/*************************    Program 9.1    ****************************/
/*                                                                      */
/************************************************************************/
/* Please Note:                                                         */
/*                                                                      */
/* (1) This computer program is written by Tao Pang in conjunction with */
/*     his book, "An Introduction to Computational Physics," published  */
/*     by Cambridge University Press in 1997.                           */
/*                                                                      */
/* (2) No warranties, express or implied, are made for this program.    */
/*                                                                      */
/************************************************************************/

#include <stdio.h>
#include <time.h>
#include <math.h>
#define MMAX 1000000
#define f(x)  (x*x)
int iseed;

main()
/* Integration with the direct sampling Monte Carlo scheme.
   The integrand is f(x) = x*x.  Copyright (c) Tao Pang 1997. */
{
int i,m;
int t1,t2,t3,t4,t5,t6;
time_t ts,tp;
struct tm t;
double x,sum1,sum2,s,ds;
double ranf();
extern int iseed;

m = MMAX;

/* Initial seed from the system time and and forced to be odd */

ts = time(&tp);
t  = *gmtime(&tp);
t1 = t.tm_sec;
t2 = t.tm_min;
t3 = t.tm_hour;
t4 = t.tm_mday;
t5 = t.tm_mon;
t6 = t.tm_year;
iseed = t6+70*(t5+12*(t4+31*(t3+23*(t2+59*t1))));

sum1 = 0;
sum2 = 0;
for(i = 0; i < m; ++i)
  {
  x = ranf();
  sum1 = sum1+f(x);
  sum2 = sum2+f(x)*f(x);
  }
s  = sum1/m;
ds = sqrt(fabs(sum2/m-(sum1/m)*(sum1/m))/m);
printf("%16.8lf %16.8lf\n", s,ds);
}

double ranf()
/* Uniform random number generator x(n+1)= a*x(n) mod c
   with a = pow(7,5) and c = pow(2,31)-1.
   Copyright (c) Tao Pang 1997. */
{
const int ia=16807,ic=2147483647,iq=127773,ir=2836;
int il,ih,it;
double rc;
extern int iseed;
ih = iseed/iq;
il = iseed%iq;
it = ia*il-ir*ih;
if (it > 0)
  {
  iseed = it;
  }
else
  {
iseed = ic+it;
  }
rc = ic;
return iseed/rc;
}