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/* -*- mode: c -*-
*
* The Great Computer Language Shootout
* http://shootout.alioth.debian.org/
*
* Contributed by Sebastien Loisel
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
inline double eval_A(int i, int j) { return 1.0/((i+j)*(i+j+1)/2+i+1); }
void eval_A_times_u(int N, const double u[], double Au[])
{
int i,j;
for(i=0;i<N;i++)
{
Au[i]=0;
for(j=0;j<N;j++) Au[i]+=eval_A(i,j)*u[j];
}
}
void eval_At_times_u(int N, const double u[], double Au[])
{
int i,j;
for(i=0;i<N;i++)
{
Au[i]=0;
for(j=0;j<N;j++) Au[i]+=eval_A(j,i)*u[j];
}
}
void eval_AtA_times_u(int N, const double u[], double AtAu[])
{
double *v = malloc(N * sizeof(double));
eval_A_times_u(N,u,v);
eval_At_times_u(N,v,AtAu);
free(v);
}
int main(int argc, char *argv[])
{
int i;
int N = ((argc == 2) ? atoi(argv[1]) : 2500);
double * u, * v, vBv, vv;
u = malloc(N * sizeof(double));
v = malloc(N * sizeof(double));
for(i=0;i<N;i++) u[i]=1;
for(i=0;i<10;i++)
{
eval_AtA_times_u(N,u,v);
eval_AtA_times_u(N,v,u);
}
vBv=vv=0;
for(i=0;i<N;i++) { vBv+=u[i]*v[i]; vv+=v[i]*v[i]; }
printf("%0.9f\n",sqrt(vBv/vv));
return 0;
}
/********
build & benchmark results
BUILD COMMANDS FOR: spectralnorm.gcc
Fri Sep 15 20:48:08 PDT 2006
/usr/bin/gcc -pipe -Wall -O3 -fomit-frame-pointer -funroll-loops -march=pentium4 -lm spectralnorm.c -o spectralnorm.gcc_run
=================================================================
COMMAND LINE (%A is single numeric argument):
spectralnorm.gcc_run %A
N=2500
PROGRAM OUTPUT
==============
1.274224153
********/
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