|Division in C++ email@example.com (Onno Garms) (2005-07-11)|
|Re: Division in C++ firstname.lastname@example.org (Antoun Kanawati) (2005-07-12)|
|Re: Division in C++ email@example.com (Marcin 'Qrczak' Kowalczyk) (2005-07-12)|
|Re: Division in C++ firstname.lastname@example.org (Michael Tiomkin) (2005-07-12)|
|Re: Division in C++ email@example.com (2005-07-12)|
|Re: Division in C++ firstname.lastname@example.org (Florian Weimer) (2005-07-12)|
|Re: Division in C++ email@example.com (Gabriel Dos Reis) (2005-07-12)|
|[3 later articles]|
|From:||Onno Garms <firstname.lastname@example.org>|
|Date:||11 Jul 2005 10:50:08 -0400|
|Keywords:||code, question, comment|
I have a short sample program that hangs on one of my
computer when I compile in debug mode. The program works
fine on my other computers or if I compile optimized.
The problematic computer and compiler are quite old (gcc2.95
on a Pentium PC running Linux), but I wonder if the problem
will occur with other compilers on other computers if I
change the numbers.
Here is the code:
int main ()
double a = 96.03755458500125997;
double b = 3.0;
c = a/b;
if (a/b<=c) break;
Can anybody explain why this hangs?
What I want the program to do in the loop is the following:
1. compute a/b
2. compute a/b again
3. compare to previous result
4. see that it is the same and break the loop
However the program runs in an endless loop.
Regardless of any rounding errors, a/b should always return
the same value, shouldn't it?
- Storing the result of the second computation in another
variable d and then comparing d to c works correctly.
- if (static_cast<double>(a/b)<=c)
- Finally, the program below prints "not yet" (and nothing
c = a/b;
if ((d=a/b)<=c) break;
std::cout << "not yet\n";
if (d<=c) break;
std::cout << "but now\n";
Does the gnu compiler apply "optimizations" that cause the
behaviour? Which ones? (I compile without options.)
Or what else causes this strange behaviour?
[This is a small enough loop that I'd just look at the assembler code.
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