Re: [ID 20000917.004] Internal floating-point rounding error?

Depending on your actual needs, you may want to consider not using FP
numbers at all, due to the rouding problems you have noticed.  You can
user Math::BigFloat or (my own) Math::FixedPrecision to do exact FP math
instead.  Since both modules ultimately depend on Math::Integer, they
are currently not fast, but you will get what you expect.  M:FP auto
rounds to a specified number of decimal places, whereas M::BF does not.

Note that the Math::BigFloat currently distributed with 5.6.0 has bugs;
a patched version should be in 5.6.1 (hopefully RSN).  It is also
included in Math::FixedPrecision v0.10.

John Peacock

Philip Andrew wrote:
> 
> This is a bug report for perl from Philip.Andrew@stanfordalumni.org,
> generated with the help of perlbug 1.28 running under perl v5.6.0.
> 
> -----------------------------------------------------------------
> [Please enter your report here]
> 
> I get strange results running the following Perl stub:
> 
>     printf "%04d\n", $ARGV[0]/$ARGV[1];
> 
> Results:
> 
>     # Command Run       =>      Output
>     # -----------               ------
>     stub.pl 0.55 0.01   =>      0055
>     stub.pl 0.56 0.01   =>      0056
>     stub.pl 0.57 0.01   =>      0056    <== HUH?
>     stub.pl 0.58 0.01   =>      0057    <== HUH?
>     stub.pl 0.59 0.01   =>      0058    <== HUH?
>     stub.pl 0.60 0.01   =>      0060
>     stub.pl 0.61 0.01   =>      0061
>     stub.pl 0.055 0.001 =>      0055
>     stub.pl 0.056 0.001 =>      0056
>     stub.pl 0.057 0.001 =>      0057
>     stub.pl 0.058 0.001 =>      0058
>     stub.pl 0.059 0.001 =>      0058    <== HUH?
>     stub.pl 0.060 0.001 =>      0060
>     stub.pl 0.061 0.001 =>      0061
> 
> I can avoid this round-off error by changing the 'printf' to
> 
>     printf "%04d\n", ($ARGV[0]/$ARGV[1])+($ARGV[1]/10);
> 
> (adding 1/10 of my resolution to the number), but what's the
> point -- the round-off is supposed to be correct, isn't it?
> 
> I ran into this trying to create an encoding string from a real
> number, thereby eliminating the decimal point from the string
> (so that I could use the encoding in a file name, and not have an
> unwanted period added in the file name).
> 
> [Please do not change anything below this line]
> -----------------------------------------------------------------
> ---
> Flags:
>     category=core
>     severity=low
> ---
> Site configuration information for perl v5.6.0:
> 
> Configured by toolsadm at Thu Aug 17 18:36:22 PDT 2000.
> 
> Summary of my perl5 (revision 5.0 version 6 subversion 0) configuration:
>   Platform:
>     osname=solaris, osvers=2.5.1, archname=sun4-solaris
>     uname='sunos chip 5.5.1 generic_103640-08 sun4u sparc sunw,ultra-1 '
>     config_args='-de -A cc=gcc'
>     hint=recommended, useposix=true, d_sigaction=define
>     usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef
>     useperlio=undef d_sfio=undef uselargefiles=define
>     use64bitint=undef use64bitall=undef uselongdouble=undef usesocks=undef
>   Compiler:
>     cc=' gcc', optimize='-O', gccversion=2.95.2 19991024 (release)
>     cppflags='-fno-strict-aliasing -I/usr/local/include -I/opt/gnu/include'
>     ccflags ='-fno-strict-aliasing -I/usr/local/include -I/opt/gnu/include '
>     stdchar='unsigned char', d_stdstdio=define, usevfork=false
>     intsize=4, longsize=4, ptrsize=4, doublesize=8
>     d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
>     ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=4
>     alignbytes=8, usemymalloc=y, prototype=define
>   Linker and Libraries:
>     ld=' gcc', ldflags =' -L/usr/local/lib -L/opt/gnu/lib '
>     libpth=/usr/local/lib /opt/gnu/lib /lib /usr/lib /usr/ccs/lib
>     libs=-lsocket -lnsl -ldl -lm -lc -lcrypt -lsec
>     libc=/lib/libc.so, so=so, useshrplib=false, libperl=libperl.a
>   Dynamic Linking:
>     dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
>     cccdlflags='-fPIC', lddlflags='-G -L/usr/local/lib -L/opt/gnu/lib'
> 
> Locally applied patches:
> 
> 
> ---
> @INC for perl v5.6.0:
>     /usr/local/lib/perl5/5.6.0/sun4-solaris
>     /usr/local/lib/perl5/5.6.0
>     /usr/local/lib/perl5/site_perl/5.6.0/sun4-solaris
>     /usr/local/lib/perl5/site_perl/5.6.0
>     /usr/local/lib/perl5/site_perl
>     .
> 
> ---
> Environment for perl v5.6.0:
>     HOME=/home/phil
>     LANG (unset)
>     LANGUAGE (unset)
>     LC_COLLATE=en_US.ISO8859-1
>     LC_CTYPE=en_US.ISO8859-1
>     LC_MESSAGES=C
>     LC_MONETARY=en_US.ISO8859-1
>     LC_NUMERIC=en_US.ISO8859-1
>     LC_TIME=en_US.ISO8859-1
>     LD_LIBRARY_PATH=/usr/openwin/lib:/usr/local/lib:/opt/gnu/lib:/usr/lib
>     LOGDIR (unset)
>     PATH=.:/home/phil/bin:/home/phil/bin/solaris:/usr/local/bin:/opt/gnu/bin:/usr/bin:/usr/sbin:/usr/ccs/bin:/usr/ucb:/etc:/usr/dt/bin:/usr/openwin/bin
>     PERL_BADLANG (unset)
>     SHELL=/usr/local/bin/tcsh

• [ID 20000917.004] Internal floating-point rounding error? by Philip Andrew
• Re: [ID 20000917.004] Internal floating-point rounding error? by John Peacock
• Re: [ID 20000917.004] Internal floating-point rounding error? by Nick Ing-Simmons
• Re: [ID 20000917.004] Internal floating-point rounding error? by Tom Christiansen