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feat: add ndarray/find-last #8724

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335 changes: 335 additions & 0 deletions lib/node_modules/@stdlib/ndarray/find-last/README.md
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<!--

@license Apache-2.0

Copyright (c) 2025 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.

-->

# findLast

> Return a new [ndarray][@stdlib/ndarray/ctor] containing the first elements which pass a test implemented by a predicate function along one or more [ndarray][@stdlib/ndarray/ctor] dimensions.

<section class="intro">

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var findLast = require( '@stdlib/ndarray/find-last' );
```

#### findLast( x\[, options], predicate\[, thisArg] )

Returns a new [ndarray][@stdlib/ndarray/ctor] containing the last elements which pass a test implemented by a predicate function along one or more [ndarray][@stdlib/ndarray/ctor] dimensions.

<!-- eslint-disable no-invalid-this, max-len -->

```javascript
var array = require( '@stdlib/ndarray/array' );

function isEven( value ) {
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ], [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ], [ 7.0, 8.0 ] ] ] );
// returns <ndarray>

// Perform reduction:
var out = findLast( x, isEven );
// returns <ndarray>

var v = out.get();
// returns 8.0
```

The function accepts the following arguments:

- **x**: input [ndarray][@stdlib/ndarray/ctor].
- **options**: function options _(optional)_.
- **predicate**: predicate function.
- **thisArg**: predicate function execution context _(optional)_.

The function accepts the following options:

- **dims**: list of dimensions over which to perform a reduction.
- **keepdims**: boolean indicating whether the reduced dimensions should be included in the returned [ndarray][@stdlib/ndarray/ctor] as singleton dimensions. Default: `false`.
- **sentinelValue**: value to return when no element passes the test. May be either a scalar value or a zero-dimensional [ndarray][@stdlib/ndarray/ctor].

By default, the function performs reduction over all all elements in a provided [ndarray][@stdlib/ndarray/ctor]. To reduce specific dimensions, set the `dims` option.

```javascript
var array = require( '@stdlib/ndarray/array' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );

function isEven( value ) {
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ], [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ], [ 7.0, 8.0 ] ] ] );
// returns <ndarray>

var opts = {
'dims': [ 0 ]
};

// Perform reduction:
var out = findLast( x, opts, isEven );
// returns <ndarray>

var v = ndarray2array( out );
// returns [ [ 8.0, NaN ], [ 6.0, NaN ] ]
```

By default, the function returns an [ndarray][@stdlib/ndarray/ctor] having a shape matching only the non-reduced dimensions of the input [ndarray][@stdlib/ndarray/ctor] (i.e., the reduced dimensions are dropped). To include the reduced dimensions as singleton dimensions in the output [ndarray][@stdlib/ndarray/ctor], set the `keepdims` option to `true`.

```javascript
var array = require( '@stdlib/ndarray/array' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );

function isEven( value ) {
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ], [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ], [ 7.0, 8.0 ] ] ] );
// returns <ndarray>

var opts = {
'dims': [ 0 ],
'keepdims': true
};

// Perform reduction:
var out = findLast( x, opts, isEven );
// returns <ndarray>

var v = ndarray2array( out );
// returns [ [ [ 8.0, NaN ], [ 6.0, NaN ] ] ]
```

To specify a custom sentinel value to return when no element passes the test, set the `sentinelValue` option.

```javascript
var array = require( '@stdlib/ndarray/array' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );

function isEven( value ) {
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 3.0 ], [ 5.0, 7.0 ] ], [ [ 9.0, 11.0 ], [ 13.0, 15.0 ] ] ] );
// returns <ndarray>

var opts = {
'sentinelValue': -999
};

// Perform reduction:
var out = findLast( x, opts, isEven );
// returns <ndarray>

var v = out.get();
// returns -999
```
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@kgryte I think there may be a bug in ndarray/find when passing a custom sentinelValue in the options object. I'm not sure how the CI for ndarray/find passed and it's missing tests.

I checked debugging ndarray/find by modifying the example in the examples/index.js, same thing. Could you confirm if there is indeed a bug in ndarray/find? I'll then open a PR for it.


To set the `predicate` function execution context, provide a `thisArg`.

<!-- eslint-disable no-invalid-this -->

