# dswap
> Interchange two double-precision floating-point vectors.
## Usage
```javascript
var dswap = require( '@stdlib/blas/base/dswap' );
```
#### dswap( N, x, strideX, y, strideY )
Interchanges vectors `x` and `y`.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0 ] );
var y = new Float64Array( [ 6.0, 7.0, 8.0, 9.0, 10.0 ] );
dswap( x.length, x, 1, y, 1 );
// x => [ 6.0, 7.0, 8.0, 9.0, 10.0 ]
// y => [ 1.0, 2.0, 3.0, 4.0, 5.0 ]
```
The function has the following parameters:
- **N**: number of indexed elements.
- **x**: first input [`Float64Array`][mdn-float64array].
- **strideX**: index increment for `x`.
- **y**: second input [`Float64Array`][mdn-float64array].
- **strideY**: index increment for `y`.
The `N` and stride parameters determine how values in the strided arrays are accessed at runtime. For example, to swap in reverse order every other value in `x` with the first `N` elements of `y`,
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
var y = new Float64Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
dswap( 3, x, -2, y, 1 );
// x => [ 9.0, 2.0, 8.0, 4.0, 7.0, 6.0 ]
// y => [ 5.0, 3.0, 1.0, 10.0, 11.0, 12.0 ]
```
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 arrays...
var x0 = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
var y0 = new Float64Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
// Create offset views...
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var y1 = new Float64Array( y0.buffer, y0.BYTES_PER_ELEMENT*3 ); // start at 4th element
// Swap in reverse order every other value from `x1` with `y1`...
dswap( 3, x1, -2, y1, 1 );
// x0 => [ 1.0, 12.0, 3.0, 11.0, 5.0, 10.0 ]
// y0 => [ 7.0, 8.0, 9.0, 6.0, 4.0, 2.0 ]
```
#### dswap.ndarray( N, x, strideX, offsetX, y, strideY, offsetY )
Interchanges vectors `x` and `y` using alternative indexing semantics.
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0 ] );
var y = new Float64Array( [ 6.0, 7.0, 8.0, 9.0, 10.0 ] );
dswap.ndarray( x.length, x, 1, 0, y, 1, 0 );
// x => [ 6.0, 7.0, 8.0, 9.0, 10.0 ]
// y => [ 1.0, 2.0, 3.0, 4.0, 5.0 ]
```
The function has the following additional parameters:
- **offsetX**: starting index for `x`.
- **offsetY**: starting index for `y`.
While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying buffer, the offset parameters support indexing semantics based on starting indices. For example, to swap every other value in `x` starting from the second value with the last `N` elements in `y` where `x[i] = y[n]`, `x[i+2] = y[n-1]`,...,
```javascript
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
var y = new Float64Array( [ 7.0, 8.0, 9.0, 10.0, 11.0, 12.0 ] );
dswap.ndarray( 3, x, 2, 1, y, -1, y.length-1 );
// x => [ 1.0, 12.0, 3.0, 11.0, 5.0, 10.0 ]
// y => [ 7.0, 8.0, 9.0, 6.0, 4.0, 2.0 ]
```
## Notes
- If `N <= 0`, both functions leave `x` and `y` unchanged.
- `dswap()` corresponds to the [BLAS][blas] level 1 function [`dswap`][dswap].
## Examples
```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var dswap = require( '@stdlib/blas/base/dswap' );
var opts = {
'dtype': 'float64'
};
var x = discreteUniform( 10, 0, 500, opts );
console.log( x );
var y = discreteUniform( x.length, 0, 255, opts );
console.log( y );
// Swap elements in `x` and `y` starting from the end of `y`:
dswap( x.length, x, 1, y, -1 );
console.log( x );
console.log( y );
```
* * *
## C APIs
### Usage
```c
#include "stdlib/blas/base/dswap.h"
```
#### c_dswap( N, \*X, strideX, \*Y, strideY )
Interchanges two double-precision floating-point vectors.
```c
double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0 };
double y[] = { 6.0, 7.0, 8.0, 9.0, 10.0 };
c_dswap( 5, x, 1, y, 1 );
```
The function accepts the following arguments:
- **N**: `[in] CBLAS_INT` number of indexed elements.
- **X**: `[inout] double*` first input array.
- **strideX**: `[in] CBLAS_INT` index increment for `X`.
- **Y**: `[inout] double*` second input array.
- **strideY**: `[in] CBLAS_INT` index increment for `Y`.
```c
void c_dswap( const CBLAS_INT N, double *X, const CBLAS_INT strideX, double *Y, const CBLAS_INT strideY );
```
#### c_dswap_ndarray( N, \*X, strideX, offsetX, \*Y, strideY, offsetY )
Interchanges two double-precision floating-point vectors using alternative indexing semantics.
```c
double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0 };
double y[] = { 6.0, 7.0, 8.0, 9.0, 10.0 };
c_dswap_ndarray( 3, x, 1, 2, y, 1, 2 );
```
The function accepts the following arguments:
- **N**: `[in] CBLAS_INT` number of indexed elements.
- **X**: `[inout] double*` first input array.
- **strideX**: `[in] CBLAS_INT` index increment for `X`.
- **offsetX**: `[in] CBLAS_INT` starting index for `X`.
- **Y**: `[inout] double*` second input array.
- **strideY**: `[in] CBLAS_INT` index increment for `Y`.
- **offsetY**: `[in] CBLAS_INT` starting index for `Y`.
```c
void c_dswap_ndarray( const CBLAS_INT N, double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX, double *Y, const CBLAS_INT strideY, const CBLAS_INT offsetY );
```
### Examples
```c
#include "stdlib/blas/base/dswap.h"
#include
int main( void ) {
// Create strided arrays:
double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 };
double y[] = { 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 };
// Specify the number of indexed elements:
const int N = 4;
// Specify stride lengths:
const int strideX = 2;
const int strideY = -2;
// Interchange elements:
c_dswap( N, x, strideX, y, strideY );
// Print the result:
for ( int i = 0; i < 8; i++ ) {
printf( "x[ %i ] = %lf\n", i, x[ i ] );
printf( "y[ %i ] = %lf\n", i, y[ i ] );
}
// Interchange elements:
c_dswap_ndarray( N, x, strideX, 0, y, strideY, 6 );
// Print the result:
for ( int i = 0; i < 8; i++ ) {
printf( "x[ %i ] = %lf\n", i, x[ i ] );
printf( "y[ %i ] = %lf\n", i, y[ i ] );
}
}
```
[blas]: http://www.netlib.org/blas
[dswap]: http://www.netlib.org/lapack/explore-html/de/da4/group__double__blas__level1.html
[mdn-float64array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Float64Array
[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray