Append Table data by columns (not rows)

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ken.c....@gmail.com

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Oct 29, 2022, 2:46:19 PM10/29/22
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I have used the Table.append() method to create an empty Table, then add multiple rows of data to it. Now I am trying to write a procedure to add data by column (field name) instead of rows. I have done this with h5py, but can't figure out how to do the same with PyTables. I create the Table with description= but can't figure out how to allocate the number of rows. Is this possible?
Why I am doing this? Some very large datasets are easier to copy on a column-by-column basis instead of row-by-row.
To demonstrate, here is a very simple example. 

import numpy as np
col_int = [ i for i in range(10) ]
col_fl = [ float(i) for i in range(10) ]
recarr = np.empty(dtype=[('col_int',int),('col_fl',float)], shape=(10,))
recarr['col_int'] = np.array(col_int)
recarr['col_fl'] = np.array(col_fl)

import tables as tb
with tb.File('file_tb.h5', 'w') as h5f:
    # Create table with data using obj parameter:
    NXgrp_tbl1 = h5f.create_table('/','data1', obj=recarr )
    
    NXgrp_tbl2 = h5f.create_table('/','data2', description=recarr.dtype )
    # Create table with data using  description parameter:
    for colName in recarr.dtype.fields:
        # this doesn't work -- how to I do it?? :
         NXgrp_tbl2[colName] = recarr[colName]

All help is appreciated. Thanks,
Ken Walker

Antonio Valentino

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Nov 18, 2022, 2:31:26 AM11/18/22
to ken.c....@gmail.com, pytables-users
Dear Ken,

Il 29/10/22 20:46, ken.c....@gmail.com ha scritto:
I'm not aware of any other method to do what you ask except pre
allocating the table as you do in your example (providing and obj).
You could also use the "append" method with the same empty object in
principle.

Your use-case is indeed interesting.
Maybe you could try to submit a PR to add this feature.

kind regards
--
Antonio Valentino
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