Read a very large .csv file, split into parts and save each part into a smaller .csv file

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GioPapas81
GioPapas81 on 26 Sep 2019
Commented: GioPapas81 on 1 Oct 2019
Deat Matlabers,
I need to read a very large .csv file with about 15.000 columns and 500.000 rows. I need to split it into chunks of rows (i.e. 20.000 rows and all 15.000 columns), and save each chunk into a separate .csv file.
  1. I have tried to use textscan, but I am not sure that this can work, as I have not only numerics, but also non-numerics and dates across separate columns. I would ideally aim to get all this information, as I will need it for different parts of my project.
2. I also attempted tabularTextDatastore, but I get an error:
Unable to determine the format of the DATETIME data.
Try adding a format to the DATETIME specifier. e.g. '%{MM/dd/uuuu}D'.
Is there any way I could provide a DATETIME specifier (this is not explained in the relevant documentation)?
Memory is not a problem here, as I currently use a supercomputer in terms of RAM.
I would be grateful for any ideas.
George

Accepted Answer

Jeremy Hughes
Jeremy Hughes on 27 Sep 2019
If your plan is to write all the small CSV files out, and do nothing with them in MATLAB, I'd say just use tabularTextDatastore, and set all of the ds.TextscanFormats(:) = {'%q'}, There should never be any errors with '%q'
Then use writetable.
ds = tabularTextDatastore(filename,'ReadSize',myReadSize);
ds.TextscanFormats(:) = {'%q'};
while hasdata(ds)
% Need to figure out the file names but other than that, this should work.
writetable(read(ds),output_filename);
end
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More Answers (1)

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 26 Sep 2019
Hi,
The answer is rather simple. You can take out all dates with string specifier: %s. E.g. file called: DATA_date.txt
DATE Row1 Row2 Row3 Row5
11/11//2019 1 1.13 2 3.33
11/11//2019 2 0.13 3.12 3.33
11/11//2019 3 2.13 -2 -5.33
11/11//2019 4 4.13 -3 -7.33
11/11//2019 5 3.13 5.5 -8.33
11/11//2019 6 2.13 2.6 -13.33
Can be imported into matlab workspace with:
FileName = 'DATA_date.txt';
FID = fopen(FileName, 'r');
SPECs = '%s%d%f%f%f';
N_header = 1;
DATA = textscan(FID, SPECs, 'headerlines', N_header);
fclose(FID);
Now all imported data will be inside a cell array DATA. DATA{1,1} contains DATE values; DATA{1,2} contains data of Row1; ... DATA{1,5} contains data of Row5.
Good luck.
  4 Comments
GioPapas81
GioPapas81 on 27 Sep 2019
Unfortunately I do have lots of missing data in my file,randomly distributed. I also don't know which columns have dates (there are tousands of columns, across houndreds of thousans of rows).
I hoped that the tabularTextDatastore option would be possible, but I think it is not possible to account for dates via that route (according to the errors I get above).
But, thank you for your responses Sulaymon.

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