@stdlib/blas-ext-base-grev

Reverse a strided array in-place.

Usage no npm install needed!

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README

grev

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Reverse a strided array in-place.

Installation

npm install @stdlib/blas-ext-base-grev

Usage

var grev = require( '@stdlib/blas-ext-base-grev' );

grev( N, x, stride )

Reverses a strided array x in-place.

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

grev( x.length, x, 1 );
// x => [ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ]

The function has the following parameters:

  • N: number of indexed elements.
  • x: input array.
  • stride: index increment.

The N and stride parameters determine which elements in x are accessed at runtime. For example, to reverse every other element

var floor = require( '@stdlib/math-base-special-floor' );

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];
var N = floor( x.length / 2 );

grev( N, x, 2 );
// x => [ -1.0, 1.0, 4.0, -5.0, 3.0, 0.0, -2.0, -3.0 ]

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

var Float64Array = require( '@stdlib/array-float64' );
var floor = require( '@stdlib/math-base-special-floor' );

// 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 N = floor( x0.length/2 );

// Reverse every other element...
grev( N, x1, 2 );
// x0 => <Float64Array>[ 1.0, -6.0, 3.0, -4.0, 5.0, -2.0 ]

grev.ndarray( N, x, stride, offset )

Reverses a strided array x in-place using alternative indexing semantics.

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

grev.ndarray( x.length, x, 1, 0 );
// x => [ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ]

The function has the following additional parameters:

  • offset: 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 of x

var x = [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ];

grev.ndarray( 3, x, 1, x.length-3 );
// x => [ 1.0, -2.0, 3.0, -6.0, 5.0, -4.0 ]

Notes

  • If N <= 0, both functions return x unchanged.
  • Both functions support array-like objects having getter and setter accessors for array element access (e.g., @stdlib/array/complex64).
  • Where possible, one should "reverse" a strided array by negating its stride, which is an O(1) operation, in contrast to performing an in-place reversal, which is O(N). However, in certain circumstances, this is not tenable, particularly when interfacing with libraries which assume and/or expect a specific memory layout (e.g., strided array elements arranged in memory in ascending order). In general, when working with strided arrays, only perform an in-place reversal when strictly necessary.
  • Depending on the environment, the typed versions (drev, srev, etc.) are likely to be significantly more performant.

Examples

var discreteUniform = require( '@stdlib/random-base-discrete-uniform' ).factory;
var Float64Array = require( '@stdlib/array-float64' );
var gfillBy = require( '@stdlib/blas-ext-base-gfill-by' );
var grev = require( '@stdlib/blas-ext-base-grev' );

var x = gfillBy( 10, new Float64Array( 10 ), 1, discreteUniform( -100, 100 ) );
console.log( x );

grev( x.length, x, 1 );
console.log( x );

See Also

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.

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License

See LICENSE.

Copyright

Copyright © 2016-2022. The Stdlib Authors.