I’ve heard lots of people say that it’s best to use a floating point number only when you really need to.  During my MSc we learnt about how floating point numbers are encoded and did little pencil-and-paper exercises to demonstrate how decimal fractions are converted into surprisingly odd floating point representations.  I’ve read aboutcomputer arithmetic errors causing the failure of a patriot missile.  But the following little problem that I’ve just bumped into seems to be a very clean, concrete way to demonstrate that floating point numbers are to be handled with care.   Here’s the example… if I subtract 0.8 from 1, the remainder is 0.2, right?  So let’s try asking Matlab or C++.  Try evalating the following:

(1 - 0.8) == 0.2

This expression will return a boolean.  It’s simply subtracting 0.8 from 1 and then asking if the answer is equal to 0.2.   Rather surprisingly, it returns false.  Why?  Because 0.2 cannot be precisely represented in binary floating point; the significand is 1100 recurring.  0.2 decimal = 3E4CCCCD in 32-bit floating point (hex representation). Now if we convert from binary floating point back to decimal, we get: 3E4CCCCD = 2.0000000298023223876953125E-1   (You can learn more about floating point arithmetic on WikiPedia and to tinker with this nifty floating point converter applet.)  The bottom line is: if the quantity you’re trying to represent can easily be represented using integers, then it’s probably best to do so.  e.g. if you’re trying to represent monetary values in C++, and you know you’ll only be interested in values of a specific precision (like 0.1 pence) then you could build a simple Money class which internally represents money as integers.

There’s lots of good discussion (and links) of the limitations of floating point here: http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

Update 18/6/2012

I’ve just learnt that Python can cope with decimal numbers if you import decimal



import decimal



Update 21/11/2013

This is a good explanation of the “leakyness” of FP:John D. Cook: Floating point numbers are a leaky abstraction.