[Numpy-discussion] problem with a binary file on OS X 10.6

Christopher Barker Chris.Barker@noaa....
Fri Nov 5 10:50:08 CDT 2010


On 11/4/10 5:15 AM, Olli Sipilä wrote:
> I have a problem reading a binary file on OS X 10.6. On my desktop mac
> (OS X 10.4.11, Python 2.5.4, Numpy 1.3.0), the command
>
> CELLS, NSIZE, NE = fromfile(fp, int32, 3), where fp is the filename,
>
> correctly prints "21 20 300". However, when I try the above on my laptop
> using Snow Leopard (Python 2.6.6, Numpy 1.5.0), I get the numbers
> "352321536 335544320 738263040". This results in a failure when trying
> to read the data that comes afterwards (Numpy fails with "array too
> big"). I assume this error may have something to do with 10.6 being
> 64-bit and 10.4 being 32-bit; however, the Python 2.6.6. distribution is
> also 32-bit. Any thoughts on this?

you've specified int32, so 32-64 bit should not be the issue. however, 
it's likely that endian issues are -- is one of your machine PPC, and 
one Intel?

yup, that's it:

In [29]: a = np.array((21, 20, 300), dtype=np.int32)

In [30]: a
Out[30]: array([ 21,  20, 300])

In [31]: a.byteswap()
Out[31]: array([352321536, 335544320, 738263040])

So, you can either call bytswap yourself, or, better yet specify the 
endianness in your dtype specifier, somethign like (untested)

dtype='>i4' for big endian and dtype='>i4' for little endian. Intel is 
little endian, PPC is bigendian (I think -- do test to make sure!)

-Chris








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