Bug #4394: Bug in floating point multiplication

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 1.9.2p136 (2010-12-25 revision 30365) [x86_64-linux]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[philomath (Stefan Lembach)]

I stumbled upon the following odd rounding error in Float multiplication while using irb to calculate the tip for the pizza service ;-):

12.4 * 1.05 yields 13.020000000000001, but should be 13.02

[jballanc (Joshua Ballanco)]

This is how floating point math works. See here for more info: http://en.wikipedia.org/wiki/Floating_point#Representable_numbers.2C_conversion_and_rounding

If you want exact decimal math, try:

ruby-1.9.2-p136 :001 > require 'bigdecimal'
 => true 
ruby-1.9.2-p136 :002 > (BigDecimal('12.4') * BigDecimal('1.05')).to_f
 => 13.02 

[naruse (Yui NARUSE)]

• Status changed from Open to Rejected

Learn floating point numbers.

What Every Computer Scientist Should Know About Floating-Point Arithmetic
http://docs.sun.com/source/806-3568/ncg_goldberg.html
http://wiki.github.com/rdp/ruby_tutorials_core/ruby-talk-faq#floats_imprecise
http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

[rogerdpack (Roger Pack)]

You can also do

(12.4 * 1.05).round(2)
=> 13.02

[StoneBritanney (Britanney Stone)]

• Description updated (diff)

• Tracker changed from Bug to Feature
• Project changed from Ruby master to 14
• Subject changed from Bug in floating point multiplication to Здесь Вас ждут большие выигрыши!
• Description updated (diff)
• Target version deleted (1.9.2)
• ruby -v deleted (ruby 1.9.2p136 (2010-12-25 revision 30365) [x86_64-linux])

[hsbt (Hiroshi SHIBATA)]

• Tracker changed from Feature to Bug
• Project changed from 14 to Ruby master
• Subject changed from Здесь Вас ждут большие выигрыши! to Bug in floating point multiplication
• Description updated (diff)
• ruby -v set to ruby 1.9.2p136 (2010-12-25 revision 30365) [x86_64-linux]
• Backport set to 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

• File 6.gif added

[hsbt (Hiroshi SHIBATA)]

• File deleted (6.gif)