# dscal
> Multiply a double-precision floating-point vector `x` by a constant `alpha`.
## Usage
```javascript
var dscal = require( '@stdlib/blas/base/dscal' );
```
#### dscal( N, alpha, x, stride )
Multiplies a double-precision floating-point vector `x` by a constant `alpha`.
```javascript
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 => [ -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`][@stdlib/array/float64].
- **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
```javascript
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 => [ -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`][mdn-typed-array] views.
```javascript
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 => [ 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.
```javascript
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 => [ -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`][mdn-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
```javascript
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 => [ 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][blas] level 1 function [`dscal`][dscal].
## Examples
```javascript
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
```c
#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.
```c
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`.
```c
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.
```c
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`.
```c
void c_dscal_ndarray( const CBLAS_INT N, const double alpha, double *X, const CBLAS_INT stride, const CBLAS_INT offset );
```
### Examples
```c
#include "stdlib/blas/base/dscal.h"
#include
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 ] );
}
}
```
[blas]: http://www.netlib.org/blas
[dscal]: http://www.netlib.org/lapack/explore-html/de/da4/group__double__blas__level1.html
[@stdlib/array/float64]: https://www.npmjs.com/package/@stdlib/array-float64
[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
[@stdlib/blas/base/daxpy]: https://github.com/stdlib-js/blas/tree/main/base/daxpy
[@stdlib/blas/base/gscal]: https://github.com/stdlib-js/blas/tree/main/base/gscal
[@stdlib/blas/base/sscal]: https://github.com/stdlib-js/blas/tree/main/base/sscal
[@stdlib/blas/base/saxpy]: https://github.com/stdlib-js/blas/tree/main/base/saxpy