how to create a function that takes a txt file with a lot of columns and saves a new file (while keeping the old one) with 3 specific columns from the original ..

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Yoanna Ivanova 2019-7-18

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评论： Peter Jarosi 2019-7-25

Hey guys!

I have a tooon of txt files that I'd like to get 3 columns from each txt file and save them in the same folder. (the old txt file has 2932 columns, I want to select 3 of those 2932 and save that into a new file -- format doesn't matter... txt and excel are both okay) is there a way to automate this to do this for all my txt files? I am still rather new to matlab..

thank you!

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Answer 1

Peter Jarosi 2019-7-18

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编辑：Peter Jarosi 2019-7-18

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You should have been more specific, so I could have given more accurate answer. For instance, a list of your filenames, names of your three columns, structure of your data (maybe these are in csv or xlsx format). Anyway, I hope that the following will help you:

myThreeCols = {'colName1', 'colName2', 'colName3'};
listOfFiles = {'myFile1.csv', 'myFile2.csv', 'myFile3.csv', 'myFile4.csv'};
numOfFiles = size(listOfFiles, 2);
for i = 1 : numOfFiles
    myTable = readtable(listOfFiles{1,i}, 'ReadVariableNames', true); % Reading your file
    myTable = myTable(:, myThreeCols); % Select your three columns
    newFile = strcat('new_', listOfFiles{1,i}); % Insert prefix 'new_' in filename
    writetable(myTable, newFile, 'WriteVariableNames', true); % Writing your file
end

If you upload some of your files I could be more specific. Perhaps your filenames could be iterable, that makes life easier.

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Yoanna Ivanova 2019-7-19

I have a lot of txt files -- a lot a lot. And those txt files have more information than what I need. The only columns I need from each txt file are called Date, Time, o2 concentration but they don't always have the headings , sometimes they do sometimes they don't. They're found in columns A, B and AB in the txt file , regardless of whether the heading is present or not. I can open these txt files with excel normally and that is how I have been dealing with the data so far.

What I want to eventually do -- in a sense is to create these new files with these new 3 columns, then consolidate the data because I have 5 sec data for the o2 concentration but I need it in minute values (so average it out per minute). I've been doing this with excel thus far (deleting the columns I don't need, then removing blank rows, then consolidating the data per minute -- keep in mind that I also have the second data present in the Time column so I first had to split the columns, then delete the seconds, then finally consolidate) but it is so repetitive and it takes so long to process that I wonder if there is another way of doing this -- perhaps with matlab.

I tried your code adjusting it for my variables but it doesn't really work..

Peter Jarosi 2019-7-22

编辑：Peter Jarosi 2019-7-22

在 MATLAB Online 中打开

The following example doesn't work perfectly, because your example file is a little bit messy: unnecessary empty rows, and the length of your header (28) doesn't match with the length of your data row (56).

myThreeCols = {'Vstpd1', 'Temp1', 'Pambient'}; % Select your three columns
listOfFiles = {'example.txt'};
numOfFiles = size(listOfFiles, 2);
for i = 1 : numOfFiles
    myTable = readtable(listOfFiles{1,i}, 'ReadVariableNames', true,...
        'Delimiter', 'tab'); % Reading your file
    newTable = myTable(:, myThreeCols); 
    newFile = strcat('new_', listOfFiles{1,i}); % Insert prefix 'new_' in filename
    writetable(newTable, newFile, 'WriteVariableNames', true,...
        'Delimiter', 'tab'); % Writing your file
    newExcelFile = strrep(newFile, '.txt', '.xlsx');
    writetable(newTable, newExcelFile, 'WriteVariableNames', true);
end

I don't know which three columns you want, so I picked up three of them randomly. You can change the first line of my code if you want to.

I am thinking about how we can fix your example file easily.

The following code fixes your data problems more or less:

myThreeCols = {'Vstpd1', 'Temp1', 'Pambient'}; % Select your three columns
listOfFiles = {'example.txt'};
numOfFiles = size(listOfFiles, 2);
for i = 1 : numOfFiles
    myTable = readtable(listOfFiles{1,i}, 'ReadVariableNames', true,...
        'Delimiter', 'tab'); % Reading your file
    myTable = myTable(~strcmp(myTable.DATE, ''), :);
    myTable.Properties.VariableNames{'Var30'} = 'TIME_3'; 
    newTable = myTable(:, myThreeCols); 
    newFile = strcat('new_', listOfFiles{1,i}); % Insert prefix 'new_' in filename
    writetable(newTable, newFile, 'WriteVariableNames', true,...
        'Delimiter', 'tab'); % Writing your file
    newExcelFile = strrep(newFile, '.txt', '.xlsx');
    writetable(newTable, newExcelFile, 'WriteVariableNames', true);
end

Notes:

This program line deletes empty data rows.

myTable = myTable(~strcmp(myTable.DATE, ''), :);

2. This program line renames column 30 to TIME_3

myTable.Properties.VariableNames{'Var30'} = 'TIME_3';

You may also want to rename other columns.

3. Runnig the code above you will get a warning message: 'Warning: Variable names were modified to make them valid MATLAB identifiers.' It means that some of your variable names in your header are not valid variable names in Matlab, so Matlab a little bit renamed them automatically according to the following rules. https://www.mathworks.com/help/matlab/ref/isvarname.html

Yoanna Ivanova 2019-7-25

Thanks Peter! Yes, now it works okay -- I removed a couple of rows in the beginning. Thank you!!

Peter Jarosi 2019-7-25

You're very welcome!

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