math not quite accurate in C program

[uman]

I am a newb, just started learning C a few days ago. I wrote a program that calculates factorials and displays them in scientific notation.

Example:
$ ./factorial
3
3 factorial is 6.00000000000000000000000000000000000000000000000e 0

It works fine. But when I try to do a number that requires the scientific notation to be used, it is slightly off.
Example:
$ ./factorial
4
4 factorial is 2.39999999999999991118215802998747676610946655273e 1

Here is the code:
main()
{
int num,e,x;
double work=1.0;
scanf("%d", &num);
for (x=1; x<=num; ++x)
{
work=work*x;
while (work>=10.0)
{
work=work/10.0;
++e;
}
}
printf ("%d factorial is %47.47fe%d\n",num,work,e);
}
Please help me to diagnose what is wrong with my program!

[Valmont]

Welcome Uman.
I can't help you in C (others might). But since you just started, why don't you start learning C++ instead?

[frrossk]

Hello Uman

Try this:

#include <stdio.h>
#include <conio.h>

int main()
{
int num,e,x;
double work=1.0;
scanf("%d", &num);
for (x=1; x<=num; ++x)
{
work=work*x;
/*while (work>=10.0)
{
work=work/10.0;
++e;
}*/
}
printf ("%d factorial is %.3e\n",num,work);
getch ();
return 0;
}

To print a number in scientific format, you can use the "e" format argument.

[Valmont]

However, beware that conio.h is not part of the C standard .

[joe_bruin]

a good c programmer, of course, always checks for error conditions.

#include <math.h>
#include <stdio.h>

int main(void)
{
  int i;
  double f = 1.0;
  
  switch(scanf("%d", &i))
  {
  case 0:
  case EOF:
    puts("invalid input");
    return 1;
  }
  if(i < 0)
  {
    puts("undefined");
    return 0;
  }
  for( ; i > 1; i--)
  {
    f *= i;
    if(isinf(f))
    {
      puts("number too large");
      return 0;
    }
  }
  printf("%.3e\n", f);
  return 0;
}