e vs exp()? / financial applications
Andrew Thompson
andrew.thompson at ashecastle.com
Mon Sep 23 13:15:37 EDT 2002
One must be aware of the representational issues and accuracy of
arithmetic within a datatype when using numerical types. This is not so
much a Python issue as one of representing the real number line with
float types.
For example, the following on Python 2.2.1 #34 running on Windows XP
Pro, Celeron Processor (probably different elsewhere)
W=0.89 / 4
Now evaluate
1+w+w+w+w
and
w+w+w+w+1
which are both *different* according to the == operator, but for the
purposes of numerical accuracy are identical.
This behaviour is quite mild, but it shows the point that rounding
errors can easily creep in, and using == to test for equality is not
always sensible.
Caveat Emptor!
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