# fillBy
> Fill an input [ndarray][@stdlib/ndarray/ctor] according to a callback function.
## Usage
```javascript
var fillBy = require( '@stdlib/ndarray/fill-by' );
```
#### fillBy( x, fcn\[, thisArg] )
Fills an input [ndarray][@stdlib/ndarray/ctor] according to a callback function.
```javascript
var zeros = require( '@stdlib/ndarray/zeros' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );
function fcn( value ) {
return value + 10.0;
}
var x = zeros( [ 3, 1, 2 ], {
'dtype': 'float64'
});
var y = fillBy( x, fcn );
// returns
var bool = ( y === x );
// returns true
var arr = ndarray2array( y );
// returns [ [ [ 10.0, 10.0 ] ], [ [ 10.0, 10.0 ] ], [ [ 10.0, 10.0 ] ] ]
```
The function accepts the following arguments:
- **x**: array-like object containing an input [ndarray][@stdlib/ndarray/ctor].
- **fcn**: callback function.
- **thisArg**: callback function execution context (_optional_).
To set the callback function execution context, provide a `thisArg`.
```javascript
var zeros = require( '@stdlib/ndarray/zeros' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );
function fcn( value ) {
return value + this.factor;
}
var x = zeros( [ 3, 1, 2 ], {
'dtype': 'float64'
});
var ctx = {
'factor': 10.0
};
var y = fillBy( x, fcn, ctx );
// returns
var arr = ndarray2array( y );
// returns [ [ [ 10.0, 10.0 ] ], [ [ 10.0, 10.0 ] ], [ [ 10.0, 10.0 ] ] ]
```
The callback function is provided the following arguments:
- **value**: current array element.
- **indices**: current array element indices.
- **arr**: the input [ndarray][@stdlib/ndarray/ctor].
## Notes
- An input [ndarray][@stdlib/ndarray/ctor] **must** be writable. If provided a **read-only** [ndarray][@stdlib/ndarray/ctor], the function throws an error.
- The function **mutates** the input [ndarray][@stdlib/ndarray/ctor].
- The function assumes that each element in the underlying input [ndarray][@stdlib/ndarray/ctor] data buffer has one, and only one, corresponding element in input [ndarray][@stdlib/ndarray/ctor] view (i.e., a provided [ndarray][@stdlib/ndarray/ctor] is not a broadcasted [ndarray][@stdlib/ndarray/ctor] view).
## Examples
```javascript
var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var zeros = require( '@stdlib/ndarray/zeros' );
var fillBy = require( '@stdlib/ndarray/fill-by' );
// Create a zero-filled ndarray:
var x = zeros( [ 5, 2 ], {
'dtype': 'generic'
});
console.log( ndarray2array( x ) );
// Fill the ndarray with random values:
fillBy( x, discreteUniform( -100, 100 ) );
console.log( ndarray2array( x ) );
```
[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/ndarray/tree/main/ctor
[@stdlib/ndarray/fill]: https://github.com/stdlib-js/ndarray/tree/main/fill
[@stdlib/ndarray/map]: https://github.com/stdlib-js/ndarray/tree/main/map
[@stdlib/ndarray/zeros]: https://github.com/stdlib-js/ndarray/tree/main/zeros