# idamax
> Find the index of the first element having the maximum absolute value.
## 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
```
## Notes
- If `N < 1`, both functions return `-1`.
- `idamax()` corresponds to the [BLAS][blas] level 1 function [`idamax`][idamax].
## Examples
```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 );
```
* * *
## C APIs
### 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 );
```
### Examples
```c
#include "stdlib/blas/base/idamax.h"
#include
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 );
}
```
[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