Wrong Ruby Float and BigDecimal subtraction result

Posted by SamChandra on Stack Overflow See other posts from Stack Overflow or by SamChandra
Published on 2010-06-08T09:20:38Z Indexed on 2010/06/08 10:12 UTC
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Filed under:

ruby-on-rails

|

ruby

I am running:

[~/ruby/rails/sas]$ ruby --version
ruby 1.8.7 (2009-06-08 patchlevel 173) [universal-darwin10.0]

on Mac Snow Leopard 10.6.3

Can anyone help to explain why the Float and BigDecimal subtraction can be this wrong.

[~/ruby/rails/sas]$ console
Loading development environment (Rails 2.1.1)
>> num = 30.0
=> 30.0
>> num.class
=> Float
>> ex = 28.04.to_d
=> #<BigDecimal:105367e40,'0.284E2',8(8)>
>> ex.class
=> BigDecimal
>> num - ex
=> 1.6
>> _.class
=> Float
>>

I was hoping that the result should be 1.96, I know that perhaps doing an arithmetic operation using 2 different data types are not recommended, but this behavior is so strange.

It seems to be wise that from now on, I have to check the variables data type before doing any arithmetic operation.

Hopefully somebody can give me an insight on what was happening.

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