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drev

Reverse a double-precision floating-point strided array in-place.

Installation

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

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

drev( N, x, strideX )

Reverses a double-precision floating-point strided array x in-place.

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

var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

drev( x.length, x, 1 );
// x => <Float64Array>[ -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 Float64Array.
• strideX: stride length for x.

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

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

var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

drev( 4, x, 2 );
// x => <Float64Array>[ -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' );

// 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

// Reverse every other element...
drev( 3, x1, 2 );
// x0 => <Float64Array>[ 1.0, -6.0, 3.0, -4.0, 5.0, -2.0 ]

drev.ndarray( N, x, strideX, offsetX )

Reverses a double-precision floating-point strided array x in-place using alternative indexing semantics.

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

var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

drev.ndarray( x.length, x, 1, 0 );
// x => <Float64Array>[ -3.0, -1.0, 0.0, 4.0, -5.0, 3.0, 1.0, -2.0 ]

The function has the following additional parameters:

• offsetX: starting index for x.

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 the strided array:

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

var x = new Float64Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );

drev.ndarray( 3, x, 1, x.length-3 );
// x => <Float64Array>[ 1.0, -2.0, 3.0, -6.0, 5.0, -4.0 ]

Notes

• If N <= 0, both functions return the strided array unchanged.
• 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.

Examples

var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var drev = require( '@stdlib/blas-ext-base-drev' );

var x = discreteUniform( 10, -100, 100, {
    'dtype': 'float64'
});
console.log( x );

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

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C APIs

Usage

#include "stdlib/blas/ext/base/drev.h"

stdlib_strided_drev( N, *X, strideX )

Reverses a double-precision floating-point strided array X in-place.

double x[] = { 1.0, 2.0, 3.0, 4.0 };

stdlib_strided_drev( 4, x, 1 );

The function accepts the following arguments:

• N: [in] CBLAS_INT number of indexed elements.
• X: [in] double* input array.
• strideX: [in] CBLAS_INT stride length for X.

double stdlib_strided_drev( const CBLAS_INT N, double *X, const CBLAS_INT strideX );

stdlib_strided_drev_ndarray( N, *X, strideX, offsetX )

Reverses a double-precision floating-point strided array X in-place using alternative indexing semantics.

double x[] = { 1.0, 2.0, 3.0, 4.0 };

stdlib_strided_drev_ndarray( 4, x, 1, 0 );

The function accepts the following arguments:

• N: [in] CBLAS_INT number of indexed elements.
• X: [in] double* input array.
• strideX: [in] CBLAS_INT stride length for X.
• offsetX: [in] CBLAS_INT starting index for X.

void stdlib_strided_drev_ndarray( const CBLAS_INT N, double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX );

Examples

#include "stdlib/blas/ext/base/drev.h"
#include <stdio.h>

int main( void ) {
    // Create a strided array:
    double x[] = { 1.0, -2.0, 3.0, -4.0, 5.0, -6.0, 7.0, -8.0 };

    // Specify the number of elements:
    const int N = 8;

    // Specify a stride:
    const int strideX = 1;

    // Reverse the array:
    stdlib_strided_drev( N, x, strideX );

    // Print the result:
    for ( int i = 0; i < 8; i++ ) {
        printf( "x[ %i ] = %lf\n", i, x[ i ] );
    }
}

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See Also

• @stdlib/blas-ext/base/grev: reverse a strided array in-place.
• @stdlib/blas-ext/base/srev: reverse a single-precision floating-point strided array in-place.

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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 © 2016-2024. The Stdlib Authors.