dnrm2
Calculate the L2-norm of a double-precision floating-point vector.
The L2-norm is defined as
Usage
var dnrm2 = require( '@stdlib/blas/base/dnrm2' );
dnrm2( N, x, stride )
Computes the L2-norm of a double-precision floating-point vector x
.
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, -2.0, 2.0 ] );
var z = dnrm2( 3, x, 1 );
// returns 3.0
The function has the following parameters:
- N: number of indexed elements.
- x: input
Float64Array
. - stride: index increment for
x
.
The N
and stride
parameters determine which elements in x
are accessed at runtime. For example, to compute the L2-norm of every other element in x
,
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, 2.0, 2.0, -7.0, -2.0, 3.0, 4.0, 2.0 ] );
var z = dnrm2( 4, x, 2 );
// returns 5.0
Note that indexing is relative to the first index. To introduce an offset, use typed array
views.
var Float64Array = require( '@stdlib/array/float64' );
var x0 = new Float64Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var z = dnrm2( 4, x1, 2 );
// returns 5.0
If N
is less than or equal to 0
, the function returns 0
.
dnrm2.ndarray( N, x, stride, offset )
Computes the L2-norm of a double-precision floating-point vector using alternative indexing semantics.
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 1.0, -2.0, 2.0 ] );
var z = dnrm2.ndarray( 3, x, 1, 0 );
// returns 3.0
The function has the following additional parameters:
- offset: 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 calculate the L2-norm for every other value in x
starting from the second value
var Float64Array = require( '@stdlib/array/float64' );
var x = new Float64Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
var z = dnrm2.ndarray( 4, x, 2, 1 );
// returns 5.0
Notes
Examples
var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var dnrm2 = require( '@stdlib/blas/base/dnrm2' );
var opts = {
'dtype': 'float64'
};
var x = discreteUniform( 10, -100, 100, opts );
console.log( x );
var out = dnrm2( x.length, x, 1 );
console.log( out );
C APIs
Usage
#include "stdlib/blas/base/dnrm2.h"
c_dnrm2( N, *X, stride )
Computes the L2-norm of a double-precision floating-point vector.
const double x[] = { 1.0, -2.0, 2.0 };
double v = c_dnrm2( 3, x, 1 );
// returns 3.0
The function accepts the following arguments:
- N:
[in] CBLAS_INT
number of indexed elements. - X:
[in] double*
input array. - stride:
[in] CBLAS_INT
index increment forX
.
double c_dnrm2( const CBLAS_INT N, const double *X, const CBLAS_INT stride );
c_dnrm2_ndarray( N, *X, stride, offset )
Computes the L2-norm of a double-precision floating-point vector using alternative indexing semantics.
const double x[] = { 1.0, -2.0, 2.0 };
double v = c_dnrm2( 3, x, -1, 2 );
// returns 3.0
The function accepts the following arguments:
- N:
[in] CBLAS_INT
number of indexed elements. - X:
[in] double*
input array. - stride:
[in] CBLAS_INT
index increment forX
. - offset:
[in] CBLAS_INT
starting index forX
.
double c_dnrm2_ndarray( const CBLAS_INT N, const double *X, const CBLAS_INT stride, const CBLAS_INT offset );
Examples
#include "stdlib/blas/base/dnrm2.h"
#include <stdio.h>
int main( void ) {
// Create a 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 elements:
const int N = 8;
// Specify a stride:
const int strideX = 1;
// Compute the L2-norm:
double l2 = c_dnrm2( N, x, strideX );
// Print the result:
printf( "L2-norm: %lf\n", l2 );
// Compute the L2-norm:
l2 = c_dnrm2_ndarray( N, x, -strideX, N-1 );
// Print the result:
printf( "L2-norm: %lf\n", l2 );
}