Precision Issue

Christopher A. Craig com-nospam at ccraig.org
Fri Aug 3 19:51:09 CEST 2001


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cstein at giftcertificates.com (Casey Stein) writes:

> Why is it converting my 66.6 to 66.599999999999994?

Because 66.6 is a binary floating point number, which must be
represented in as powers of two by the computer.  Specifically (in
this case) 4686558362232422 * (2**-46).  

Though this probably doesn't make much sense to you, it makes a great
deal of sense to the computer, which has to think of everything as
some power of 2.  You think of 12 as 10+2 (1*(10**1)+2*(10**0)), but
your computer thinks of it as 8+4 (1*(2**3)+1*(2**2)).

There are other, more acuarate, ways to do this (such as representing
66.6 as 333/5), but they are all _much_ slower than using floats, so
Python currently uses floats (There are people who would like to
change that.)

See 
http://www.python.org/cgi-bin/faqw.py?req=show&file=faq04.098.htp
for more information.

- -- 
Christopher A. Craig <com-nospam at ccraig.org>
"The world would be a better place if Larry Wall had been born in Iceland, 
 or any other country where the native language actually has syntax." 
                -- Peter da Silva (in alt.sysadmin.recovery) 
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