```javascript
var array = require( '@stdlib/ndarray/array' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );

function isEven( value ) {
this.count += 1;
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ], [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ], [ 7.0, 8.0 ] ] ] );
// returns <ndarray>

var ctx = {
'count': 0
};

// Perform reduction:
var out = findLast( x, isEven, ctx );
// returns <ndarray>

var v = out.get();
// returns 8.0

var count = ctx.count;
// returns 1
```

#### findLast.assign( x, out\[, options], predicate\[, thisArg] )

Finds the last elements which pass a test implemented by a predicate function along one or more [ndarray][@stdlib/ndarray/ctor] dimensions and assigns results to a provided output [ndarray][@stdlib/ndarray/ctor].

```javascript
var array = require( '@stdlib/ndarray/array' );
var empty = require( '@stdlib/ndarray/empty' );

function isEven( value ) {
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ], [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ], [ 7.0, 8.0 ] ] ] );
// returns <ndarray>

// Create an output ndarray:
var y = empty( [], {
'dtype': x.dtype
});

// Perform reduction:
var out = findLast.assign( x, y, isEven );
// returns <ndarray>

var bool = ( out === y );
// returns true

var v = y.get();
// returns 8.0
```

The function accepts the following arguments:

- **x**: input [ndarray][@stdlib/ndarray/ctor].
- **out**: output [ndarray][@stdlib/ndarray/ctor].
- **options**: function options _(optional)_.
- **predicate**: predicate function.
- **thisArg**: predicate function execution context _(optional)_.

The function accepts the following options:

- **dims**: list of dimensions over which to perform a reduction.
- **sentinelValue**: value to return when no element passes the test. May be either a scalar value or a zero-dimensional [ndarray][@stdlib/ndarray/ctor].

```javascript
var array = require( '@stdlib/ndarray/array' );
var empty = require( '@stdlib/ndarray/empty' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );

function isEven( value ) {
return value % 2.0 === 0.0;
}

// Create an input ndarray:
var x = array( [ [ [ 1.0, 2.0 ], [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ], [ 7.0, 8.0 ] ] ] );
// returns <ndarray>

// Create an output ndarray:
var y = empty( [ 2, 2 ], {
'dtype': x.dtype
});

var opts = {
'dims': [ 0 ]
};

// Perform reduction:
var out = findLast.assign( x, y, opts, isEven );

var bool = ( out === y );
// returns true

var v = ndarray2array( y );
// returns [ [ 8.0, NaN ], [ 6.0, NaN ] ]
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

- By default, when no `sentinelValue` is provided, the function returns a default sentinel value based on the input [ndarray][@stdlib/ndarray/ctor] [data-type][@stdlib/ndarray/dtypes]:

- real-valued floating-point data types: `NaN`.
- complex-valued floating-point data types: `NaN + NaNj`.
- integer data types: maximum value.
- boolean data types: `false`.

- The `predicate` function is provided the following arguments:

- **value**: current array element.
- **indices**: current array element indices.
- **arr**: the input [ndarray][@stdlib/ndarray/ctor].

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var uniform = require( '@stdlib/random/uniform' );
var isPositive = require( '@stdlib/assert/is-positive-number' ).isPrimitive;
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var findLast = require( '@stdlib/ndarray/find-last' );

var x = uniform( [ 2, 4, 5 ], -10.0, 10.0, {
'dtype': 'float64'
});
console.log( ndarray2array( x ) );

var y = findLast( x, isPositive );
console.log( y.get() );
```

</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">

[@stdlib/ndarray/ctor]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/ctor

[@stdlib/ndarray/dtypes]: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/dtypes

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

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

</section>

<!-- /.links -->
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