Using trunc function in Erlang

Erlang Questions <>
Fri Dec 30 20:50:29 CET 2005


I was afraid about that.
Unfortunately, these numbers represent money in my application :o(

Regards, Carlos.-

----- Original Message ----- 
From: "Richard Cameron" <>
To: "Erlang Questions" <>
Cc: "Erlang Questions" <>
Sent: Friday, December 30, 2005 3:35 PM
Subject: Re: Using trunc function in Erlang


> 
> On 30 Dec 2005, at 18:15, Erlang Questions wrote:
> 
> > Hello everybody, I have one question about Erlang trunc function.
> >
> > Why Erlang behaves like this?
> >
> > 1> trunc(1.3/0.1).
> > 13
> > 2> trunc(1.4/0.1).
> > 13
> > 3> trunc(1.5/0.1).
> > 15
> 
> Unfortunately, this isn't an Erlang problem at all. Look at the  
> output of this little C program on my machine:
> 
> wilt:/tmp rcameron$ cat floor.c
> #include <math.h>
> #include <stdio.h>
> main() {
>          printf("%f\n", floor(1.4/0.1));
> }
> wilt:/tmp rcameron$ gcc -o floor floor.c
> wilt:/tmp rcameron$ ./floor
> 13.000000
> 
> Computers have a nasty habit of doing things you might not  
> intuitively expect with floating point operations. You sort of just  
> have to accept that float operations are not exact, so the computation:
> 
> 1.4/0.1
> 
> actually produces a result in the range
> 
> 14.0000000000000 +/- epsilon
> 
> where epsilon is (very) small. Unfortunately, in your case, it's  
> every so slightly negative, so
> 
> erlang:trunc(14.0000000000000 - Epsilon).
> 
> evaluates to 13.
> 
> > And which you think is the best way to manage this in order to  
> > solve this kind of problems?
> 
> It depends very much what you're trying to do. What do these numbers  
> represent in your application?
> 
> Richard.
> 
> 




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