# Force calculation result to BigDecimal **URL:** \ **Category:** Ruby\ **Created:** [July 17, 2007, 1:34am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579 "2007-07-17T01:34:28Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 1:34am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/1 "2007-07-17T01:34:28Z")
Hi all, I’m doing some complex math which I need to be based on BigDecimal accuracy (not Float). It seems that ruby defaults to Float when there’s a mixture of type in there, e.g. Code : 1. irb(main):001:0\> require ‘bigdecimal’ 2. =\> true 3. irb(main):002:0\> a = 5.123 4. =\> 5.123 5. irb(main):003:0\> b = BigDecimal(“6.789”) 6. =\> #BigDecimal:8b4c0,‘0.6789E1’,8(8) 7. irb(main):004:0\> c = a \* b 8. =\> 34.780047 9. irb(main):005:0\> c.class 10. =\> Float Is there a way I can force a BigDecimal outcome. One way I guess would work is to make sure all args in the calculation are BigDecimal, but that’s a bit tiresome. For a start, I’d have to type check each arg and convert to BigDecimal if it was not BigDecimal - that’s gotta be slow… On a related note, how do I initalize a BigDecimal from a Float (without converting to string first). How could I extend BigDecimal for this? Thanks, Rob ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 7:50am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/2 "2007-07-17T07:50:02Z")
> Here’s a quick fix. There may be a better way to handle this, though. > Again, I’ll give the usual warning about modifying core classes: you > shouldn’t normally do it unless you can be certain it won’t break > yours or anyone else’s code. Thanks Todd… What are the risks in modifying core classes? I’ve already done this a little by adding .round(n) to BigDecimal, where n is the decimal places. Appreciate your tip, but I was thinking the other way round: have BigDecimal take Float as an initializer, not have Float output BigDecimal. But this is all by-the-by. What I’m really looking for is BigDecimal math, not Float, in my calculations (when there’s a mixture of BigDecimal, Float and Fixnum types). Still looking for suggestions on how I can do this better than manual type-checking and casting… Thanks, ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 9:09am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/3 "2007-07-17T09:09:07Z")
Hi, Am Dienstag, 17. Jul 2007, 14:50:18 +0900 schrieb Robert B.: > Appreciate your tip, but I was thinking the other way round: have > BigDecimal take Float as an initializer, not have Float output > BigDecimal. -From time to time I wonder why there is no BigDecimal initializer taking a Float argument. There seems to be a point as Float values aren’t unambiguous and you have to explicitly mention which one you mean of “%f” % 1.8 # =\> “1.800000” “%32.24f” % 1.8 # =\> " 1.800000000000000044408921" So I’d rather say it’s disencouraged to convert Float to BigDecimal. Bertram ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 9:30am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/4 "2007-07-17T09:30:01Z")
Bertram S. wrote: > So I’d rather say it’s disencouraged to convert Float to > BigDecimal. Indeed! I’d rather not use Float at all in this project as it’s financial math and float quirks will cause me problems. I’d be prepared to take the performance hit and just turn off Float if I could… ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 3:18am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/5 "2007-07-17T03:18:39Z")
On 7/16/07, Robert B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 1. irb(main):002:0\> a = 5.123 > One way I guess would work is to make sure all args in the calculation > > Rob Here’s a quick fix. There may be a better way to handle this, though. Again, I’ll give the usual warning about modifying core classes: you shouldn’t normally do it unless you can be certain it won’t break yours or anyone else’s code. class Float require ‘bigdecimal’ def big BigDecimal self.to\_s end end p pi = Math::PI.big # prints out something like # #\ **Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 9:31am UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/6 "2007-07-17T09:31:12Z") 2007/7/17, Todd B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > > 1. =\> true > > > > Rob > > Here’s a quick fix. There may be a better way to handle this, though. Hm… I don’t think this is a fix because once you have a float you lost all the precision. A better fix is probably to change #coerce to work properly. Another solution is to make sure all values are converted to BD before starting the calculation. Note, that BigDecimal is returned when working with integers. > Again, I’ll give the usual warning about modifying core classes: you > shouldn’t normally do it unless you can be certain it won’t break > yours or anyone else’s code. Very good point! Kind regards robert ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 2:14pm UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/7 "2007-07-17T14:14:10Z")
