Using limits to give an error message for a negative, non-integer

[martinhiggs]

Hey!

I'm currently writing a program that calculates the factorial of a given number. This number has to be positive, and it has to be an integer, ie, not a decimal.

I got the integer part working, using limits, which is shown in the code below. I also then needed to add a bit where if the user enters a negative number that an error message appears and the user is prompted to re-enter a number.

I must stress that I have literally been using C++ for one week, so I am extremely new to this!

When I enter in a decimal number, it displays the error message and asks the user to re-enter. However, when I enter in a negative number, it gives the factorial to be 1, rather than displaying the error message.

In trying to fix this problem I now receive the error message when compiling:

"Stirling.cc: In function `int main()':
Stirling.cc:14: error: expected primary-expression before "else"
Stirling.cc:14: error: expected `;' before "else"
"

#include <cmath>
#include <limits>
#include <iostream>

using namespace std; 
int main(){
  int n;
    while (true) {
        cout << "Enter a positive integer" << endl;
        cin >> n;
    if (cin.peek() == '\n' && cin.good()) break;
       cout << "Invalid input!  Try again...\n" << endl;
       do { cin.ignore(numeric_limits<streamsize>::max(), '\n'); } while (!cin);
    else if (n<0)
      cout << "Invalid imput! Try again!" << endl;
      cin.clear();
    do { cin.ignore(numeric_limits<streamsize>::max(), '\n'); } while (!cin);
    }
  

    if (n <= 50)
    {
      double result=1;
      for (double i=1; i<=n; ++i){
      result=result*=i;
      }
      cout << "The factorial of this is " << result << endl;
    }
    else if (n > 50);
    {
      const double PI = 3.141592;
      double factorial1 =(n*log(n));
      double factorial2 =(n);
      double factorial3 =(0.5*log(2*PI*n));
      double x = factorial1-factorial2+factorial3;
      double xfac = exp(x);
      cout << "The factorial of this number is " << xfac << endl;
    }

}

Any help would be greatly appreciated!

[ne555]

You've got unpaired else with if. (that's the compiler complaint)
The problem is that if you read a negative number cin will not fail, the if success and the while break.
What about if(cin>>n && n>=0) to check for good input

[1yvXoG1T]

And just to let you know that you can, you could actually crunch that entire first section down in to something like this:

while(cin.good() == false || n < 0) {
    cin.clear();
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
    cout << "Invalid input! Try again...\n\n";
    cout << "Enter a positive integer: ";
    cin >> n;
}

and still get the same result. It just saves you some time on typing more than anything.