feat: add ndarray/base/where

lib/node_modules/@stdlib/ndarray/base/where/README.md

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+<!--
+
+@license Apache-2.0
+
+Copyright (c) 2026 The Stdlib Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+-->
+
+# Where
+
+> Apply a condition to elements in two input ndarrays and assign results to elements in an output ndarray.
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var where = require( '@stdlib/ndarray/base/where' );
+```
+
+#### where( arrays )
+
+Applies a condition to elements in two input ndarrays and assigns results to elements in an output ndarray.
+
+<!-- eslint-disable max-len -->
+
+```javascript
+var Float64Array = require( '@stdlib/array/float64' );
+var Uint8Array = require( '@stdlib/array/uint8' );
+
+// Create data buffers:
+var cbuf = new Uint8Array( [ 1, 0, 0, 1, 0, 1 ] );
+var xbuf = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );
+var ybuf = new Float64Array( [ -1.0, -2.0, -3.0, -4.0, -5.0, -6.0 ] );
+var obuf = new Float64Array( 6 );
+
+// Define the shape of the input and output arrays:
+var shape = [ 3, 1, 2 ];
+
+// Define the array strides:
+var sc = [ 2, 2, 1 ];
+var sx = [ 2, 2, 1 ];
+var sy = [ 2, 2, 1 ];
+var so = [ 2, 2, 1 ];
+
+// Define the index offsets:
+var oc = 0;
+var ox = 0;
+var oy = 0;
+var oo = 0;
+
+// Create the input and output ndarrays:
+var condition = {
+    'dtype': 'uint8',
+    'data': cbuf,
+    'shape': shape,
+    'strides': sc,
+    'offset': oc,
+    'order': 'row-major'
+};
+var x = {
+    'dtype': 'float64',
+    'data': xbuf,
+    'shape': shape,
+    'strides': sx,
+    'offset': ox,
+    'order': 'row-major'
+};
+var y = {
+    'dtype': 'float64',
+    'data': ybuf,
+    'shape': shape,
+    'strides': sy,
+    'offset': oy,
+    'order': 'row-major'
+};
+var out = {
+    'dtype': 'float64',
+    'data': obuf,
+    'shape': shape,
+    'strides': so,
+    'offset': oo,
+    'order': 'row-major'
+};
+
+// Apply the condition:
+where( [ condition, x, y, out ] );
+
+console.log( out.data );
+// => <Float64Array>[ 1.0, -2.0, -3.0, 4.0, -5.0, 6.0 ]
+```
+
+The function accepts the following arguments:
+
+-   **arrays**: array-like object containing three input ndarrays and one output ndarray.
+
+Each provided ndarray should be an object with the following properties:
+
+-   **dtype**: data type.
+-   **data**: data buffer.
+-   **shape**: dimensions.
+-   **strides**: stride lengths.
+-   **offset**: index offset.
+-   **order**: specifies whether an ndarray is row-major (C-style) or column major (Fortran-style).
+
+</section>
+
+<!-- /.usage -->
+
+<section class="notes">
+
+## Notes
+
+-   `condition` ndarray must be a `boolean` or `uint8` ndarray.
+-   `condition`, `x`, `y`, and `out` ndarrays must have the same shape.
+-   For very high-dimensional ndarrays which are non-contiguous, one should consider copying the underlying data to contiguous memory before conditionally assigning elements in order to achieve better performance.
+
+</section>
+
+<!-- /.notes -->
+
+<section class="examples">
+
+## Examples
+
+<!-- eslint no-undef: "error" -->
+
+```javascript
+var discreteUniform = require( '@stdlib/random/base/discrete-uniform' ).factory;
+var bernoulli = require( '@stdlib/random/base/bernoulli' ).factory;
+var filledarray = require( '@stdlib/array/filled' );
+var filledarrayBy = require( '@stdlib/array/filled-by' );
+var shape2strides = require( '@stdlib/ndarray/base/shape2strides' );
+var ndarray2array = require( '@stdlib/ndarray/base/to-array' );
+var where = require( '@stdlib/ndarray/base/where' );
+
+var N = 10;
+var shape = [ 5, 2 ];
+var condition = {
+    'dtype': 'uint8',
+    'data': filledarrayBy( N, 'uint8', bernoulli( 0.5 ) ),
+    'shape': shape,
+    'strides': [ 2, 1 ],
+    'offset': 0,
+    'order': 'row-major'
+};
+console.log( ndarray2array( condition.data, condition.shape, condition.strides, condition.offset, condition.order ) ); // eslint-disable-line max-len
+
+var x = {
+    'dtype': 'generic',
+    'data': filledarrayBy( N, 'generic', discreteUniform( 0, 100 ) ),
+    'shape': shape,
+    'strides': [ 2, 1 ],
+    'offset': 0,
+    'order': 'row-major'
+};
+console.log( ndarray2array( x.data, x.shape, x.strides, x.offset, x.order ) );
+
+var y = {
+    'dtype': 'generic',
+    'data': filledarrayBy( N, 'generic', discreteUniform( -100, 0 ) ),
+    'shape': shape,
+    'strides': [ 2, 1 ],
+    'offset': 0,
+    'order': 'row-major'
+};
+console.log( ndarray2array( y.data, y.shape, y.strides, y.offset, y.order ) );
+
+var out = {
+    'dtype': 'generic',
+    'data': filledarray( 0, N, 'generic' ),
+    'shape': shape.slice(),
+    'strides': shape2strides( shape, 'column-major' ),
+    'offset': 0,
+    'order': 'column-major'
+};
+
+where( [ condition, x, y, out ] );
+console.log( ndarray2array( out.data, out.shape, out.strides, out.offset, out.order ) ); // eslint-disable-line max-len
+
+```
+
+</section>
+
+<!-- /.examples -->
+
+<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
+
+<section class="related">
+
+</section>
+
+<!-- /.related -->
+
+<section class="links">
+
+<!-- <related-links> -->
+
+<!-- </related-links> -->
+
+</section>
+
+<!-- /.links -->