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# indexOf

> Return the first index of a specified search element along an [ndarray][@stdlib/ndarray/ctor] dimension.

<section class="usage">

## Usage

```javascript
var indexOf = require( '@stdlib/blas/ext/index-of' );
```

#### indexOf( x, searchElement\[, fromIndex]\[, options] )

Returns the first index of a specified search element along an [ndarray][@stdlib/ndarray/ctor] dimension.

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

// 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 = indexOf( x, 2.0 );
// returns <ndarray>[ 1 ]
```

The function has the following parameters:

-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have at least one dimension.
-   **searchElement**: search element. May be either a scalar value or an [ndarray][@stdlib/ndarray/ctor]. If provided a scalar value, the value is cast to the data type of the input [ndarray][@stdlib/ndarray/ctor]. If provided an [ndarray][@stdlib/ndarray/ctor], the value must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the non-reduced dimensions of the input [ndarray][@stdlib/ndarray/ctor]. For example, given the input shape `[2, 3, 4]` and `options.dim=0`, the search element [ndarray][@stdlib/ndarray/ctor] must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the shape `[3, 4]`.
-   **fromIndex**: index from which to begin searching (_optional_). May be either a scalar value or an [ndarray][@stdlib/ndarray/ctor] having an integer index or "generic" [data type][@stdlib/ndarray/dtypes]. If provided an [ndarray][@stdlib/ndarray/ctor], the value must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the non-reduced dimensions of the input [ndarray][@stdlib/ndarray/ctor]. For example, given the input shape `[2, 3, 4]` and `options.dim=0`, a provided [ndarray][@stdlib/ndarray/ctor] must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the shape `[3, 4]`. If provided a negative integer, the index at which to begin searching along a dimension is determined by counting backward from the last element (where `-1` refers to the last element). Default: `0`.
-   **options**: function options (_optional_).

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 the function is unable to find a search element along an [ndarray][@stdlib/ndarray/ctor], the corresponding element in the returned [ndarray][@stdlib/ndarray/ctor] is `-1`.

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

// 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 = indexOf( x, 10.0 );
// returns <ndarray>[ -1 ]
```

By default, the function begins searching from the first element along the reduction dimension. To begin searching from a different index, provide a `fromIndex` argument.

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

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

// Perform operation:
var out = indexOf( x, 2.0, 2 );
// returns <ndarray>[ 4 ]
```

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 array = require( '@stdlib/ndarray/array' );

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

var out = indexOf( x, -3.0, {
    'dim': 0
});
// returns <ndarray>[ 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 x = array( [ [ -1.0, 2.0 ], [ -3.0, 4.0 ] ] );

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

var out = indexOf( x, -3.0, opts );
// returns <ndarray>[ [ 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' );

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

var idx = indexOf( x, 2.0, {
    'dtype': 'generic'
});
// returns <ndarray>

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

#### indexOf.assign( x, searchElement\[, fromIndex], out\[, options] )

Returns the first index of a specified search element along an [ndarray][@stdlib/ndarray/ctor] dimension and assigns results to a provided output [ndarray][@stdlib/ndarray/ctor].

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

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

var out = indexOf.assign( x, 3.0, y );
// returns <ndarray>[ 2 ]

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

The method has the following parameters:

-   **x**: input [ndarray][@stdlib/ndarray/ctor]. Must have at least one dimension.
-   **searchElement**: search element. May be either a scalar value or an [ndarray][@stdlib/ndarray/ctor]. If provided a scalar value, the value is cast to the data type of the input [ndarray][@stdlib/ndarray/ctor]. If provided an [ndarray][@stdlib/ndarray/ctor], the value must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the non-reduced dimensions of the input [ndarray][@stdlib/ndarray/ctor]. For example, given the input shape `[2, 3, 4]` and `options.dim=0`, the search element [ndarray][@stdlib/ndarray/ctor] must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the shape `[3, 4]`.
-   **fromIndex**: index from which to begin searching (_optional_). May be either a scalar value or an [ndarray][@stdlib/ndarray/ctor] having an integer index or "generic" [data type][@stdlib/ndarray/dtypes]. If provided an [ndarray][@stdlib/ndarray/ctor], the value must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the non-reduced dimensions of the input [ndarray][@stdlib/ndarray/ctor]. For example, given the input shape `[2, 3, 4]` and `options.dim=0`, a provided [ndarray][@stdlib/ndarray/ctor] must have a shape which is [broadcast-compatible][@stdlib/ndarray/base/broadcast-shapes] with the shape `[3, 4]`. If provided a negative integer, the index at which to begin searching along a dimension is determined by counting backward from the last element (where `-1` refers to the last element). Default: `0`.
-   **out**: output [ndarray][@stdlib/ndarray/ctor].
-   **options**: function options (_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

-   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 indexOf = require( '@stdlib/blas/ext/index-of' );

// Generate an array of random numbers:
var xbuf = discreteUniform( 10, 0, 20, {
    'dtype': 'float64'
});

// Wrap in an ndarray:
var x = new ndarray( 'float64', xbuf, [ 5, 2 ], [ 2, 1 ], 0, 'row-major' );
console.log( ndarray2array( x ) );

// Perform operation:
var idx = indexOf( x, 10.0, {
    'dim': 0
});

// 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 -->
