Subj : Serious Problem with precision or if statement
To   : borland.public.cpp.borlandcpp
From : "Chris" <mudd1_wheels@hotmail.com>
Date : Wed Aug 13 2003 08:47 am

Hi everyone.
I am having a problem with the following program. It's very
simple but the results are very unexpected (at least I think so).

code
-----------------------
#include <stdio.h>

int main() {

  int Phase = 3;

  if( Phase/3.0 == 1.0 )
    printf("equal");
  else
    printf("not equal");

  return 1;
}
-----------------------

Now we would expect "equal" to be printed, but to my 
(everyone's) surprise, "not equal" is printed.

An even more extraordinary thing occurs when tracing the program.
The expression ( Phase/3.0 == 1.0 ) evaluates to TRUE. However,
the else(FALSE) branch is taken printing "not equal"!

I have tried several modifications to the if statement 
to no avail.

if( (double)Phase/3 == 1.0 )
if( Phase/3.0 == (double) 1 )

It is worthwhile to note that
if( Phase/3 == 1 ) does return the correct answer.

Assigning the value (Phase/3.0) to an intermediate double x
and then checking if( x == 1.0 ) also works. 

If anyone has time or wants some amusement, run my program.
I am using Borland C++ 5.02 on an Intel Pentium III with Windows
XP. I get the same results with a AMD Duron with Windows 98.

Under LINUX compiled for i586, compiling and running my program
with gcc returns "equal", the correct answer.

Thanks. Please post your answers or results.

.