Bug #4022: Mistake in this cocient of logs followed by %1

• Status: Rejected
• Assignee: -
• Target version: 1.9.1
• ruby -v: 1.9.1

[dsilber (Daniel Silberschmidt)]

=begin
I get a wrong answer when I apply the following expression: Math.log(4913)/Math.log(17) % 1
This happens in ruby 1.9.1 and in ruby 1.8.7
With other set of numbers the answers are correct

irb(main):001:0> Math.log(4913)/Math.log(17) % 1
=> 1.0
irb(main):002:0> Math.log(4913)/Math.log(17)
=> 3.0
irb(main):003:0> 3.0 % 1
=> 0.0
irb(main):004:0> (Math.log(4913)/Math.log(17)) % 1
=> 1.0
=end

[judofyr (Magnus Holm)]

=begin
That's just how floats work:

Math.log(4913)/Math.log(17) == 3.0
=> false
"%.50f" % (Math.log(4913)/Math.log(17))
=> "2.99999999999999955591079014993738383054733276367188"
"%.50f" % 3.0
=> "3.00000000000000000000000000000000000000000000000000"

// Magnus Holm

On Wed, Nov 3, 2010 at 17:52, Daniel Silberschmidt [e-mail]:

Bug #4022: Mistake in this cocient of logs followed by %1
http://redmine.ruby-lang.org/issues/show/4022

Author: Daniel Silberschmidt
Status: Open, Priority: Normal
Target version: 1.9.1
ruby -v: 1.9.1

I get a wrong answer when I apply the following expression:
Math.log(4913)/Math.log(17) % 1
This happens in ruby 1.9.1 and in ruby 1.8.7
With other set of numbers the answers are correct

irb(main):001:0> Math.log(4913)/Math.log(17) % 1
=> 1.0
irb(main):002:0> Math.log(4913)/Math.log(17)
=> 3.0
irb(main):003:0> 3.0 % 1
=> 0.0
irb(main):004:0> (Math.log(4913)/Math.log(17)) % 1
=> 1.0

---

http://redmine.ruby-lang.org

That's just how floats work:

>> Math.log(4913)/Math.log(17) == 3.0

=> false

>> "%.50f" % (Math.log(4913)/Math.log(17))

=> "2.99999999999999955591079014993738383054733276367188"

>> "%.50f" % 3.0

=> "3.00000000000000000000000000000000000000000000000000"

// Magnus Holm

On Wed, Nov 3, 2010 at 17:52, Daniel Silberschmidt <[e-mail]> wrote:

Bug #4022: Mistake in this cocient of logs followed by %1

http://redmine.ruby-lang.org/issues/show/4022



Author: Daniel Silberschmidt

Status: Open, Priority: Normal

Target version: 1.9.1

ruby -v: 1.9.1



I get a wrong answer when I apply the following expression: Math.log(4913)/Math.log(17) % 1

This happens in ruby 1.9.1 and in ruby 1.8.7

With other set of numbers the answers are correct



irb(main):001:0> Math.log(4913)/Math.log(17) % 1

=> 1.0

irb(main):002:0> Math.log(4913)/Math.log(17)

=> 3.0

irb(main):003:0> 3.0 % 1

=> 0.0

irb(main):004:0> (Math.log(4913)/Math.log(17)) % 1

=> 1.0





----------------------------------------

http://redmine.ruby-lang.org

=end

[naruse (Yui NARUSE)]

• Status changed from Open to Rejected

=begin
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
=end