dnrm2.c (1405B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2020 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 #include "stdlib/blas/base/dnrm2.h" 20 #include <math.h> 21 22 /** 23 * Computes the L2-norm of a double-precision floating-point vector. 24 * 25 * @param N number of values over which to compute the norm 26 * @param X input array 27 * @param stride stride length 28 * @return output value 29 */ 30 double c_dnrm2( const int N, const double *X, const int stride ) { 31 double scale; 32 double ssq; 33 double ax; 34 int i; 35 36 if ( N <= 0 || stride <= 0 ) { 37 return 0.0; 38 } 39 if ( N == 1 ) { 40 return fabs( X[ 0 ] ); 41 } 42 scale = 0.0; 43 ssq = 1.0; 44 for ( i = 0; i < N*stride; i += stride ) { 45 if ( X[ i ] != 0.0 ) { 46 ax = fabs( X[ i ] ); 47 if ( scale < ax ) { 48 ssq = 1.0 + ( ssq * pow( scale/ax, 2 ) ); 49 scale = ax; 50 } else { 51 ssq += pow( ax/scale, 2 ); 52 } 53 } 54 } 55 return scale * sqrt( ssq ); 56 }