无法从 function 返回值？

Question

I wrote this code and there is a problem that I can't seem to work out. The function is supposed to return T1 and cout it from the main but it always gives me an error "T1 is undeclared identifier". Why?

#include<iostream>
#include<math.h>
#include<time.h>
using namespace std;
double factorial()
{
   int i;
   double T1,total=0;
   for(i=0;i<200;i++) 
   {
      clock_t start = clock();

      int a,N,f;
  N=99999;
      f=N;
  for(a=N-1;a>0;a--)
  {
         f=f*a;
      }

      clock_t end = clock();
      T1=double(end-start)/(double) CLOCKS_PER_SEC;
      total=total+T1;
   }
   T1=total/200;
   return T1;
}
int main()
{
   factorial();
   cout<<T1<<endl;

   return 0;
}

Answer 1

Each function only knows it local variables (and globals, which you don't have any defined). You have to create a variable for the result from in main:

int main()
{
   double answer = factorial();
   cout << answer << endl;

   return 0;
}

I don't know if you care, but the factorial value is going to overflow.

Answer 2

Because T1 is not defined under the main() scope, it is only defined under the scope of your function factorial .

You should do this instead:

cout<<factorial()<<endl;

or define T1 like this within your main function:

double T1 = factorial();
cout<<T1<<endl;

Answer 3

You have to define T1 first locally or make it a global variable (not recommended).

int main()
{
double T1=factorial();

cout<<T1<<endl;

return 0;
}