factorial of negative integer

[cams91]

here's my program. my problem is that, everytime i put a negative integer the ouput is 1.
how can i make it a negative ouput??
for exmple,

input: -4
output: -24

ive tried to use if else statemnt but, when i execute it, the result is zero.

#include <iostream.h>

int factorial(int);

void main(void) {
 int number;
 
 cout<<"\n\n"<<" Please enter a positive integer: ";
 cin>> number;
 cout<<"\n\n";
 if (number > 0)
  
  cout<<" The "<<number<<"! is "<<factorial(number)<<"\n\n\n\n";
}

int factorial(int number) {
 int temp;

 if(number <= 1) return 1;

 temp = number * factorial(number - 1);
 return temp;
}

[olredixsis]

Hmmm... well just make an exception like

void main(void) {
    int number;

    while(true){
         int number;
 
         cout<<"\n\n"<<" Please enter a positive integer: ";
         cin>> number;
         cout<<"\n\n";
         if(number < 0)
             cout<<"Invalid Input!"<<endl;
         else
             break;
    }

    cout<<" The "<<number<<"! is "<<factorial(number)<<"\n\n\n\n";
}

Hopes this helps.

[gcampton]

I tried this but to no avail.... I'm not so sure you can use negativs like this:

Definition of Factorial
Let n be a positive integer. n factorial, written n!, is defined by

n! = 1 * 2 * 3 * ... (n - 1) * n


The special case when n = 0, 0 factorial is given by: 0! = 1

http://www.analyzemath.com/statistics/factorial.html

anyway here's the test I did: and cleaned up a bit of your code, eg void main() // is damn ugly.

#include <iostream>
using namespace std;

int factorial(int);
int negFactorial(int);

int main()
{
   int number;

   cout<<"\n\n"<<" Please enter a positive integer: ";
   cin>> number;
   cout<<"\n\n";
   if (number>0)
      cout<<" The "<<number<<"! is "<<factorial(number)<<"\n\n\n\n";
   if (number<0)
      cout << " The " << number << "! is " << negFactorial(number) << endl;
   else
      cout << " 0! is 1 " << endl;

   return 0;
}

int factorial(int number)
{
  if(number <= 1)
   return 1;
  else
   return (number * factorial(number - 1));
}

int negFactorial(int number)
{
   if (number >=0)
      return 0;
   else
      return (number * negFactorial(number+1));
}

[gcampton]

ah ok I figured it out, had to use a calculator to realize what I was doing wrong:

change negFactorial to this:

int negFactorial(int number)
{
   if (number >= -1)
      return -1;
   else
      return (number * negFactorial(number+1));
}

the output of this will be a positive number, however on the calculator this is the same if you press the multiply sign twice, switching between negative and positive 24, so all you need to do is add a negative sign in output: " is -"

   else if (number<0)
      cout << " The " << number << "! is -" << negFactorial(number) << endl;
   else

end result:

#include <iostream>
using namespace std;

int factorial(int);
int negFactorial(int);

int main()
{
   int number;

   cout << endl << " Please enter ANY! whole number: " ;
   cin >> number;
   cout << endl;
   if (number>0)
      cout << " The " << number << "! is " << factorial(number) << endl;
   else if (number<0)
      cout << " The " << number << "! is -" << negFactorial(number) << endl;
   else
      cout << " The 0! is 1" << endl;

   return 0;
}

int factorial(int number)
{
  if(number <= 1)
   return 1;
  else
   return (number * factorial(number - 1));
}

int negFactorial(int number)
{
   if (number >=-1)
      return -1;
   else
      return (number * negFactorial(number+1));
}

[cams91]

thanks! anyway, what do you by void main () is ugly? how it affects my progrm?

[firedraco]

http://www.gidnetwork.com/b-66.html

[olredixsis]

Basically, as much as possible your main must be of type int (default? correct me if I'm wrong) It must return something. (to end the program? return 0).

[cams91]

thanks guys. :D

[gcampton]

np, also note that the program does not validate input, so if the user enters jumbled text "hello my name is bob", or fractions "6.34332" this will probably not work as intented, a better way to rewrite this would be to validate using a loop, maybe getting even up to a fractional and then casting to an int. compiler will give warnings but these can be ignored as this is what we want.

as far as main goes, void main() is just poor programming, as the program makes no attempt to be automated. for instance, say I write a much larger program, and part of my big huge program takes your little factorial program and uses it, if an error was to occur your program will not let me know (no return 1...119) in fact if it runs smoothly and everything works it doesn't let me know this either. (no return 0) Return 0 is an exit status, 0 means the program exited correctly, it went though all the code and then passed 0 back to the program or operating system.

Take the C++ compiler you use, when you compile code that compiler is a running program... If you have errors in your code it returns the errors and usually an exit status 1 on the last line (error). However when there is no errors, the code simply compiles. The compiler does send back an exit status of 0 but we never see this, as the operating system you use considers exit status 0 to be successful. The operating system will only tell you if you get an exit status of 1 or higher(error), not exit status 0 (no error).