### how can one make a project?

i tried to creat a project and i have the main function:

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#include <cmath>
#include <iostream>
#include "lib.h"
using namespace std;

// Here we define various functions called by the main program
// this function defines the function to integrate
double func(double x);
// Main function begins here
int main()
{
int i, n;
long idum;
double crude_mc, x, sum_sigma, fx, variance, exact;
printf("Read in the number of Monte-Carlo samples\n");
scanf("%d", &n);
crude_mc = sum_sigma=0. ; idum=-1 ; exact=acos(-1.);
// evaluate the integral with a crude Monte-Carlo method
for ( i = 1; i <= n; i++){
x=ran0(&idum);
fx=func(x);
crude_mc += fx;
sum_sigma += fx*fx;
}
crude_mc = crude_mc/((double) n );
sum_sigma = sum_sigma/((double) n );
variance=sum_sigma-crude_mc*crude_mc;
// final output
printf("%d variance= %12.5E Inum= %12.5E pi= %12.5E\n", n, variance, crude_mc, exact);
return 0;
} // end of main program

// this function defines the function to integrate

double func(double x)
{
double value;
value = 4/(1.+x*x);
return value;
} // end of function to evaluate
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and the function ran0() which must be called in the main function:

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/*
** The function
** ran0()
** is an "Minimal" random number generator of Park and Miller
** Set or reset the input value
** idum to any integer value (except the unlikely value MASK)
** to initialize the sequence; idum must not be altered between
** calls for sucessive deviates in a sequence.
** The function returns a uniform deviate between 0.0 and 1.0.
*/
double ran0(long &idum)
{
const int a = 16807, m = 2147483647, q = 127773;
const int r = 2836, MASK = 123459876;
const double am = 1./m;
long k;
double ans;
k = (*idum)/q;
idum = a*(idum - k*q) - r*k;
// add m if negative difference
if(idum < 0) idum += m;
ans=am*(idum);
return ans;
} // End: function ran0()
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when i try to compiille my code i gott error so if some can try that or give me an ideas?
Hi, use code tags please :D

You won't actually need a pointer in ran0()...
http://cplusplus.com/doc/tutorial/pointers/
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