[Numpy-discussion] Integer Overflow?

Dave dave.hirschfeld@gmail....
Tue Jul 14 08:16:31 CDT 2009


I got stung when taking an ordinary python integer to the power of a numpy
integer - the result wasn't what I was expecting (see below)!

Taking a wild guess I expect this is due to integer overflow (since it doesn't
show up with int64). When working with an int32 type one has to be aware of such
issues and I'm guessing the casting behaviour is a design decision, nevertheless
I thought I'd post to make others aware of such considerations.

HTH,
Dave


Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit (Intel)]
Type "copyright", "credits" or "license" for more information.

IPython 0.10.bzr.r1163 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object'. ?object also works, ?? prints more.

In [2]: import numpy as np

In [3]: np.__version__
Out[3]: '1.4.0.dev7056'

In [4]: 10**-10
Out[4]: 1e-010

In [5]: 10**np.float32(-10)
Out[5]: 1e-010

In [6]: 10**np.int32(-10)
Out[6]: 7.0918695992859933e-010 ????????????????????????????????????

In [7]: 10**np.int32(-1)
Out[7]: 0.10000000000000001

In [8]: 10**np.int32(-2)
Out[8]: 0.01

In [9]: 10**np.int32(-3)
Out[9]: 0.001

In [10]: 10**np.int32(-4)
Out[10]: 0.0001

In [11]: 10**np.int32(-5)
Out[11]: 1.0000000000000001e-005

In [12]: 10**np.int32(-6)
Out[12]: 9.9999999999999995e-007

In [13]: 10**np.int32(-7)
Out[13]: 9.9999999999999995e-008

In [14]: 10**np.int32(-8)
Out[14]: 1e-008

In [15]: 10**np.int32(-9)
Out[15]: 1.0000000000000001e-009




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