# gfillBy
> Fill a strided array according to a provided callback function.
## Usage
```javascript
var gfillBy = require( '@stdlib/blas/ext/base/gfill-by' );
```
#### gfillBy( N, x, stride, clbk\[, thisArg] )
Fills a strided array `x` 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`][mdn-array], [`typed array`][mdn-typed-array], or an array-like object (excluding strings and functions).
- **stride**: index increment.
- **clbk**: callback function.
- **thisArg**: execution context (_optional_).
The invoked callback is provided four arguments:
- **value**: array element.
- **aidx**: array index.
- **sidx**: strided index (`offset + aidx*stride`).
- **array**: input array/collection.
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 `x` are accessed at runtime. For example, to fill every other element
```javascript
var floor = require( '@stdlib/math/base/special/floor' );
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 ];
var N = floor( x.length / 2 );
gfillBy( N, 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' );
var floor = require( '@stdlib/math/base/special/floor' );
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
var N = floor( x0.length/2 );
// Fill every other element...
gfillBy( N, x1, 2, fill );
// x0 => [ 1.0, 0.0, 3.0, -4.0, 5.0, -12.0 ]
```
#### gfillBy.ndarray( N, x, stride, offset, clbk\[, thisArg] )
Fills a strided array `x` 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:
- **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 access only the last three elements of `x`
```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 ]
```
## Notes
- If `N <= 0`, both functions return `x` unchanged.
- 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.
## Examples
```javascript
var round = require( '@stdlib/math/base/special/round' );
var randu = require( '@stdlib/random/base/randu' );
var Float64Array = require( '@stdlib/array/float64' );
var gfillBy = require( '@stdlib/blas/ext/base/gfill-by' );
function fill() {
var rand = round( randu()*100.0 );
var sign = randu();
if ( sign < 0.5 ) {
sign = -1.0;
} else {
sign = 1.0;
}
return sign * rand;
}
var x = new Float64Array( 10 );
console.log( x );
gfillBy( x.length, x, 1, fill );
console.log( x );
```
[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/blas/ext/base/dfill]: https://www.npmjs.com/package/@stdlib/blas/tree/main/ext/base/dfill
[@stdlib/blas/ext/base/gfill]: https://www.npmjs.com/package/@stdlib/blas/tree/main/ext/base/gfill
[@stdlib/blas/ext/base/sfill]: https://www.npmjs.com/package/@stdlib/blas/tree/main/ext/base/sfill