isamax

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

Installation

npm install @stdlib/blas-base-isamax

Usage

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

isamax( N, x, strideX )

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

var Float32Array = require( '@stdlib/array-float32' );

var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

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

The function has the following parameters:

• N: number of indexed elements.
• x: input Float32Array.
• 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 Float32Array = require( '@stdlib/array-float32' );

var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

var idx = isamax( 4, x, 2 );
// returns 2

Note that indexing is relative to the first index. To introduce an offset, use typed array views.

var Float32Array = require( '@stdlib/array-float32' );

// Initial array:
var x0 = new Float32Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );

// Create an offset view:
var x1 = new Float32Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

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

isamax.ndarray( N, x, strideX, offset )

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

var Float32Array = require( '@stdlib/array-float32' );

var x = new Float32Array( [ -2.0, 1.0, 3.0, -5.0, 4.0, 0.0, -1.0, -3.0 ] );

var idx = isamax.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 Float32Array = require( '@stdlib/array-float32' );

var x = new Float32Array( [ 1.0, -2.0, 3.0, -4.0, 5.0, -6.0 ] );

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

Notes

• If N < 1, both functions return -1.
• isamax() corresponds to the BLAS level 1 function isamax.

Examples

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

var opts = {
    'dtype': 'float32'
};
var x = discreteUniform( 10, -100, 100, opts );
console.log( x );

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

C APIs

Usage

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

c_isamax( N, *X, strideX )

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

const float x[] = { 4.0f, 2.0f, -3.0f, 5.0f, -1.0f };

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

The function accepts the following arguments:

• N: [in] CBLAS_INT number of indexed elements.
• X: [in] float* input array.
• strideX: [in] CBLAS_INT index increment for X.

CBLAS_INT c_isamax( const CBLAS_INT N, const float *X, const CBLAS_INT strideX );

c_isamax_ndarray( N, *X, strideX, offsetX )

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

const float x[] = { 4.0f, 2.0f, -3.0f, 5.0f, -1.0f };

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

The function accepts the following arguments:

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

CBLAS_INT c_isamax_ndarray( const CBLAS_INT N, const float *X, const CBLAS_INT strideX, const CBLAS_INT offsetX );

Examples

#include "stdlib/blas/base/isamax.h"
#include <stdio.h>

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

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

    // Specify stride:
    const int strideX = 1;

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

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

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

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

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.