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

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!

Answer 1

[Malik]

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!

[Walter Roberson]

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.

Answer 2

[Walter Roberson]

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.

[Malik]

Thank you for your response.

Following you link, when I try to load a specific table I get an error. The steps I take, similar to the linked example are as follows:

exampleObject = matfile('filename.mat','Writable',true);
firstRowB = exampleObject.data_unstacked(1,:);

Error:: The variable 'data_unstacked' is of class 'table'. To use 'data_unstacked', load the entire variable.

Although, I can access the entire table, which then gets loaded into the workspace, without any issues as follows:

access_table = exampleObject.data_unstacked; 

I would simply like to access parts of this table without loading the entire table into memory.

Appreciate your support.

[Walter Roberson]

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]

Thank you for your response.