Math irregularity ... is this normal?

[Daniel Yoo]

Some One Else <nospam at any.time> wrote:

: Hello!

: I just compiled and installed Python 2.2.1 on Linux-Mandrake 8.0.  When I 
: do the following:

:>>> 2/5.0
: 0.40000000000000002

: In  'Learning Python', a similar example says I should get the more logical 
: answer of 0.4.  Did I install something wrong? I can see where this could 
: result in creeping math errors.

Hello!

The behavior of displaying floating point numbers changed slightly
between Python 1.52 and Python 2.0 to reflect more of the nature of
floating point: almost all floating point numbers can't be represented
precisely on computers.  The Python tutorial has a good section that
taks more about floating point issues:

    http://www.python.org/doc/tut/node14.html


To make this inprecision more visible to programmers, the implementers
of Python 2.0 made a small change in Python's repr() string
representation function:

    http://www.amk.ca/python/2.0/

Look around Section 10, and you should see something about this.



By the way, if you call the 'str()' function on your number, Python
will display the number in a way that you'd expect:

###
>>> 2/5.0
0.40000000000000002
>>> str(2/5.0)
'0.4'
>>> print 2/5.0
0.4
###

and when you encounter the 'print' statement later on in Learning
Python, you'll find that it does an implicit str() of anything you
give it.



Hope this helps!