Plz help Write a program to calculate (20 number) ! ( factorial) with c?

hi im learning C ( with turbo C++ 4.5 )


our teacher ask us if any one can bring a simple source code of


factorial of 20 digit number i mean ( its over a belion number i think )


he asked us only C code not php or VB or anything else


so if any one can write such a program please help me with it


and i dont need any guide i need the program :D of course i need the headers to becuse i`m new to c and i want to learn the code


( headers like %26lt;stdio.h%26gt; and ...%26gt;


i also found a code in the internet but its a calculator and it has geraphic on it if any one want to watch it go to


http://arashmidos2006.googlepages.com/Ca...


i didnt understand it


ok guys im realy depending on u please help me with this

Plz help Write a program to calculate (20 number) ! ( factorial) with c?
hmm a 20 digit number would be X * x-1 * x-2... x-19





I guess the big question is, does it ONLY do 20 digit numbers? heheh. been a while since i did c, i'm a c# dev... but here goes. Not even sure if a 'long' would hold your result or not...


this won't work if a 'long' won't hold the result, it may overflow.





void main()


{


long value;


long result;


cout %26lt;%26lt; "Enter your number";


value %26lt;%26lt; cin;//can't remember if you can do this in c or not...


result = value;


if(value %26gt; 1)


{


while(value %26gt; 1)


{


value = value - 1;//decrement value


result = result * value;//multiply result by value


}


cout %26lt;%26lt; result;





}


else


{


cout %26lt;%26lt; "Must be a positive whole number greater than 1";


}





}








You know, thinking more about it, i don't think a long will hold the results... the Best way to do it would be a bit array and use binary multiplication. (can't say i remember how to do binary multiplication but it was something we covered in school...)





basically make a bit array that holds enough bits to calculate 99999999999999999999! and do binary math the same as the above code. :) good luck
Reply:Sorry about that. I fixed the problem:





#include %26lt;iostream%26gt;





// Need the climits file in order to know the maximum size of an unsigned


// long int.


#include %26lt;climits%26gt;





// Declare the class.


class Factorial {


public:


Factorial(unsigned short num = 1);


bool inRange();


unsigned long getFactorial();





private:


unsigned short num;





}; // End of class.





// Implement the methods:


Factorial::Factorial (unsigned short num) {


this-%26gt;num = num;


}





bool Factorial::inRange() {





// Maximum possible value for validation.


unsigned long max = ULONG_MAX;





// Loop from 1 to num, dividing the number from max.


for (int i = num; i %26gt;= 1; --i) {





max /= i;





} // End of the for loop.





// Return a true/false value.


if (max %26lt; 1) {


return false;


} else {


return true;


}





} // End of the factorialInRange() function.





unsigned long Factorial::getFactorial() {





// For the factorial.


unsigned long sum = 1;





// Loop from 1 to num, adding the result to the sum.


for (int i = 1; i %26lt;= num; ++i) {





// Multiply current sum times i.


sum *= i;





} // End of the for loop.





return sum;





} // end of the returnFactorial() function.





// Start the main() function.


int main() {





// Declare the variable for the user input.


unsigned short numberIn = 0;





// Prompt the user, take, and validate the input.


std::cout %26lt;%26lt; "Enter a small, positive integer: [##] ";


while (!(std::cin %26gt;%26gt; numberIn) || ((numberIn %26lt; 1) || (numberIn %26gt; 20))) {





// Problem! Clear cin and reprompt.


std::cin.clear();


std::cin.ignore(100, '\n');


std::cout %26lt;%26lt; "I said, 'Please enter a positive integer.' Please: [##] ";





} // End of WHILE.





// Discard any extraneous input.


std::cin.ignore(100, '\n');





// Create the object.


Factorial f(numberIn);





// Print the results.


if (f.inRange()) {








std::cout %26lt;%26lt; "The factorial of " %26lt;%26lt; numberIn %26lt;%26lt; " is "


%26lt;%26lt; f.getFactorial() %26lt;%26lt; ".\n\n";





} else {





std::cout %26lt;%26lt; "The factorial of " %26lt;%26lt; numberIn


%26lt;%26lt; " cannot be calculated.\n"


%26lt;%26lt; "Use a smaller number.\n\n";





}





std::cout %26lt;%26lt; "Press enter. \n";


std::cin.get();


return 0;





} // End of the main() function.