Why can not a BigDecimal be compared to a Float via "==". How should I handle this?

Testing RSpec
1

Hi,
Any suggestions on how to write an rspec expectation for equality when
you
get a BigDecimal back. I see that “big_decimal_variable.should ==
123.23”
doesn’t work as the ruby BigDecimal comparison (via ==) to the same
Float
value gives false (not true as you’d expect). For background / as an
example see below:

    ?> bd
    => #<BigDecimal:221832c,'-0.32303E3',8(12)>
    >> f
    => -323.03
    >> f.class
    => Float
    >> bd.should == f
    Spec::Expectations::ExpectationNotMetError: expected: -323.03,
         got: #<BigDecimal:221832c,'-0.32303E3',8(12)> (using ==)
            from

/opt/local/lib/ruby/gems/1.8/gems/rspec-1.1.11/lib/spec/expectations.rb:52:in
fail_with' from /opt/local/lib/ruby/gems/1.8/gems/rspec-1.1.11/lib/spec/matchers/operator_matcher.rb:46:infail_with_message’
from
/opt/local/lib/ruby/gems/1.8/gems/rspec-1.1.11/lib/spec/matchers/operator_matcher.rb:61:in
__delegate_method_missing_to_given' from /opt/local/lib/ruby/gems/1.8/gems/rspec-1.1.11/lib/spec/matchers/operator_matcher.rb:12:in==’
from (irb):73
>> bd == f
=> false
>> f == bd
=> false

thanks
Greg

2

On Sun, Jan 11, 2009 at 12:17 AM, Greg H. <
[email protected]> wrote:

Any suggestions on how to write an rspec expectation for equality when you
get a BigDecimal back. I see that “big_decimal_variable.should == 123.23”
doesn’t work as the ruby BigDecimal comparison (via ==) to the same Float
value gives false (not true as you’d expect).

How about bd.to_s.should == 123.23.to_s ?

///ark

3

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?

4

On Sun, Jan 11, 2009 at 4:05 AM, Greg H.
[email protected] 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.

Personally, I think this seeming annoyance is actually a good thing.

FWIW,
David

5

On Sun, Jan 11, 2009 at 9:21 AM, David C.
[email protected]wrote:

require ‘bigdecimal’
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

6

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?

7

2009-01-11 18:17, Greg H.:

Any suggestions on how to write an rspec expectation for equality when you
get a BigDecimal back. I see that “big_decimal_variable.should ==
123.23”

If you register keywords “comparison” and “float”, you should train
yourself to cry out “delta” without even thinking. :wink:

Would this work for you?

big_decimal_variable.should be_close(123.23, 0.005)

8

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 H.

9

ok - thanks - seems like if one remembers to use “BigDecimal(‘10.1’)”,
then
instead of 10.1 in your code you should be ok then.

On Mon, Jan 12, 2009 at 6:45 AM, Maurício Linhares <

10

On Sun, Jan 11, 2009 at 12:17 AM, Greg H. <
[email protected]> wrote:

Any suggestions on how to write an rspec expectation for equality when you
get a BigDecimal back. I see that “big_decimal_variable.should == 123.23”
doesn’t work as the ruby BigDecimal comparison (via ==) to the same Float
value gives false (not true as you’d expect).

Noting there are two decimal places in your data, I might suggest using
the
Money plugin.

///ark