npm package: @stdlib/blas-base-idamax
Description: Find the index of the first element having the maximum absolute value.
Author: The Stdlib Authors
Latest version: 0.1.1

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</details>

# idamax

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

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

<section class="installation">

## Installation

```bash
npm install @stdlib/blas-base-idamax
```

</section>

<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>

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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

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

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

</section>

<!-- /.stdlib -->

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[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

[deno-url]: https://github.com/stdlib-js/blas-base-idamax/tree/deno
[deno-readme]: https://github.com/stdlib-js/blas-base-idamax/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/blas-base-idamax/tree/umd
[umd-readme]: https://github.com/stdlib-js/blas-base-idamax/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/blas-base-idamax/tree/esm
[esm-readme]: https://github.com/stdlib-js/blas-base-idamax/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/blas-base-idamax/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/blas-base-idamax/main/LICENSE

[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>

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