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# gfillBy

> Fill a strided array according to a provided callback function.

<section class="usage">

## Usage

```javascript
var gfillBy = require( '@stdlib/blas/ext/base/gfill-by' );
```

#### gfillBy( N, x, strideX, clbk\[, thisArg] )

Fills a strided array according to a provided callback function.

```javascript
function fill( v, i ) {
    return v * i;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

gfillBy( x.length, x, 1, fill );
// x => [ 0.0, 1.0, 6.0, -15.0, 16.0, 0.0, -6.0, -21.0 ]
```

The function has the following parameters:

-   **N**: number of indexed elements.
-   **x**: input array.
-   **strideX**: stride length.
-   **clbk**: callback function.
-   **thisArg**: execution context (_optional_).

The callback function is provided the following arguments:

-   **value**: current array element.
-   **aidx**: array index.
-   **sidx**: strided index (`offset + aidx*stride`).
-   **array**: the input array.

To set the callback execution context, provide a `thisArg`.

```javascript
function fill( v, i ) {
    this.count += 1;
    return v * i;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

var context = {
    'count': 0
};

gfillBy( x.length, x, 1, fill, context );
// x => [ 0.0, 1.0, 6.0, -15.0, 16.0, 0.0, -6.0, -21.0 ]

var cnt = context.count;
// returns 8
```

The `N` and stride parameters determine which elements in the strided array are accessed at runtime. For example, to fill every other element:

```javascript
function fill( v, i ) {
    return v * i;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

gfillBy( 4, x, 2, fill );
// x => [ 0.0, 1.0, 3.0, -5.0, 8.0, 0.0, -3.0, -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' );

function fill( v, i ) {
    return v * i;
}

// 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

// Fill every other element...
gfillBy( 3, x1, 2, fill );
// x0 => <Float64Array>[ 1.0, 0.0, 3.0, -4.0, 5.0, -12.0 ]
```

#### gfillBy.ndarray( N, x, strideX, offsetX, clbk\[, thisArg] )

Fills a strided array according to a provided callback function and using alternative indexing semantics.

```javascript
function fill( v, i ) {
    return v * i;
}

var x = [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

gfillBy.ndarray( x.length, x, 1, 0, fill );
// x => [ 0.0, 1.0, 6.0, -15.0, 16.0, 0.0, -6.0, -21.0 ]
```

The function has the following additional parameters:

-   **offsetX**: 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 access only the last three elements:

```javascript
function fill( v, i ) {
    return v * i;
}

var x = [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ];

gfillBy.ndarray( 3, x, 1, x.length-3, fill );
// x => [ 1.0, -2.0, 3.0, 0.0, 5.0, -12.0 ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions return `x` unchanged.
-   Both functions support array-like objects having getter and setter accessors for array element access (e.g., [`@stdlib/array/complex64`][@stdlib/array/complex64]).
-   When filling a strided array with a scalar constant, prefer using [`dfill`][@stdlib/blas/ext/base/dfill], [`sfill`][@stdlib/blas/ext/base/sfill], and/or [`gfill`][@stdlib/blas/ext/base/gfill], as, depending on the environment, these interfaces are likely to be significantly more performant.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
var Float64Array = require( '@stdlib/array/float64' );
var gfillBy = require( '@stdlib/blas/ext/base/gfill-by' );

var x = new Float64Array( 10 );
console.log( x );

gfillBy( x.length, x, 1, discreteUniform( -100, 100 ) );
console.log( x );
```

</section>

<!-- /.examples -->

<!-- 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/ext/base/gfill`][@stdlib/blas/ext/base/gfill]</span><span class="delimiter">: </span><span class="description">fill a strided array with a specified scalar constant.</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="links">

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

[@stdlib/array/complex64]: https://www.npmjs.com/package/@stdlib/array-complex64

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

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

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

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

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

</section>

<!-- /.links -->
