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# gwxsa

> Subtract a scalar constant from each element in a strided array `x` and assign the results to elements in a strided array `w`.

<section class="intro">

This BLAS extension implements the operation

<!-- <equation class="equation" label="eq:wxsa" align="center" raw="\mathbf{w} = \mathbf{x} - \alpha" alt="Equation for wxsa operation."> -->

<!-- </equation> -->

This API is complementary to the package [`@stdlib/blas/ext/base/gwapx`][@stdlib/blas/ext/base/gwapx].

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var gwxsa = require( '@stdlib/blas/ext/base/gwxsa' );
```

#### gwxsa( N, alpha, x, strideX, w, strideW )

Subtracts a scalar constant from each element in a strided array `x` and assigns the results to elements in a strided array `w`.

```javascript
var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
var w = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

gwxsa( x.length, 5.0, x, 1, w, 1 );
// w => [ -4.0, -3.0, -2.0, -1.0, 0.0 ]
```

The function has the following parameters:

-   **N**: number of indexed elements.
-   **alpha**: scalar constant.
-   **x**: input [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideX**: stride length for `x`.
-   **w**: output [`Array`][mdn-array] or [`typed array`][mdn-typed-array].
-   **strideW**: stride length for `w`.

The `N` and stride parameters determine which elements in the strided arrays are accessed at runtime. For example, to subtract `alpha` from every other element in `x` and assign the results to every other element in `w`:

```javascript
var x = [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ];
var w = [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ];

gwxsa( 3, 5.0, x, 2, w, 2 );
// w => [ -4.0, 0.0, -2.0, 0.0, 0.0, 0.0 ]
```

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 w0 = new Float64Array( [ 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 ] );

// Create offset views...
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var w1 = new Float64Array( w0.buffer, w0.BYTES_PER_ELEMENT*2 ); // start at 3rd element

gwxsa( 3, 5.0, x1, 1, w1, 1 );
// w0 => <Float64Array>[ 0.0, 0.0, -3.0, -2.0, -1.0, 0.0 ]
```

#### gwxsa.ndarray( N, alpha, x, strideX, offsetX, w, strideW, offsetW )

Subtracts a scalar constant from each element in a strided array `x` and assigns the results to elements in a strided array `w` using alternative indexing semantics.

```javascript
var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
var w = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

gwxsa.ndarray( x.length, 5.0, x, 1, 0, w, 1, 0 );
// w => [ -4.0, -3.0, -2.0, -1.0, 0.0 ]
```

The function has the following additional parameters:

-   **offsetX**: starting index for `x`.
-   **offsetW**: starting index for `w`.

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 subtract `alpha` from the last three elements of `x` and assign the results to the last three elements of `w`:

```javascript
var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
var w = [ 0.0, 0.0, 0.0, 0.0, 0.0 ];

gwxsa.ndarray( 3, 5.0, x, 1, x.length-3, w, 1, w.length-3 );
// w => [ 0.0, 0.0, -2.0, -1.0, 0.0 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions return `w` unchanged.
-   Both functions support array-like objects having getter and setter accessors for array element access (e.g., [`@stdlib/array/base/accessor`][@stdlib/array/base/accessor]).

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var gwxsa = require( '@stdlib/blas/ext/base/gwxsa' );

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

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

gwxsa( x.length, 5.0, x, 1, w, 1 );
console.log( w );
```

</section>

<!-- /.examples -->

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

<section class="related">

</section>

<!-- /.related -->

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<section class="links">

[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

[@stdlib/array/base/accessor]: https://www.npmjs.com/package/@stdlib/array-base-accessor

[@stdlib/blas/ext/base/gwapx]: https://github.com/stdlib-js/blas/tree/main/ext/base/gwapx

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

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

</section>

<!-- /.links -->
