {
    'shape': [ 5, 2 ],
    'dtype': 'generic'
});
console.log( ndarray2array( x ) );

var y = filter( x, naryFunction( isPositive, 1 ) );
console.log( ndarray2array( y ) );

See Also

• @stdlib/ndarray-filter-map: filter and map elements in an input ndarray to elements in a new output ndarray according to a callback function.
• @stdlib/ndarray-map: apply a callback to elements in an input ndarray and assign results to elements in a new output ndarray.
• @stdlib/ndarray-reject: return a shallow copy of an ndarray containing only those elements which fail a test implemented by a predicate function.
• @stdlib/ndarray-slice: return a read-only view of an input ndarray.

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.