Bug #10850: BigDecimal division incorrect

• Status: Rejected
• Assignee: core
• Target version: -
• ruby -v: ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-linux]
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN

[Elyasin (Elyasin Shaladi)]

I tried this in Ruby 2.1.4, 2.1.5 and 2.2.0, but in none of the versions it seems correct to me.

irb(main):041:0* amount1 = BigDecimal("241.3")
=> #BigDecimal:7f49bcb03558,'0.2413E3',18(18)

irb(main):042:0> amount2 = BigDecimal("1800")
=> #BigDecimal:7f49bcaf3400,'0.18E4',9(18)

irb(main):043:0> rate = amount1 / amount2
=> #<BigDecimal:7f49bcae8398,'0.1340555555 5555555555 5555556E0',27(45)>

irb(main):044:0> rate * amount2 #should return amount1 = 241.3 in BigDecimal, but it does not
=> #<BigDecimal:7f49bcad6a30,'0.2413000000 0000000000 00000008E3',36(45)>

My guess is that there is a bug in the BigDecimal implementation.

[shugo (Shugo Maeda)]

• Priority changed from 5 to Normal

Elyasin Shaladi wrote:

I tried this in Ruby 2.1.4, 2.1.5 and 2.2.0, but in none of the versions it seems correct to me.

irb(main):041:0* amount1 = BigDecimal("241.3")
=> #BigDecimal:7f49bcb03558,'0.2413E3',18(18)

irb(main):042:0> amount2 = BigDecimal("1800")
=> #BigDecimal:7f49bcaf3400,'0.18E4',9(18)

irb(main):043:0> rate = amount1 / amount2
=> #<BigDecimal:7f49bcae8398,'0.1340555555 5555555555 5555556E0',27(45)>

irb(main):044:0> rate * amount2 #should return amount1 = 241.3 in BigDecimal, but it does not
=> #<BigDecimal:7f49bcad6a30,'0.2413000000 0000000000 00000008E3',36(45)>

My guess is that there is a bug in the BigDecimal implementation.

The result of 241.3 / 1800 is a recurring decimal 0.1340555555.....,
which can't be represented exactly by a BigDecimal.
Please use Rational if you need an exact representation of 241.3 / 1800.

[Elyasin (Elyasin Shaladi)]

Thanks for the answer Shugo,

however I am not sure if that is easily possible for me in that case. I use BigDecimal as a fiedl type in a Rails application and don't see how I can ues Rational here :-(
Any suggestions or tips or references? would be very helpful.

[shugo (Shugo Maeda)]

• Status changed from Open to Rejected

Elyasin Shaladi wrote:

Thanks for the answer Shugo,

however I am not sure if that is easily possible for me in that case. I use BigDecimal as a fiedl type in a Rails application and don't see how I can ues Rational here :-(
Any suggestions or tips or references? would be very helpful.

You can use BigDecimal#to_r to convert a BigDecimal to a Rational.

[Elyasin (Elyasin Shaladi)]

Maybe you have an advice for the following I am thinking to use.

I could convert BigDecimal to Rational, do the calculation in Rational numbers and then convert that to back to BigDecimal.

Do you see any caveats with this approach?

Shugo Maeda wrote:

Elyasin Shaladi wrote:

Thanks for the answer Shugo,

however I am not sure if that is easily possible for me in that case. I use BigDecimal as a fiedl type in a Rails application and don't see how I can ues Rational here :-(
Any suggestions or tips or references? would be very helpful.

You can use BigDecimal#to_r to convert a BigDecimal to a Rational.