npm package: @stdlib/blas-ext-base-gfind-index
Description: Return the index of the first element which passes a test implemented by a predicate function.
Author: The Stdlib Authors
Latest version: 0.1.1

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# gfindIndex

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

<section class="installation">

## Installation

```bash
npm install @stdlib/blas-ext-base-gfind-index
```

</section>

<section class="usage">

## Usage

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

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

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

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

var x = [ 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

var idx = gfindIndex( 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 = gfindIndex( 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, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

var ctx = {
    'count': 0
};

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

var count = ctx.count;
// returns 4
```

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 = [ 1.0, 3.0, -5.0, 0.0, 4.0, -1.0, -3.0 ];

var idx = gfindIndex( 4, x, 2, isEven );
// returns 2
```

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, 3.0, 4.0, -5.0, 7.0, 6.0 ] );

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

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

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

Returns the index of the first 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, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ];

var idx = gfindIndex.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, -4.0, 5.0, -7.0, 6.0 ];

var idx = gfindIndex.ndarray( 3, x, 1, x.length-3, isEven );
// returns 2
```

</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 gfindIndex = require( '@stdlib/blas-ext-base-gfind-index' );

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

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

var idx = gfindIndex( x.length, x, 1, isEven );
console.log( idx );
```

</section>

<!-- /.examples -->

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<section class="references">

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

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---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

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