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simple float numbers problem

Bengt Richter bokr at oz.net
Sat Nov 8 19:46:00 EST 2003 
On Fri, 7 Nov 2003 17:50:16 +0100, "Fredrik Lundh" <fredrik at pythonware.com> wrote:

>Vio <vmilitaru at sympatico.ca> wrote:
>
>> I need to test for equality between simple 2 decimal numbers. For example:
>>
>> if (10 + 15.99) == 25.99:
>>        do some stuff...
>>
>> The preceding sentence should be TRUE, but to Python it appears FALSE.
>> Which is wrong.
>
With exact decimals you can do it:

 >>> from exactdec import ED
 >>> ED(10)+ED('15.99') == ED('25.99')
 True

but if I get all the bits from floating point values instead of exactly interpreting string literals,
we can see what happened to you:

 >>> print '%s\n%s'%(ED(10+15.99, 'all'), ED(25.99,'all'))
 ED('25.99000000000000198951966012828052043914794921875')
 ED('25.989999999999998436805981327779591083526611328125')

Grabbing less bits:

 >>> print '%s\n%s'%(ED(10+15.99, 15), ED(25.99, 15))
 ED('25.990000000000002')
 ED('25.989999999999998')

15 fractional decimals still shows a difference, but

 >>> print '%s\n%s'%(ED(10+15.99, 14), ED(25.99, 14))
 ED('25.99')
 ED('25.99')

So we can expect

 >>> ED(10+15.99, 14) == ED(25.99, 14)
 True



>that's how floating point numbers work.  see:
>
> http://www.python.org/doc/current/tut/node14.html
>
Or play with the new toy. I posted it to the "prePEP: Decimal data type" thread ;-)

>> Perhaps because Python translates "25.99" to "25.98999999999999998" and
>> not "25.99", which may be the reason for this error (me guessing...). If
>> that's the case, how do I force Python to only use 2 decimal points, and
>> not "make up" superfluous decimals? Or if that's not the cause for the
>> problem, how do I make Python see my math expression as TRUE (as it
>> "should" be)?
>
>convert both terms to strings, and compare the strings.
>
>or calculate the absolute value of the difference between the two
>numbers (abs(x-y)) and compare that to a small constant.
>
>or use a data type designed to handle decimal numbers, such as:
>
>    http://fixedpoint.sourceforge.net/
>
Still, note the difference beween enforcing rounding to a number of fractional bits
as you go (maybe it doesn't do that??) vs keeping exactness until rounding is explicitly
invoked by a method or during construction from another value.

Also note that exact values can be expensive (time, space) to keep, because there is
a kind of rational type hiding in there ;-)

Regards,
Bengt Richter
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