@stdlib/utils
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Standard utilities.
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# mapRight
> Apply a function to each element in an array and assign the result to an element in an output array, iterating from right to left.
<!-- 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 -->
<!-- Package usage documentation. -->
<section class="usage">
## Usage
```javascript
var mapRight = require( '@stdlib/utils/map-right' );
```
<a name="fcn-map-right"></a>
#### mapRight( arr, fcn\[, thisArg] )
Applies a function to each element in an array and assigns the result to an element in a new array, iterating from right to left.
```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var abs = require( '@stdlib/math/base/special/abs' );
var arr = [ -1, -2, -3, -4, -5, -6 ];
var out = mapRight( arr, naryFunction( abs, 1 ) );
// returns [ 1, 2, 3, 4, 5, 6 ]
```
The function accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.
```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var abs = require( '@stdlib/math/base/special/abs' );
var array = require( '@stdlib/ndarray/array' );
var opts = {
'dtype': 'generic'
};
var arr = array( [ [ -1, -2, -3 ], [ -4, -5, -6 ] ], opts );
var out = mapRight( arr, naryFunction( abs, 1 ) );
// returns <ndarray>
var v = out.get( 1, 1 );
// returns 5
```
The applied function is provided the following arguments:
- **value**: array element.
- **index**: element index.
- **arr**: input array.
To set the `this` context when invoking the input function, provide a `thisArg`.
<!-- eslint-disable no-invalid-this -->
```javascript
var abs = require( '@stdlib/math/base/special/abs' );
function fcn( v ) {
this.count += 1;
return abs( v );
}
var arr = [ -1, -2, -3, -4, -5, -6 ];
var ctx = {
'count': 0
};
var out = mapRight( arr, fcn, ctx );
// returns [ 1, 2, 3, 4, 5, 6 ]
var cnt = ctx.count;
// returns 6
```
<a name="method-map-right-assign"></a>
#### mapRight.assign( arr, out, fcn\[, thisArg] )
Applies a function to each element in an array and assigns the result to an element in an output array, iterating from right to left.
```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var abs = require( '@stdlib/math/base/special/abs' );
var arr = [ -1, -2, -3, -4, -5, -6 ];
var out = [ 0, 0, 0, 0, 0, 0 ];
mapRight.assign( arr, out, naryFunction( abs, 1 ) );
console.log( out );
// => [ 1, 2, 3, 4, 5, 6 ]
```
The method accepts both array-like objects and [`ndarray`][@stdlib/ndarray/ctor]-like objects.
```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var abs = require( '@stdlib/math/base/special/abs' );
var array = require( '@stdlib/ndarray/array' );
var opts = {
'dtype': 'generic',
'shape': [ 2, 3 ]
};
var arr = array( [ [ -1, -2, -3 ], [ -4, -5, -6 ] ], opts );
var out = array( opts );
mapRight.assign( arr, out, naryFunction( abs, 1 ) );
var v = out.get( 1, 1 );
// returns 5
```
Input and output arrays must be either both array-like objects or both [`ndarray`][@stdlib/ndarray/ctor]-like objects. If input and output arrays are both array-like objects, both arrays **must** have the same number of elements.
If input and output arrays are both [`ndarray`][@stdlib/ndarray/ctor]-like objects, the arrays **must** be [broadcast compatible][@stdlib/ndarray/base/broadcast-shapes]. To map from an input [`ndarray`][@stdlib/ndarray/ctor] to an output [`ndarray`][@stdlib/ndarray/ctor] which has the same rank (i.e., dimensionality) and the same number of elements, but which is not [broadcast compatible][@stdlib/ndarray/base/broadcast-shapes], reshape the arrays prior to invocation.
```javascript
var naryFunction = require( '@stdlib/utils/nary-function' );
var abs = require( '@stdlib/math/base/special/abs' );
var array = require( '@stdlib/ndarray/array' );
var opts = {
'dtype': 'generic',
'shape': [ 2, 3 ]
};
var arr = array( [ [ -1, -2, -3 ], [ -4, -5, -6 ] ], opts );
opts = {
'dtype': 'generic',
'shape': [ 2, 2, 3 ] // broadcast compatible shape
};
var out = array( opts );
mapRight.assign( arr, out, naryFunction( abs, 1 ) );
var v = out.get( 0, 1, 1 );
// returns 5
v = out.get( 1, 1, 1 );
// returns 5
```
In general, avoid providing output [`ndarray`][@stdlib/ndarray/ctor]-like objects which are [non-contiguous][@stdlib/ndarray/base/assert/is-contiguous] views containing one or more elements referring to the **same** memory location. Writing to an overlapping [non-contiguous][@stdlib/ndarray/base/assert/is-contiguous] view is likely to simultaneously affect multiple elements and yield unexpected results.
