Python float representation error?

[mailpitches]

try running this in python:

print [39.95]

the output i get is:
[39.950000000000003]

what's up with that?

 

[James Stroud]

try running this in python:

print [39.95]

the output i get is:
[39.950000000000003]

what's up with that?

This comes from the imprecision in representing base-10 fractions as
binary. Try "print [str(39.95)]".

If you want to print a list, maybe:

print map(str, alist)

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

 

[nikie]

try running this in python:

print [39.95]

the output i get is:
[39.950000000000003]

what's up with that?

That's perfectly normal - some numbers aren't exactly expressible as
binary floating points, just like 1/3 isn't expressible as a decimal
floating point number.

Here's a good link on floating point arithmetic:
http://docs.sun.com/source/806-3568/ncg_goldberg.html

 

[Alex Martelli]

try running this in python:

print [39.95]

the output i get is:
[39.950000000000003]

what's up with that?

Why not start by reading the Python online tutorial? Specifically
<http://docs.python.org/tut/node16.html>?


Alex

 

[Dennis Lee Bieber]

try running this in python:

print [39.95]

the output i get is:
[39.950000000000003]

what's up with that?

Nothing's up...

Standard computer science -- floating point numbers are not exact.

You shoved the number into a list (the []), and since lists don't
have a "clean" printable form, you get the repr() format for the list
and its contents.

Python normally cleans up the "noise" in floating point numbers when
printed directly. repr(), however, shows the number to the best of the
internal storage format.

alst = [anum]
print alst, str(alst), repr(alst) [39.950000000000003] [39.950000000000003] [39.950000000000003]

for an in alst:

.... print an, str(an), repr(an)
....
39.95 39.95 39.950000000000003.... def __str__(self):
.... return "I'm a C"
.... def __repr__(self):
.... return "I think I'm a C at %s" % id(self)
....