>
>From: FrancescAlted <fal...@pytables.org>
>To: Discussion list for PyTables <pytables-users@lists.sourceforge.net>
>Sent: Wed, March 23, 2011 9:32:04 AM
>Subject: Re: [Pytables-users] Problem writing strings to a CArray. Could this
>be
>
>a bug?
>
>
>2011/3/23 Adriano Vilela Barbosa <adriano.vil...@yahoo.com>
>
>Hello,
>>
>>I'm having a problem with PyTables that's driving me nuts. I'm probably doing
>>something wrong, but I wonder if it could be a bug.
>>
>>Basically, I need to write a series of numpy arrays to an HDF file. Because of
>>performance and compression requirements, I decided to convert each numpy
array
>>to a binary string and then write the resulting string to a CArray in the
>output
>>HDF file. This works fine, as long as there are no trailing \x00 elements in
>the
>>string. For some reason, trailing \x00 elements are NOT copied to the Carray.
>>
[clip]
This is not a bug, but rather a feature of NumPy. Look at this:
>>> import numpy as np
>>> a = np.array(['aa\x00\x00'])
>>> a[0]
'aa' # hey! were have my trailing 0's gone?
>>> a.data[:]
'aa\x00\x00' # yeah, they still are in the data area of the array
I'd recommend you using the byte ('i1') type for achieving what you want:
>>> a.view('i1')
array([97, 97, 0, 0], dtype=int8)
Hope this helps,
--
>FrancescAlted
>
Thank you very much for your explanation, but I still don't get it.
Let's forget numpy for a moment and just say I want to store the string
'aa\x00\x00' in a CArray. Each element of the CArray is a 4 element string.
First, I create the CArray:
>>> import tables
>>> fid = tables.openFile("carray_test.hdf","w")
>>> fid.createGroup("/", 'table', 'Binary table')
>>> array_atom = tables.StringAtom(itemsize=4)
>>> array_shape = (1,)
>>> fid.createCArray(fid.root.table,'bin_table',array_atom,array_shape)
Now, I store the string 'aa\x00\x00' in the first row (which is the only row in
this example) of the CArray:
>>> fid.root.table.bin_table[0] = 'aa\x00\x00'
Now, I do
>>> fid.root.table.bin_table[0].data[:]
'aa'
So, it looks to me that the trailing \x00 elements of the string are not being
stored in the CArray. From my side, there's no numpy involved; I'm just trying
to store a string. What am I missing?
Thanks a lot for your help.
Adriano
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