Re: g++/gcc : Floating point issue

1 msg	Suggestion required for appropriate implementation
11 msg	An article I alluded to
3 msg	Strange floating point problems on SH4 with gcc...

g++/gcc : Floating point issue
\ abuzer.salik (26 Jul 2006)
. \ Daniel Llorens del Río (26 Jul 2006)
. \ Neil Ferguson (26 Jul 2006)
. \ Bill McEnaney (26 Jul 2006)
. \ miridian (28 Jul 2006)

1 msg	Creating a Windows dll in Linux?
2 msg	output to a directory which doesn't exit yet
1 msg	GCC 3.4.4 compilation error for arm-linux target
1 msg	problems with precompiled header - vector and a...
1 msg	builtin bounds checking how-to
1 msg	help installing gcc 4.0.3
2 msg	help spawning/executing a command line utility
1 msg	gcc symbols
3 msg	static variables, the stack, the heap, and speed
1 msg	g++ -- can unused constructors affect generated...
2 msg	sending structure containing a pointer to anoth...
4 msg	ARM Assembler
9 msg	CygWin - GCC compiler error
1 msg	GCC: Linking problem
1 msg	Fully qualified lib name.
2 msg	gcc -print-search-dirs

Subject:	Re: g++/gcc : Floating point issue
Group:	Gcc-help
From:	miridian
Date:	28 Jul 2006

abuzer.salik wrote:

> I have an application which uses double values a lot. If I try to put say
> 401.74 in a double variable it is getting stored as 401.74000000000001
> which creates problem in the calculations down the line. ( I am checking
> these values using gdb debugger).
>
> So, I am wondering if there is a way by which I can store the correct
> values (i.e. 401.74) in the double.

No.

Only dyadic rationals (a rational number of the form a/(2^b) where a is
an integer and b is a natural number) have a finite binary expansion.

40174/100 = 200087/50 (and 50 is not 2^b for a natural number b)

See the links:
http://en.wikipedia.org/wiki/Binary_numeral_system#Representing_real_numbers
http://en.wikipedia.org/wiki/Dyadic_fraction

> Do we need to look for another compiler/hardware or current setup can
> handle this?

You can use an arbitrary precision numeric library, for ex. GMP
http://www.swox.com/gmp/

Regars,
Luca.