idamax

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

Usage

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

idamax( N, x, strideX )

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

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.
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,

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 views.

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.

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 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,

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 level 1 function idamax.

Examples

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

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

c_idamax( N, *X, strideX )

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

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.

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.

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.

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

Examples

#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 );
}