@stdlib/blas-base-dswap/README.md

Interchange two double-precision floating-point vectors.

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# dswap

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> Interchange two double-precision floating-point vectors.

<section class="installation">

## Installation

```bash
npm install @stdlib/blas-base-dswap
```

</section>

<section class="usage">

## 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 => <Float64Array>[ 6.0, 7.0, 8.0, 9.0, 10.0 ]
// y => <Float64Array>[ 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 => <Float64Array>[ 9.0, 2.0, 8.0, 4.0, 7.0, 6.0 ]
// y => <Float64Array>[ 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.

<!-- eslint-disable stdlib/capitalized-comments -->

```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 => <Float64Array>[ 1.0, 12.0, 3.0, 11.0, 5.0, 10.0 ]
// y0 => <Float64Array>[ 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 => <Float64Array>[ 6.0, 7.0, 8.0, 9.0, 10.0 ]
// y => <Float64Array>[ 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 => <Float64Array>[ 1.0, 12.0, 3.0, 11.0, 5.0, 10.0 ]
// y => <Float64Array>[ 7.0, 8.0, 9.0, 6.0, 4.0, 2.0 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions leave `x` and `y` unchanged.
-   `dswap()` corresponds to the [BLAS][blas] level 1 function [`dswap`][dswap].

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```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 );
```

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- C usage documentation. -->

<section class="usage">

### 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 };
double y[] = { 0.0, 0.0, 0.0, 0.0 };

c_dswap( 4, 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*` first 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 );
```

</section>

<!-- /.usage -->

<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

```c
#include "stdlib/blas/base/dswap.h"
#include <stdio.h>

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 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 ] );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/blas-base/dcopy`][@stdlib/blas/base/dcopy]</span><span class="delimiter">: </span><span class="description">copy values from x into y.</span>
-   <span class="package-name">[`@stdlib/blas-base/gswap`][@stdlib/blas/base/gswap]</span><span class="delimiter">: </span><span class="description">interchange two vectors.</span>
-   <span class="package-name">[`@stdlib/blas-base/sswap`][@stdlib/blas/base/sswap]</span><span class="delimiter">: </span><span class="description">interchange two single-precision floating-point vectors.</span>
-   <span class="package-name">[`@stdlib/blas-dswap`][@stdlib/blas/dswap]</span><span class="delimiter">: </span><span class="description">interchange two double-precision floating-point vectors.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

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---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2024. The Stdlib [Authors][stdlib-authors].

</section>

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/blas-base-dswap/main/LICENSE

[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

<!-- <related-links> -->

[@stdlib/blas/base/dcopy]: https://www.npmjs.com/package/@stdlib/blas-base-dcopy

[@stdlib/blas/base/gswap]: https://www.npmjs.com/package/@stdlib/blas-base-gswap

[@stdlib/blas/base/sswap]: https://www.npmjs.com/package/@stdlib/blas-base-sswap

[@stdlib/blas/dswap]: https://www.npmjs.com/package/@stdlib/blas-dswap

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</section>

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