Extract specific table columns of table variable from a .mat file

Question

Malik 2022-2-16

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评论： Malik 2022-2-22

Hello there, Support would be very much apprecaited with the following:

Background: I have append a large number of .mat files that contain tables. These files are very large and I would like to load only specific table columns e.g. a, d, f of the 96 columns, without opening the entire file, to reduce the memory hit. Each .mat file has the same variable 'data_unstacked', of different time periods, that I would later like to append. So,

I have a filename.mat file that contains a table unstacked_data
table data_unstacked has a number of colums with 96 variable names a, b, c and so on...
when I load('filename.mat'), it reveals a 1x1 struct in my workspace that on opening contains the data_unstacked 1123456x96 table
I would like to only load columns Timestamp, a, d, f etc. of table data_unstacked when I load the file
Later, once I have loaded, say, twelve of these mat files I would like to append the downselected tables, since they are part of the same timeseries.

Inside the .mat file:

Many thanks!

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Answer 1

Malik 2022-2-20

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编辑：Malik 2022-2-20

I have found a similar request from user in the past. See:

https://uk.mathworks.com/matlabcentral/answers/344709-is-there-any-way-to-load-parts-of-a-large-table-from-a-mat-file

It seems my request, which is very similar to above, is a Matlab limitation.

Would be great if someone could confirm, or provide a workaround.

Thank you!

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Walter Roberson 2022-2-21

As @Steven Lord pointed out there, it is a documented limitation https://www.mathworks.com/help/matlab/ref/matlab.io.matfile.html#mw_cd9f9130-9398-4df9-9729-070d19d4c781 that matfile cannot index into variables of tables. Indexing in this case includes fetching the whole variable.

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Answer 2

Walter Roberson 2022-2-16

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https://ww2.mathworks.cn/matlabcentral/answers/1651105-extract-specific-table-columns-of-table-variable-from-a-mat-file#answer_897660

编辑：Walter Roberson 2022-2-20

https://www.mathworks.com/help/matlab/import_export/load-parts-of-variables-from-mat-files.html

A -v7.3 mat file and using matfile() is your only hope.

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Walter Roberson 2022-2-20

I guess because the table has the header information about variable types and descriptions and so on, that this is just something that is not possible at this time.

Malik 2022-2-22