Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!sdd.hp.com!wuarchive!uunet!convex!usenet
From: tchrist@convex.COM (Tom Christiansen)
Newsgroups: comp.lang.perl
Subject: Re: Addition of floating point problem
Message-ID: <1991May07.202640.21635@convex.com>
Date: 7 May 91 20:26:40 GMT
References: <1991May07.174519.13307@dircon.co.uk>
Sender: usenet@convex.com (news access account)
Reply-To: tchrist@convex.COM (Tom Christiansen)
Organization: CONVEX Software Development, Richardson, TX
Lines: 51
Nntp-Posting-Host: pixel.convex.com

From the keyboard of uaa1006@dircon.co.uk (Peter Miles):
:
:Hi...I'm trying to write some maths scripts for perl and have 
:been have a major problem. This is best summed up by the following:
:
:$ perl -e 'print 0.5 + 0.2,"\n"'
:0.6999999999999996
:$
:
:What's going on? I'm running perl verion 4.0.1.1 patchlevel 3 on
:SCO UNIX 386 v 3.2.2. It's the version from comp.sources.misc.

What's going is that your floating point hardware is giving you a value
that isn't getting rounded enough for your tastes.  Even though you say .7
and .2, you can't represent these values precisely in the internal
floating point notation.

Remember: God created the integers; and all else is the work 
of Man (and it shows :-).

You could set $# to something like '%.1g' to change the default
output format or better yet use a printf.

(From the man page:)

     $#      The output format for printed numbers.  This vari-
             able is a half-hearted attempt to emulate awk's OFMT
             variable.  There are times, however, when awk and
             perl have differing notions of what is in fact
             numeric.  Also, the initial value is %.20g rather
             than %.6g, so you need to set $# explicitly to get
             awk's value.  (Mnemonic: # is the number sign.)

On a Convex, this statement:

    perl -e 'print .7, "\n"'

yields 

    0.7

but on a Sun, I get this:

    .69999999999999995559



--tom
--
Tom Christiansen		tchrist@convex.com	convex!tchrist
		"So much mail, so little time."