dscal

Multiply a double-precision floating-point vector x by a constant alpha.

Usage

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

dscal( N, alpha, x, stride )

Multiplies a double-precision floating-point vector x by a constant alpha.

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 ] );

dscal( x.length, 5.0, x, 1 );
// x => <Float64Array>[ -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 Float64Array.
  • 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 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 ] );

dscal( 4, 5.0, x, 2 );
// x => <Float64Array>[ -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' );

// 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

// Scale every other value...
dscal( 3, 5.0, x1, 2 );
// x0 => <Float64Array>[ 1.0, -10.0, 3.0, -20.0, 5.0, -30.0 ]

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

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

Multiplies a double-precision floating-point vector x by a constant alpha 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 ] );

dscal.ndarray( x.length, 5.0, x, 1, 0 );
// x => <Float64Array>[ -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 Float64Array = require( '@stdlib/array/float64' );

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

dscal.ndarray( 3, 5.0, x, 1, x.length-3 );
// x => <Float64Array>[ 1.0, -2.0, 3.0, -20.0, 25.0, -30.0 ]

Notes

  • If N <= 0, both functions return x unchanged.
  • dscal() corresponds to the BLAS level 1 function dscal.

Examples

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

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

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

C APIs

Usage

#include "stdlib/blas/base/dscal.h"

c_dscal( N, alpha, *X, stride )

Multiplies each element of a double-precision floating-point vector by a constant.

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

c_dscal( 4, 5.0, x, 1 );

The function accepts the following arguments:

  • N: [in] CBLAS_INT number of indexed elements.
  • alpha: [in] double scalar constant.
  • X: [inout] double* input array.
  • stride: [in] CBLAS_INT index increment for X.
void c_dscal( const CBLAS_INT N, const double alpha, double *X, const CBLAS_INT stride );

c_dscal_ndarray( N, alpha, *X, stride, offset )

Multiplies each element of a double-precision floating-point vector by a constant using alternative indexing semantics.

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

c_dscal_ndarray( 4, 5.0, x, 1, 0 );

The function accepts the following arguments:

  • N: [in] CBLAS_INT number of indexed elements.
  • alpha: [in] double scalar constant.
  • X: [inout] double* input array.
  • stride: [in] CBLAS_INT index increment for X.
  • offset: [in] CBLAS_INT starting index for X.
void c_dscal_ndarray( const CBLAS_INT N, const double alpha, double *X, const CBLAS_INT stride, const CBLAS_INT offset );

Examples

#include "stdlib/blas/base/dscal.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 stride = 1;

    // Scale the vector:
    c_dscal( N, 5.0, x, stride );

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

    // Scale the vector:
    c_dscal_ndarray( N, 5.0, x, -stride, N-1 );

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

See Also

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