How do I find a factorial? [closed]

Answer 1

Example in C using recursion

unsigned long factorial(unsigned long f)
{
        if (f) return(f * factorial(f - 1));
        return 1;
}

printf("%lu", factorial(5));

Answer 2

I don't think I'd use this in most cases, but one well-known practice which is becoming less widely used is to have a look-up table. If we're only working with built-in types, the memory hit is tiny.

Just another approach, to make the poster aware of a different technique. Many recursive solutions also can be memoized whereby a lookup table is filled in when the algorithm runs, drastically reducing the cost on future calls (kind of like the principle behind .NET JIT compilation I guess).

Answer 3

I would do this with a pre-calculated lookup table as suggested by Mr. Boy. This would be faster to calculate than an iterative or recursive solution. It relies on how fast n! grows, because the largest n! you can calculate without overflowing an unsigned long long (max value of 18,446,744,073,709,551,615) is only 20!, so you only need an array with 21 elements. Here's how it would look in c:

long long factorial (int n) {
    long long f[22] = {1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800, 39916800, 479001600, 6227020800, 87178291200, 1307674368000, 20922789888000, 355687428096000, 6402373705728000, 121645100408832000, 2432902008176640000, 51090942171709440000};
    return f[n]; 
}

See for yourself!

Answer 4

You use the following code to do it.

#include    <stdio.h>
#include    <stdlib.h>

int main()
{
   int x, number, fac;
   fac = 1;
   printf("Enter a number:\n");
   scanf("%d",&number);

   if(number<0)
   {
      printf("Factorial not defined for negative numbers.\n");
      exit(0);
   }

   for(x = 1; x <= number; x++)
   {
      if (number >= 0)
         fac = fac * x;
      else
         fac=1;
   }

   printf("%d! = %d\n", number, fac);
} 

Answer 5

Simple solution:

unsigned int factorial(unsigned int n)
{
   return (n == 1 || n == 0) ? 1 : factorial(n - 1) * n;
}

Answer 6

I used this code for Factorial:

#include<stdio.h>
int main(){
  int i=1,f=1,n;

  printf("\n\nEnter a number: ");
  scanf("%d",&n);
  while(i<=n){
      f=f*i;
      i++;
  }
  printf("Factorial of is: %d",f);
  getch();
}