[rspec-users] Why can not a BigDecimal be compared to a Float via "==". How should I handle this???

Maurício Linhares mauricio.linhares at gmail.com
Sun Jan 11 15:45:28 EST 2009


This is a very bad idea, as you can break the whole runtime by doing
this, as many internal classes use floating point math. You would also
slow the world down, as operations using BigDecimals are some order of
magnitude slower than pure floating point math.

-
Maurício Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/ (en)



On Sun, Jan 11, 2009 at 6:33 PM, Greg Hauptmann
<greg.hauptmann.ruby at gmail.com> wrote:
> it would be nice in one's project if you could basically say "use
> BigDecimal wherever you normally would have used a float", just to get the
> benefit but maintain ease of coding/readability etc - anyone know if this is
> possible?   Would it be enough to override Float's initializer and return a
> BigDecimal instead?
>
>
> On Mon, Jan 12, 2009 at 4:53 AM, Rick DeNatale <rick.denatale at gmail.com>
> wrote:
>>
>> On Sun, Jan 11, 2009 at 9:21 AM, David Chelimsky <dchelimsky at gmail.com>
>> wrote:
>>>
>>> On Sun, Jan 11, 2009 at 4:05 AM, Greg Hauptmann
>>> <greg.hauptmann.ruby at gmail.com> wrote:
>>> > I've gone with the following
>>> >      ai.amount.should == BigDecimal('-323.03')
>>> > However I'm still a bit surprised that Ruby itself does allow a good
>>> > "=="
>>> > test between a Float and a BigDecimal.  Perhaps there's a reason that
>>> > I'm
>>> > missing?
>>>
>>> Very telling is this:
>>>
>>> >> require 'bigdecimal'
>>> => true
>>> >> BigDecimal.new(3333333.0) == 3333333.0
>>> TypeError: can't convert Float into String
>>>        from (irb):4:in `new'
>>>        from (irb):4
>>>
>>> As for why, I think you'll get some good insights if you post the
>>> ruby-lang mailing list, but I can take shot.
>>>
>>> BigDecimal has explicit precision. Float does not. Imagine the
>>> developer at the bank explaining that the thousands of dollars
>>> discrepancy last year was due to an average miscalculation of 0.00005
>>> per transaction because sometimes the code used BigDecimal, and
>>> sometimes it used Float.
>>
>> Even more telling is this:
>> irb(main):001:0> 1.0 / 3.0
>> => 0.333333333333333
>> irb(main):002:0> (1.0 / 3.00) ==  0.333333333333333
>> => false
>>
>> This has little to do with rspec or Ruby, and everything to do with
>> floats.
>>
>> Floats are approximations, it's a mistake to thing of them as equivalent
>> to the mathematical concept of real numbers, or even rational numbers. There
>> are several issues here including
>>
>> 1. Floats are not infinite precision, they have a fixed number of bits or
>> digits, this means that in-between any two consecutive real number which CAN
>> be represented by a float, there are an infinite number of reals which
>> cannot.
>>
>> 2. As is the case in decimal fractions, where some rational numbers such
>> as 1/3 cannot be represented without an infinite number of decimal digits,
>> there are similar values dependent on the base used for the float
>> representation.
>>
>> There's a whole branch of computer science, Numerical Analysis, comprised
>> in large part of understanding how Floats differ from the mathematical
>> ideal.
>>
>>
>> --
>> Rick DeNatale
>>
>> Blog: http://talklikeaduck.denhaven2.com/
>> Twitter: http://twitter.com/RickDeNatale
>>
>> _______________________________________________
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>
>
>
> --
> Greg
> http://blog.gregnet.org/
>
>
>
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