C in 7 days - The Monte-Carlo algorithm

6

The Monte-Carlo algorithm

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main( ) {
const long NTHROWS = 1000000; // how many times do we throw the dart
unsigned long inside = 0;
unsigned seed = time( 0 );
double x, y;
unsigned long i;
srand( seed );
for ( i = 0; i < NTHROWS; i++ ) {
x = (double)rand()/RAND_MAX;
y = (double)rand()/RAND_MAX;
if ( x*x + y*y < 1.0 )
inside++;
}
printf("PI=%f\n", 4.0*inside/NTHROWS);
}

$ gcc -o mc mc.c
$ ./mc
PI=3.139084

Change NTHROWS to 10000000 to obtain a closer value (and test the speed of your CPU):

$ gcc -o mc mc.c
$ ./mc
PI=3.141202

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