The applied function is provided the same arguments as with [`mapRight`](#fcn-map-right).
</section>
<!-- /.usage -->
<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
<section class="notes">
## Notes
- The [`mapRight`](#fcn-map-right) function **always** returns an output value having a "generic" data type. For example, if provided an array-like object, the function returns a generic `array`. If provided an [`ndarray`][@stdlib/ndarray/ctor], the function returns an [`ndarray`][@stdlib/ndarray/ctor] having a "generic" data type.
Accordingly, in contrast to [`TypedArray.prototype.map()`][mdn-typedarray-map], when provided a typed array, the [`mapRight`](#fcn-map-right) function does **not** return a typed array of the same type. To assign results to a typed array, use the [`mapRight.assign`](#method-map-right-assign) method.
- Both [`mapRight`](#fcn-map-right) and [`mapRight.assign`](#method-map-right-assign) accept array-like objects exposing getters and setters for array element access (e.g., [`Complex64Array`][@stdlib/array/complex64], [`Complex128Array`][@stdlib/array/complex128], etc).
```javascript
var Complex64Array = require( '@stdlib/array/complex64' );
var Complex64 = require( '@stdlib/complex/float32/ctor' );
var realf = require( '@stdlib/complex/float32/real' );
var imagf = require( '@stdlib/complex/float32/imag' );
function scale( z ) {
return new Complex64( realf(z)*10.0, imagf(z)*10.0 );
}
var x = new Complex64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 ] );
var y = new Complex64Array( 4 );
mapRight.assign( x, y, scale );
var v = y.get( 0 );
var re = realf( v );
// returns 10.0
var im = imagf( v );
// returns 20.0
```
- When applying a function to [`ndarray`][@stdlib/ndarray/ctor]-like objects, performance will be best for [`ndarray`][@stdlib/ndarray/ctor]-like objects which are single-segment contiguous. For non-contiguous arrays, see [`@stdlib/ndarray/base/unary`][@stdlib/ndarray/base/unary].
- Both [`mapRight`](#fcn-map-right) and [`mapRight.assign`](#method-map-right-assign) do **not** skip `undefined` elements.
</section>
<!-- /.notes -->
<!-- Package usage examples. -->
<section class="examples">
## Examples
<!-- eslint no-undef: "error" -->
```javascript
var filledarrayBy = require( '@stdlib/array/filled-by' );
var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
var naryFunction = require( '@stdlib/utils/nary-function' );
var abs2 = require( '@stdlib/math/base/special/abs2' );
var array = require( '@stdlib/ndarray/array' );
var mapRight = require( '@stdlib/utils/map-right' );
function fill( i ) {
var rand = discreteUniform( -10*(i+1), 10*(i+1) );
return filledarrayBy( 10, 'generic', rand );
}
// Create a two-dimensional ndarray (i.e., a matrix):
var x = array( filledarrayBy( 10, 'generic', fill ), {
'dtype': 'generic',
'flatten': true
});
// Create an explicit unary function:
var f = naryFunction( abs2, 1 );
// Compute the element-wise squared absolute value...
var y = mapRight( x, f );
console.log( 'x:' );
console.log( x.data );
console.log( 'y:' );
console.log( y.data );
```
</section>
<!-- /.examples -->
<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
<section class="references">
</section>
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<!-- 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/utils/map`][@stdlib/utils/map]</span><span class="delimiter">: </span><span class="description">apply a function to each element in an array and assign the result to an element in an output array.</span>
- <span class="package-name">[`@stdlib/utils/reduce`][@stdlib/utils/reduce]</span><span class="delimiter">: </span><span class="description">apply a function against an accumulator and each element in an array and return the accumulated result.</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">
[@stdlib/ndarray/ctor]: https://www.npmjs.com/package/@stdlib/ndarray-ctor
[@stdlib/ndarray/base/unary]: https://www.npmjs.com/package/@stdlib/ndarray-base-unary
[@stdlib/ndarray/base/broadcast-shapes]: https://www.npmjs.com/package/@stdlib/ndarray-base-broadcast-shapes
[@stdlib/ndarray/base/assert/is-contiguous]: https://www.npmjs.com/package/@stdlib/ndarray-base-assert-is-contiguous
[@stdlib/array/complex64]: https://www.npmjs.com/package/@stdlib/array-complex64
[@stdlib/array/complex128]: https://www.npmjs.com/package/@stdlib/array-complex128
[mdn-typedarray-map]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/map
<!-- <related-links> -->
[@stdlib/utils/map]: https://github.com/stdlib-js/utils/tree/main/map
[@stdlib/utils/reduce]: https://github.com/stdlib-js/utils/tree/main/reduce
<!-- </related-links> -->
</section>
<!-- /.links -->