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# idamax

> Find the index of the first element having the maximum absolute value.

<section class="usage">

## Usage

```javascript
var idamax = require( '@stdlib/blas/base/idamax' );
```

#### idamax( N, x, strideX )

Finds the index of the first element having the maximum absolute value.

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

var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

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

The function has the following parameters:

-   **N**: number of indexed elements.
-   **x**: input [`Float64Array`][@stdlib/array/float64].
-   **strideX**: index increment for `x`.

The `N` and `strideX` parameters determine which elements in `x` are accessed at runtime. For example, to traverse every other value,

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

var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

var idx = idamax( 4, x, 2 );
// 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' );

// 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

// Find index of element having the maximum absolute value:
var idx = idamax( 3, x1, 2 );
// returns 2
```

#### idamax.ndarray( N, x, strideX, offset )

Finds the index of the first element having the maximum absolute value using alternative indexing semantics.

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

var x = new Float64Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

var idx = idamax.ndarray( x.length, x, 1, 0 );
// 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 start from the second index,

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

var x = new Float64Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );

var idx = idamax.ndarray( 5, x, 1, 1 );
// returns 4
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   If `N < 1`, both functions return `-1`.
-   `idamax()` corresponds to the [BLAS][blas] level 1 function [`idamax`][idamax].

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var idamax = require( '@stdlib/blas/base/idamax' );

var opts = {
    'dtype': 'float64'
};
var x = discreteUniform( 10, -100, 100, opts );
console.log( x );

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

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- C usage documentation. -->

<section class="usage">

### Usage

```c
#include "stdlib/blas/base/idamax.h"
```

#### c_idamax( N, \*X, strideX )

Finds the index of the first element having the maximum absolute value.

```c
const double x[] = { 4.0, 2.0, -3.0, 5.0, -1.0 };

int idx = c_idamax( 5, x, 1 );
// returns 3
```

The function accepts the following arguments:

-   **N**: `[in] CBLAS_INT` number of indexed elements.
-   **X**: `[in] double*` input array.
-   **strideX**: `[in] CBLAS_INT` index increment for `X`.

```c
CBLAS_INT c_idamax( const CBLAS_INT N, const double *X, const CBLAS_INT strideX );
```

#### c_idamax_ndarray( N, \*X, strideX, offsetX )

Finds the index of the first element having the maximum absolute value using alternative indexing semantics.

```c
const double x[] = { 4.0, 2.0, -3.0, 5.0, -1.0 };

int idx = c_idamax_ndarray( 5, x, 1, 0 );
// returns 3
```

The function accepts the following arguments:

-   **N**: `[in] CBLAS_INT` number of indexed elements.
-   **X**: `[in] double*` input array.
-   **strideX**: `[in] CBLAS_INT` index increment for `X`.
-   **offsetX**: `[in] CBLAS_INT` starting index for `X`.

```c
CBLAS_INT c_idamax_ndarray( const CBLAS_INT N, const double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX );
```

</section>

<!-- /.usage -->

<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

```c
#include "stdlib/blas/base/idamax.h"
#include <stdio.h>

int main( void ) {
    // Create strided array:
    const double x[] = { 1.0, -2.0, 3.0, -4.0, 5.0, -6.0, 7.0, -8.0 };

    // Specify the number of element:
    const int N = 8;

    // Specify stride:
    const int strideX = 1;

    // Compute the index of the maximum absolute value:
    int idx = c_idamax( N, x, strideX );

    // Print the result:
    printf( "index value: %d\n", idx );

    // Compute the index of the maximum absolute value:
    idx = c_idamax_ndarray( N, x, -strideX, N-1 );

    // Print the result:
    printf( "index value: %d\n", idx );
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<!-- 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">

[blas]: http://www.netlib.org/blas

[idamax]: https://netlib.org/lapack/explore-html/dd/de0/idamax_8f_source.html

[@stdlib/array/float64]: https://www.npmjs.com/package/@stdlib/array-float64

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

</section>

<!-- /.links -->
