#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#define NSEED 100
int main()
{
double x, pi, f, dv;
double sum, sum_pi, sum_pi2;
int itry, iseed, ntry;
printf("Input the number of MC trials\n");
scanf("%d",&ntry);
srand((unsigned)time((long *)0));
sum_pi = sum_pi2 = 0.0;
for (iseed=0; iseed<NSEED; iseed++)
{
sum = 0.0;
for (itry=0; itry<ntry; itry++)
{
x = rand()/(double)RAND_MAX;
f = 4.0/(1.0 + x*x);
sum += f;
}
pi = sum/ntry;
sum_pi += pi;
sum_pi2 += pi*pi;
}
dv = sqrt((sum_pi2/NSEED) - (sum_pi/NSEED)*(sum_pi/NSEED));
printf("MC estimate for PI = %f +- %e \n", sum_pi/NSEED, dv);
return 0;
}
