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# findLastIndex

> Return the index of the last element along an [ndarray][@stdlib/ndarray/ctor] dimension which passes a test implemented by a predicate function.

<section class="usage">

## Usage

```javascript
var findLastIndex = require( '@stdlib/blas/ext/find-last-index' );
```

#### findLastIndex( x\[, options], clbk\[, thisArg] )

Returns the index of the last element along an [ndarray][@stdlib/ndarray/ctor] dimension which passes a test implemented by a predicate function.

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

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

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

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

var idx = out.get();
// returns 5
```

The function has the following parameters:

-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have at least one dimension.
-   **options**: function options (_optional_).
-   **clbk**: callback function.
-   **thisArg**: callback execution context (_optional_).

The invoked callback is provided three arguments:

-   **value**: current array element.
-   **idx**: current array element index.
-   **array**: input ndarray.

To set the callback execution context, provide a `thisArg`.

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

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

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

var x = array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );

var ctx = {
    'count': 0
};
var out = findLastIndex( x, isEven, ctx );
// returns <ndarray>

var idx = out.get();
// returns 5

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

The function accepts the following options:

-   **dtype**: output ndarray [data type][@stdlib/ndarray/dtypes]. Must be an integer index or generic [data type][@stdlib/ndarray/dtypes].
-   **dim**: dimension over which to perform operation. If provided a negative integer, the dimension along which to perform the operation is determined by counting backward from the last dimension (where `-1` refers to the last dimension). Default: `-1`.
-   **keepdims**: boolean indicating whether the reduced dimensions should be included in the returned [ndarray][@stdlib/ndarray/ctor] as singleton dimensions. Default: `false`.

If no element along an [ndarray][@stdlib/ndarray/ctor] dimension passes a test implemented by the predicate function, the corresponding element in the returned [ndarray][@stdlib/ndarray/ctor] is `-1`.

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

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

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

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

var idx = out.get();
// returns -1
```

By default, the function performs the operation over elements in the last dimension. To perform the operation over a different dimension, provide a `dim` option.

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

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

var x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );

var opts = {
    'dim': 0
};

var out = findLastIndex( x, opts, isEven );
// returns <ndarray>

var idx = ndarray2array( out );
// returns [ -1, 1 ]
```

By default, the function excludes reduced dimensions from the output [ndarray][@stdlib/ndarray/ctor]. To include the reduced dimensions as singleton dimensions, set the `keepdims` option to `true`.

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

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

var x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );

var opts = {
    'dim': 0,
    'keepdims': true
};

var out = findLastIndex( x, opts, isEven );
// returns <ndarray>

var idx = ndarray2array( out );
// returns [ [ -1, 1 ] ]
```

By default, the function returns an [ndarray][@stdlib/ndarray/ctor] having a [data type][@stdlib/ndarray/dtypes] determined by the function's output data type [policy][@stdlib/ndarray/output-dtype-policies]. To override the default behavior, set the `dtype` option.

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

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

var x = array( [ 1.0, 2.0, 3.0, 4.0 ] );

var opts = {
    'dtype': 'generic'
};

var idx = findLastIndex( x, opts, isEven );
// returns <ndarray>

var dt = dtype( idx );
// returns 'generic'
```

#### findLastIndex.assign( x, out\[, options], clbk\[, thisArg] )

Returns the index of the last element along an [ndarray][@stdlib/ndarray/ctor] dimension which passes a test implemented by a predicate function and assigns results to a provided output [ndarray][@stdlib/ndarray/ctor].

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

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

var x = array( [ 1.0, 2.0, 3.0, 4.0 ] );
var y = zeros( [], {
    'dtype': 'int32'
});

var out = findLastIndex.assign( x, y, isEven );
// returns <ndarray>

var idx = out.get();
// returns 3

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

The method has the following parameters:

-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have at least one dimension.
-   **out**: output [ndarray][@stdlib/ndarray/ctor].
-   **options**: function options (_optional_).
-   **clbk**: callback function.
-   **thisArg**: callback execution context (_optional_).

The method accepts the following options:

-   **dim**: dimension over which to perform operation. If provided a negative integer, the dimension along which to perform the operation is determined by counting backward from the last dimension (where `-1` refers to the last dimension). Default: `-1`.

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   A provided callback function should return a boolean.
-   Setting the `keepdims` option to `true` can be useful when wanting to ensure that the output [ndarray][@stdlib/ndarray/ctor] is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with ndarrays having the same shape as the input [ndarray][@stdlib/ndarray/ctor].
-   The output data type [policy][@stdlib/ndarray/output-dtype-policies] only applies to the main function and specifies that, by default, the function must return an [ndarray][@stdlib/ndarray/ctor] having an integer index or "generic" [data type][@stdlib/ndarray/dtypes]. For the `assign` method, the output [ndarray][@stdlib/ndarray/ctor] is allowed to have any supported output [data type][@stdlib/ndarray/dtypes].

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var ndarray2array = require( '@stdlib/ndarray/to-array' );
var ndarray = require( '@stdlib/ndarray/ctor' );
var findLastIndex = require( '@stdlib/blas/ext/find-last-index' );

// Define a callback function:
function isEven( v ) {
    return v % 2.0 === 0.0;
}

// Generate an array of random numbers:
var xbuf = discreteUniform( 10, 0, 20, {
    'dtype': 'generic'
});

// Wrap in an ndarray:
var x = new ndarray( 'generic', xbuf, [ 5, 2 ], [ 2, 1 ], 0, 'row-major' );
console.log( ndarray2array( x ) );

var opts = {
    'dim': 0
};

// Perform operation:
var idx = findLastIndex( x, opts, isEven );

// Print the results:
console.log( ndarray2array( idx ) );
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

</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/dtypes]: https://www.npmjs.com/package/@stdlib/ndarray-dtypes

[@stdlib/ndarray/output-dtype-policies]: https://www.npmjs.com/package/@stdlib/ndarray-output-dtype-policies

[@stdlib/ndarray/base/broadcast-shapes]: https://www.npmjs.com/package/@stdlib/ndarray-base-broadcast-shapes

</section>

<!-- /.links -->