On 7/17/07, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hm… I don’t think this is a fix because once you have a float you > lost all the precision. Can you elaborate on this? I was under the impression that converting to BigDecimal before arithmetic operations would get around this. Using class Float def big; BigDecimal(self.to\_s); end end irb \> Math::PI.big \* 2.0 =\> 6.283185307178 irb \> Math::PI.big \* 2.0.big =\> #\ > A better fix is probably to change #coerce to > work properly. Another solution is to make sure all values are > converted to BD before starting the calculation. This would be the best solution (maybe through #induced\_from?), but I don’t know how to do it. > Note, that BigDecimal is returned when working with integers. Todd ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 3:34pm UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/8 "2007-07-17T15:34:22Z")
2007/7/17, Todd B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > On 7/17/07, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > Hm… I don’t think this is a fix because once you have a float you > > lost all the precision. > > Can you elaborate on this? I was under the impression that converting > to BigDecimal before arithmetic operations would get around this. Actually you are right. I somehow thought you intended #big to be called on the result, i.e. after the calculation. Stupid me. Sorry for the confusion. > > work properly. Another solution is to make sure all values are > > converted to BD before starting the calculation. > > This would be the best solution (maybe through #induced\_from?), but I > don’t know how to do it. It would take me too much time ATM to do it and it should also be checked whether changing #coerce causes issues somewhere else… Kind regards robert ---
**Author:** ![rjberry](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rjberry](https://www.ruby-forum.com/u/rjberry)\ **Post date:** [July 17, 2007, 3:04pm UTC](https://www.ruby-forum.com/t/force-calculation-result-to-bigdecimal/109579/9 "2007-07-17T15:04:15Z")
Dear Robert, > Indeed! I’d rather not use Float at all in this project as it’s > financial math and float quirks will cause me problems. I’d be prepared > to take the performance hit and just turn off Float if I could… you could use a “continued fractions” approximation > **[Continued Fraction -- from Wolfram MathWorld](https://mathworld.wolfram.com/ContinuedFraction.html)** > > The term "continued fraction" is used to refer to a class of expressions of which generalized continued fraction of the form b\_0+(a\_1)/(b\_1+(a\_2)/(b\_2+(a\_3)/(b\_3+...))) =b\_0+K\_(n=1)^infty(a\_n)/(b\_n) (and the terms may be integers, reals,... to your Float number and then work with fractions without any further loss in accuracy. Continued fractions are in a sense the best possible approximations to irrational numbers (and of course to rational numbers, the latter being fractions themselves). Once you have a fraction (use the code below to get it from a continued fraction), you can calculate with it in Ruby using Rational: require “rational” a=Rational(1,2) b=Rational(3,4) p a+b # =\> Rational (5,4) , as 5/4 == 1/2+3/4 p (a+b).to\_f # =\> 1.25 Below is some code to give you the continued fraction representation (4) in the webpage cited above. From the continued fraction representation, you can obtain the partial quotients mentioned in (11) of the cited webpage by the method cont\_fract\_to\_fract below. Best regards, Axel * * * class Float def cont\_fract\_appr(_prec) a=[self.floor] t=[self-self.floor] res=[] if prec==[] prec=10_\*-8 else prec=prec[0] end k=1 rem=t[-1] while rem.abs\>prec r\_new=1/t[-1] if (r\_new+prec).floor\>r\_new.floor r\_new=r\_new+prec end a[k]=r\_new.floor t[k]=r\_new-r\_new.floor k=k+1 p,q=(a.cont\_fract\_to\_fract) rem=(self-p[-1].to\_f/q[-1].to\_f).abs end return a end end class Array def cont\_fract\_to\_fract # produce the partial fractions (see eq. 11 of webpage) p=[0,1] q=[1,0] for n in 2…self.length+1 p\<\