About stdlib...

We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.

The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.

When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.

To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!

filterMap

Filter and map elements in an input ndarray to elements in a new output ndarray according to a callback function.

Installation

npm install @stdlib/ndarray-filter-map

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

filterMap( x[, options], fcn[, thisArg] )

Filters and maps elements in an input ndarray to elements in a new output ndarray according to a callback function.

var Float64Array = require( '@stdlib/array-float64' );
var ndarray = require( '@stdlib/ndarray-ctor' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );

function fcn( z ) {
    if ( z > 5.0 ) {
        return z * 10.0;
    }
}

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 = filterMap( x, fcn );
// returns <ndarray>

var arr = ndarray2array( y );
// returns [ 80.0, 90.0, 100.0 ]

The function accepts the following arguments:

• x: input ndarray.
• options: function options (optional).
• fcn: callback function.
• thisArg: callback 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 manor, 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' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );

function fcn( z ) {
    if ( z > 5.0 ) {
        return z * 10.0;
    }
}

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 = filterMap( x, opts, fcn );
// returns <ndarray>

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

var arr = ndarray2array( y );
// returns [ 80.0, 90.0, 100.0 ]

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

var Float64Array = require( '@stdlib/array-float64' );
var ndarray = require( '@stdlib/ndarray-ctor' );
var ndarray2array = require( '@stdlib/ndarray-to-array' );

function fcn( z ) {
    this.count += 1;
    if ( z > 5.0 ) {
        return z * 10.0;
    }
}

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 = filterMap( x, fcn, ctx );
// returns <ndarray>

var arr = ndarray2array( y );
// returns [ 80.0, 90.0, 100.0 ]

var count = ctx.count;
// returns 6

The callback 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 by a provided callback function.

  var Float64Array = require( '@stdlib/array-float64' );
  var ndarray = require( '@stdlib/ndarray-ctor' );
  var Complex128 = require( '@stdlib/complex-float64-ctor' );
  var ndarray2array = require( '@stdlib/ndarray-to-array' );
  
  function fcn( z ) {
      if ( z > 5.0 ) {
          return new Complex128( z, 0.0 );
      }
  }
  
  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': 'complex128'
  };
  var y = filterMap( x, opts, fcn );
  // returns <ndarray>

• If a provided callback function returns undefined, the function skips the respective ndarray element. If the callback function returns a value other than undefined, the function stores the callback's return value in the output ndarray.

• 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 array = require( '@stdlib/ndarray-array' );
var filterMap = require( '@stdlib/ndarray-filter-map' );

function fcn( v ) {
    if ( v > 0 ) {
        return v * 100;
    }
}

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

var y = filterMap( x, fcn );
console.log( ndarray2array( y ) );

---

See Also

• @stdlib/ndarray-filter: return a shallow copy of an ndarray containing only those elements which pass a test implemented by a predicate 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 © 2016-2024. The Stdlib Authors.