grev
Reverse a strided array in-place.
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 returnx
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 isO(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 );