C/C++ What is causing my C++ program to truncate Euler's number?

[Superposed_Cat]

Hi, just started c++ and I made this program to calculate the value of eulers number. It works in c# but truncates the decimal points as far as I can tell. What am I doing wrong?

#include <iostream>

using namespace std;
int factorial(int a){
    int b=1;
while(a>0){
    b=b*a;
    a--;
    
    }
    return(b);
}

int main()
{
 float x=1.0;
 for(int i=1;i<25;i++){
     x=x+1/factorial(i);
 }
   cout <<x; 

   return 0;
}

 

[UltrafastPED]

factorial is an integer function; cast it to float ...

 

[jedishrfu]

The 1/factorial will do integer division. You need to use 1.0 to get floating pt division.

 

[Superposed_Cat]

Thanks. Those must be the fastest responses I've ever had.

 

[Mark44]

For more precision, use double instead of float.

Also, many programmers would write the assignment statement in the for loop as

x += 1.0/factorial(i);

You can do something similar in the code in your factorial function.

Your indentation could be tweaked a bit to make your code more readable. On the plus side you used code tags - good!

Here's how I would indent this code. I have also added more spaces

#include <iostream>

using namespace std;

int factorial(int a)
{
    int b = 1;
    while(a > 0){
       b *= a;
       a--;
    
   }
   return(b);
}

int main()
{
   float x = 1.0;
   for(int i = 1; i < 25; i++){
      x += 1.0/factorial(i);
   }
   cout <<x; 

   return 0;
}

 

[Superposed_Cat]

noticed that after posting.(the += thing the double I completely forgot about, thanks)

 

[rcgldr]

Note that 25! is greater than 2^83. 18! is greater than 2^52, so truncation will occur at i >= 18 when adding 1/i! to a double >= 2.0. You could reverse the order of the loop (for i = 20; i >= 1; i--) and use 64 bit integers for factorial (20! < 2^62), but it wouldn't make much difference.