drev

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

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 );

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: [inout] 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: [inout] 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 ] );
    }
}

See Also

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