Factorial Help

[Nexrus]

I'm having 2 problems. First of all I can't figure out why it's not displaying the factorial of number. When I try to compile this code I get an error on the line cout << "Factorial of " << number << " is " << factorial(number) << endl; but I can't figure out whats wrong for the life of me.

Also its repeating "Enter a number between 2 and 30: " regardless of what number I enter. The only time I want it to repeat this is when the number is not between 2 and 30.

Any help is much appreciated.

#include <iostream>


using namespace std;
int factorial (int);

int main()

{
    int number;
    do {
    cout << "Enter a number between 2 and 30: ";
    cin >> number;
    cout << "Factorial of " << number << " is " << factorial(number) << endl;;
    
    
}
   while (number );
    
    system("PAUSE");
    return EXIT_SUCCESS;
}

[xkcd83]

Where is the implementation of the factorial function??

[Nexrus]

Im sorry I'm not sure what you mean. I'm very new to this.

[whitenite1]

@Nexrus

You declared a function named

int factorial (int);

but the routine to get the factorial, is not there. You should have a

// declare another variable
int fact;
// After your cin >> number, put in
fact = factorial(number);
cout << "Factorial of " << number << " is " << fact << endl;


int factorial( number)  // The called function
{
int factorial; 
// code to get the factorial
return factorial; // This value is returned as the variable fact 
}

[Nexrus]

This is my new code. I don't get any errors when I compile it but the exe will not run either.

#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;
int factorial (int);


int main()


{
    int number;
    int fact;
    
    do {
    cout << "Enter a number between 2 and 30: ";
    cin >> number; 
    fact = factorial(number);
    cout << "Factorial of " << number << " is " << fact << endl;
    
}
   while (number);
    system("PAUSE");
    return EXIT_SUCCESS;
}

[whitenite1]

@Nexrus

You still haven't written the routine to show the factorial. As I said, you declared a function named

int factorial( int );

but you never created it. You still need the

int factorial( number)  // The called function
{
int factorial; 
// code to get the factorial
return factorial; // This value is returned as the variable fact 
}

.

[Gaminic]

You declared a function "factorial", but it has no meaning, as you haven't defined it.
Imagine I want a function to double my number:

int doubleMyNumber(int); // Declares function

int main() {
  int myNumber = 8
  int doubleNumber doubleMyNumber(myNumber); // Calls function
  return 0;
}

int doubleMyNumber(int t) { // Defines function
  return t*2;
}

Each 'step' has a point:
-Declaration tells the compiler the function exists.
-Calling tells the compiler the function has to be executed (and with which parameters).
-Defining tells the compiler what the function does, in this case: returning the double of a number.

Declaration and definition can be done simultaneously (i.e. move the definition upwards before the first time it's called), but splitting both can improve readability.

[nathan10]

I get the impression that Nexrus believes that C++ has a math function called 'factorial' that can be implemented at will in the same way there are math functions called 'power', 'sine', or 'cosine'.

@Nexrus,

There is NO math function called 'factorial' in C++. You must define it yourself by writing code for it as Whitenite1 and Gaminic explained in their posts.

[Nexrus]

Thats exactly what I thought. Let me take the info you guys have shared with me and see what I can do. Thank you very much.

[Nexrus]

I'm not entirely sure how but I messed around with it until I finally got it to give me the factorial of the number I enter. Now I need to use do-while loop so that if I enter any number thats not between 2-15 it will cout "Enter a number between 2 and 30: "

#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;
int number;

int factorial (int a)
{
  if (a > 1)
   return (a * factorial (a-1));
  else
   return (1);
}

int main()
{
    cout << "Enter a number between 2 and 30: ";
    cin >> number; 
    cout << "Factorial of " << number << " is " << factorial (number) << endl;

    system("PAUSE");
    return EXIT_SUCCESS;
}

[whitenite1]

@Nexrus, congrats on the program. I don't know anything about factorials, so I don't really know if the results are correct, but I'll take your word, ( and program ) they are.