idamax

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

Usage

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

idamax.main( 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( [ 1.0, -2.0, 2.0 ] );

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

The function has the following parameters:

N: number of indexed elements.
x: input Float64Array.
strideX: index increment for x.

The N and stride parameters determine which elements in the input strided array 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.main( 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.main( 3, x1, 2 );
// returns 2

idamax.ndarray( N, x, strideX, offsetX )

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( [ 1.0, -2.0, 2.0 ] );

var idx = idamax.ndarray( 3, x, 1, 0 );
// returns 1

The function has the following additional parameters:

offsetX: starting index for x.

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

Module

idamax.Module( memory )

Returns a new WebAssembly module wrapper instance which uses the provided WebAssembly memory instance as its underlying memory.

var Memory = require( '@stdlib/wasm/memory' );

// Create a new memory instance with an initial size of 10 pages (640KiB) and a maximum size of 100 pages (6.4MiB):
var mem = new Memory({
    'initial': 10,
    'maximum': 100
});

// Create a BLAS routine:
var mod = new idamax.Module( mem );
// returns <Module>

// Initialize the routine:
mod.initializeSync();

idamax.Module.prototype.main( N, xp, sx )

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

var Memory = require( '@stdlib/wasm/memory' );
var oneTo = require( '@stdlib/array/one-to' );

// Create a new memory instance with an initial size of 10 pages (640KiB) and a maximum size of 100 pages (6.4MiB):
var mem = new Memory({
    'initial': 10,
    'maximum': 100
});

// Create a BLAS routine:
var mod = new idamax.Module( mem );
// returns <Module>

// Initialize the routine:
mod.initializeSync();

// Define a vector data type:
var dtype = 'float64';

// Specify a vector length:
var N = 5;

// Define a pointer (i.e., byte offset) to the first vector element:
var xptr = 0;

// Write vector values to module memory:
mod.write( xptr, oneTo( N, dtype ) );

// Perform computation:
var idx = mod.main( N, xptr, 1 );
// returns 4

The function has the following parameters:

N: number of indexed elements.
xp: input Float64Array pointer (i.e., byte offset).
sx: index increment for x.

idamax.Module.prototype.ndarray( N, xp, sx, ox )

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

var Memory = require( '@stdlib/wasm/memory' );
var oneTo = require( '@stdlib/array/one-to' );

// Create a new memory instance with an initial size of 10 pages (640KiB) and a maximum size of 100 pages (6.4MiB):
var mem = new Memory({
    'initial': 10,
    'maximum': 100
});

// Create a BLAS routine:
var mod = new idamax.Module( mem );
// returns <Module>

// Initialize the routine:
mod.initializeSync();

// Define a vector data type:
var dtype = 'float64';

// Specify a vector length:
var N = 5;

// Define a pointer (i.e., byte offset) to the first vector element:
var xptr = 0;

// Write vector values to module memory:
mod.write( xptr, oneTo( N, dtype ) );

// Perform computation:
var idx = mod.ndarray( N, xptr, 1, 0 );
// returns 4

The function has the following additional parameters:

ox: starting index for x.

Notes

If N < 1, both main and ndarray methods return -1.
This package implements routines using WebAssembly. When provided arrays which are not allocated on a idamax module memory instance, data must be explicitly copied to module memory prior to computation. Data movement may entail a performance cost, and, thus, if you are using arrays external to module memory, you should prefer using @stdlib/blas/base/idamax. However, if working with arrays which are allocated and explicitly managed on module memory, you can achieve better performance when compared to the pure JavaScript implementations found in @stdlib/blas/base/idamax. Beware that such performance gains may come at the cost of additional complexity when having to perform manual memory management. Choosing between implementations depends heavily on the particular needs and constraints of your application, with no one choice universally better than the other.
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-wasm' );

var opts = {
    'dtype': 'float64'
};
var x = discreteUniform( 10, 0, 100, opts );
console.log( x );

var idx = idamax.ndarray( x.length, x, 1, 0 );
console.log( idx );