# Bad maths **URL:** \ **Category:** ruby-talk\ **Created:** [19 May 2004 22:56 UTC](https://rubytalk.org/t/bad-maths/11634 "2004-05-19T22:56:01Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Martin\_Larsson1](https://avatars.discourse-cdn.com/v4/letter/m/90db22/32.png) [@Martin\_Larsson1](https://rubytalk.org/u/Martin_Larsson1)\ **Post date:** [19 May 2004 22:56 UTC](https://rubytalk.org/t/bad-maths/11634/1 "2004-05-19T22:56:01Z")
Hi i = 0 100.times { p i += 0.6 } results in: 5.6 5.7 5.8 5.9 5.99999999999999 6.09999999999999 6.19999999999999 on my XP maching with ruby 1.8.1-13. I used rubyinstaller to install it if that matters anyone more have this problem? anyway to fix it? ---
**Author:** ![Martin\_Larsson1](https://avatars.discourse-cdn.com/v4/letter/m/90db22/32.png) [@Martin\_Larsson1](https://rubytalk.org/u/Martin_Larsson1)\ **Post date:** [19 May 2004 22:58 UTC](https://rubytalk.org/t/bad-maths/11634/2 "2004-05-19T22:58:21Z")
Martin larsson wrote: > Hi > > i = 0 > 100.times { > p i += 0.6 > } > > results in: > 5.6 > 5.7 > 5.8 > 5.9 > 5.99999999999999 > 6.09999999999999 > 6.19999999999999 > > on my XP maching with ruby 1.8.1-13. I used rubyinstaller to install > it if that matters > > anyone more have this problem? anyway to fix it? sry, i was testing around with different numbers so i copied wrong result for that litte script, just change 0.6 to 0.1 and the the result is right ---
**Author:** ![HAL\_9000](https://avatars.discourse-cdn.com/v4/letter/h/90ced4/32.png) [@HAL\_9000](https://rubytalk.org/u/HAL_9000)\ **Post date:** [19 May 2004 23:10 UTC](https://rubytalk.org/t/bad-maths/11634/3 "2004-05-19T23:10:02Z")
Martin larsson wrote: > Hi > > i = 0 > 100.times { > p i += 0.6 > } > > results in: > 5.6 > 5.7 > 5.8 > 5.9 > 5.99999999999999 > 6.09999999999999 > 6.19999999999999 > > on my XP maching with ruby 1.8.1-13. I used rubyinstaller to install it > if that matters > > anyone more have this problem? anyway to fix it? Look for a thread about a week or so ago. 🙂 I suggest you round off the numbers for output and/or for calculations. And use integer arithmetic wherever you can. Hal ---
**Author:** ![Florian\_Gross](https://avatars.discourse-cdn.com/v4/letter/f/e9a140/32.png) [@Florian\_Gross](https://rubytalk.org/u/Florian_Gross)\ **Post date:** [20 May 2004 00:18 UTC](https://rubytalk.org/t/bad-maths/11634/4 "2004-05-20T00:18:48Z")
Martin larsson wrote: > Hi Moin! > i = 0 > 100.times { > p i += 0.6 > } Try this: 100.times { |i| p (i + 1) \* 0.6 } Regards, Florian Gross ---
**Author:** ![Lloyd\_Zusman](https://avatars.discourse-cdn.com/v4/letter/l/8baadc/32.png) [@Lloyd\_Zusman](https://rubytalk.org/u/Lloyd_Zusman)\ **Post date:** [19 May 2004 23:20 UTC](https://rubytalk.org/t/bad-maths/11634/5 "2004-05-19T23:20:47Z")
Martin larsson [morg@borgeby.se](mailto:morg@borgeby.se) writes: > Martin larsson wrote: > > > Hi > > > > i = 0 > > 100.times { > > p i += 0.6 > > } > > > > results in: > > 5.6 > > 5.7 > > 5.8 > > 5.9 > > 5.99999999999999 > > 6.09999999999999 > > 6.19999999999999 > > > > on my XP maching with ruby 1.8.1-13. I used rubyinstaller to install > > it if that matters > > > > anyone more have this problem? anyway to fix it? > > sry, i was testing around with different numbers so i copied wrong > result for that litte script, just change 0.6 to 0.1 and the the result > is right First, look up “floating point arithmetic” on Google (include the double quotes in your search). This link is also quite useful: [http://docs.sun.com/source/806-3568/ncg\_goldberg.html](http://docs.sun.com/source/806-3568/ncg_goldberg.html) Then, read the recent thread in this list called “Major addition bug?”. This thread starts with the following message ID: [200405120948.51036.sean@celsoft.com](mailto:200405120948.51036.sean@celsoft.com) > **···** > > – > Lloyd Zusman > [ljz@asfast.com](mailto:ljz@asfast.com) > God bless you. ---
**Author:** ![oinkoink](https://avatars.discourse-cdn.com/v4/letter/o/8e8cbc/32.png) [@oinkoink](https://rubytalk.org/u/oinkoink)\ **Post date:** [20 May 2004 04:53 UTC](https://rubytalk.org/t/bad-maths/11634/6 "2004-05-20T04:53:48Z")
Martin larsson [morg@borgeby.se](mailto:morg@borgeby.se) wrote in message news:[40ABE654.7070601@borgeby.se](mailto:40ABE654.7070601@borgeby.se)… > Martin larsson wrote: > [repeated adding of 0.1 not coming out exact] The problem is that the computer thinks in binary and 0.1 has an infinitely long binary representation. (You won’t see this “problem” if you instead add 0.5, which has a finitely long representation) This is not a ruby problem, but a limitation of the computer’s floating point. In general, you should regard floating point numbers as being slightly inexact. If you need exactness, require the mathn library and use rationals. Imitating your example, you’d get something like: require ‘mathn’ i = 5 11.times {puts i += 1/10} # =\> exact rational results As an alternative, you could limit the precision of your output, which effectively rounds the numbers to output precision: i = 5.5 5.times { puts “%5.3f” % (i += 0.1)} #output follows: 5.600 5.700 5.800 5.900 6.000 Regards, Bret Bret Jolly (Jou Lii Bair) ---
**Author:** ![oinkoink](https://avatars.discourse-cdn.com/v4/letter/o/8e8cbc/32.png) [@oinkoink](https://rubytalk.org/u/oinkoink)\ **Post date:** [20 May 2004 06:08 UTC](https://rubytalk.org/t/bad-maths/11634/7 "2004-05-20T06:08:49Z")
A small addendum to my previous answer. If you need exact decimal arithmetic (as you do if the decimals represent dollars and cents) you should use the BigDecimal class. That works in base 10 rather than base 2. (Actually, it works in base 10000, which is mathematically equivalent to base 10.) Imitating your example: require 'bigdecimal’ a = BigDecimal(“5.5”, 20) # the 20 indicates the precision 5.times {puts (a += BigDecimal(“0.1”)).to\_s} # output follows: 0.56E1 0.57E1 0.58E1 0.59E1 0.6E1 No nonsense about 5.99999… here. The BigDecimal class is useful when you want the arithmetic to be decimal rather than binary, and especially when you need extra precision beyond what floating-point can achieve. It is, however, slower than floating point, a bit trickier to use (since you have to set precision or make sure the implicit settings are acceptable) and its method library is less rich than what the Math module provides for Float. Regards, Bret