time-to-botec/squiggle/node_modules/@stdlib/blas/base/gscal/README.md

gscal

Multiply a vector x by a constant alpha.

Usage

var gscal = require( '@stdlib/blas/base/gscal' );

gscal( N, alpha, x, stride )

Multiplies a vector x by a constant alpha.

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

gscal( x.length, 5.0, x, 1 );
// x => [ -10.0, 5.0, 15.0, -25.0, 20.0, 0.0, -5.0, -15.0 ]

The function has the following parameters:

• N: number of indexed elements.
• alpha: scalar constant.
• x: input Array or typed array.
• stride: index increment.

The N and stride parameters determine which elements in x are accessed at runtime. For example, to multiply every other value by a constant

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 );
var alpha = 5.0;
var stride = 2;

gscal( N, alpha, x, stride );
// x => [ -10.0, 1.0, 15.0, -5.0, 20.0, 0.0, -5.0, -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 );
var alpha = 5.0;
var stride = 2;

// Scale every other value...
gscal( N, alpha, x1, stride );
// x0 => <Float64Array>[ 1.0, -10.0, 3.0, -20.0, 5.0, -30.0 ]

If either N or stride is less than or equal to 0, the function returns x unchanged.

gscal.ndarray( N, alpha, x, stride, offset )

Multiplies a vector x by a constant alpha using alternative indexing semantics.

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

gscal.ndarray( x.length, 5.0, x, 1, 0 );
// x => [ -10.0, 5.0, 15.0, -25.0, 20.0, 0.0, -5.0, -15.0 ]

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 multiply the last three elements of x by a constant

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

gscal.ndarray( 3, alpha, x, 1, x.length-3 );
// x => [ 1.0, -2.0, 3.0, -20.0, 25.0, -30.0 ]

Notes

• If N <= 0, both functions return x unchanged.
• gscal() corresponds to the BLAS level 1 function dscal with the exception that this implementation works with any array type, not just Float64Arrays. Depending on the environment, the typed versions (dscal, sscal, etc.) are likely to be significantly more performant.

Examples

var round = require( '@stdlib/math/base/special/round' );
var randu = require( '@stdlib/random/base/randu' );
var Float64Array = require( '@stdlib/array/float64' );
var gscal = require( '@stdlib/blas/base/gscal' );

var rand;
var sign;
var x;
var i;

x = new Float64Array( 100 );
for ( i = 0; i < x.length; i++ ) {
    rand = round( randu()*100.0 );
    sign = randu();
    if ( sign < 0.5 ) {
        sign = -1.0;
    } else {
        sign = 1.0;
    }
    x[ i ] = sign * rand;
}
console.log( x );

gscal( x.length, 5.0, x, 1 );
console.log( x );