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# gfindLastIndex

> Return the index of the last element which passes a test implemented by a predicate function.

<section class="usage">

## Usage

```javascript
var gfindLastIndex = require( '@stdlib/blas/ext/base/gfind-last-index' );
```

#### gfindLastIndex( N, x, strideX, clbk\[, thisArg] )

Returns the index of the last element which passes a test implemented by a predicate function.

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

var x = [ 1.0, 2.0, -3.0, 4.0, -5.0 ];

var idx = gfindLastIndex( x.length, x, 1, isEven );
// returns 3
```

If no element passes a test implemented by a predicate function, the function returns `-1`.

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

var x = [ 1.0, 3.0, 5.0, 7.0, 9.0 ];

var idx = gfindLastIndex( x.length, x, 1, isEven );
// returns -1
```

The function has the following parameters:

-   **N**: number of indexed elements.
-   **x**: input array.
-   **strideX**: stride length.
-   **clbk**: callback function.
-   **thisArg**: execution context (_optional_).

The callback function is provided the following arguments:

-   **value**: current array element.
-   **aidx**: array index.
-   **sidx**: strided index (`offset + aidx*stride`).
-   **array**: the input array.

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

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

var x = [ 1.0, 2.0, -3.0, 4.0, -5.0 ];

var ctx = {
    'count': 0
};

var idx = gfindLastIndex( x.length, x, 1, isEven, ctx );
// returns 3

var count = ctx.count;
// returns 2
```

The `N` and stride parameters determine which elements in the strided array are accessed at runtime. For example, to test every other element:

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

var x = [ 2.0, 1.0, 4.0, 3.0, 5.0 ];

var idx = gfindLastIndex( 3, x, 2, isEven );
// returns 1
```

Note that indexing is relative to the first index. To introduce an offset, use [`typed array`][mdn-typed-array] views.

```javascript
var Float64Array = require( '@stdlib/array/float64' );

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

// Initial array:
var x0 = new Float64Array( [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ] );

// Create an offset view:
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

var idx = gfindLastIndex( 3, x1, 2, isEven );
// returns 2
```

#### gfindLastIndex.ndarray( N, x, strideX, offsetX, clbk\[, thisArg] )

Returns the index of the last element which passes a test implemented by a predicate function using alternative indexing semantics.

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

var x = [ 1.0, 2.0, -3.0, 4.0, -5.0 ];

var idx = gfindLastIndex.ndarray( x.length, x, 1, 0, isEven );
// returns 3
```

The function has the following additional parameters:

-   **offsetX**: starting index.

While [`typed array`][mdn-typed-array] views mandate a view offset based on the underlying buffer, the offset parameter supports indexing semantics based on a starting index. For example, to access only the last three elements:

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

var x = [ 1.0, 2.0, -3.0, 4.0, -5.0 ];

var idx = gfindLastIndex.ndarray( 3, x, 1, x.length-3, isEven );
// returns 1
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N <= 0`, both functions return `-1`.
-   Both functions support array-like objects having getter and setter accessors for array element access (e.g., [`@stdlib/array/base/accessor`][@stdlib/array/base/accessor]).

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var gfindLastIndex = require( '@stdlib/blas/ext/base/gfind-last-index' );

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

var x = discreteUniform( 10, -100, 100, {
    'dtype': 'generic'
});
console.log( x );

var idx = gfindLastIndex.ndarray( x.length, x, 1, 0, isEven );
console.log( idx );
```

</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>

<!-- /.references -->

<!-- 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">

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

[@stdlib/array/base/accessor]: https://www.npmjs.com/package/@stdlib/array-base-accessor

</section>

<!-- /.links -->
