Bug #5477: to_f function / multiplication bug

• Status: Rejected
• Assignee: -
• Target version: 1.9.2
• ruby -v: ruby 1.9.2p174 (2011-01-28 revision 30696) [x86_64-darwin10.6.0]

[notomodungwa (Noto Modungwa)]

This is posted on stack overflow but I thought I should report it here
Hi I just ran into an issue where ruby's to_f function is giving me inconsistent results.

ruby-1.9.2-head :026 > 8.45.to_f * 100 => 844.9999999999999

ruby-1.9.2-head :027 > 4.45.to_f * 100 => 445.0

ruby-1.9.2-head :028 > 4.35.to_f * 100 => 434.99999999999994

My workaround is to simply round the result this way

ruby-1.9.2-head :029 > (4.35.to_f * 100).round => 435

After more playing around I realised that the issue might be with the multiplication operator "* 100"

[notomodungwa (Noto Modungwa)]

I have been educated that this is more a floating number calculation issue than an issue with ruby itself by guys on stackoverflow see http://stackoverflow.com/questions/7879606/ruby-to-f-bug-or-multiplication-operator-bug

This document Floating-Point Arithmetic's section on "Rounding Error" mentions that you should round the result of a floating point calculation. =>"...Therefore the result of a floating-point calculation must often be rounded in order to fit back into its finite representation..."
http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html

[drbrain (Eric Hodel)]

• Status changed from Open to Rejected

Yes, see also:

http://wiki.github.com/rdp/ruby_tutorials_core/ruby-talk-faq#floats_imprecise
http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems