How Can I Append Multiple Lined Header to a CSV File?

I am looking to create a csv file with the date/time, variable names, and a table of data - something like:

Date, 01/11/20

A, B

1, 4

2, 5

3, 7

I've created a 2x2 cell for the header, and have an array of doubles containing the data. However, I cannot understand the error message I receive when trying to use the following code. What do I need to change in the code to make this work?

savetofile = 'test.csv';

date = datetime(now,'ConvertFrom','datenum');
data = [1 4; 2 5; 3 7];
data = array2table(data);

headerlines = {'Date:', date;
                ' A', 'B'};

fid = fopen(savetofile, 'wt');
for l = 1:numel(headerlines)
    fprintf(fid,'%s\n', headerlines{l});
end
fprintf(fid, '%f; %f\n', data{:});
fclose(fid);

The error message I get is:

Subscripting into a table using one subscript (as in t(i)) or three or more subscripts (as in t(i,j,k)) is not supported. Always specify a row subscript and a variable subscript, as in t(rows,vars).

What am I doing wrong the subscripting?

Answer 1

[Ameer Hamza]

The indexing of table using data{:} is incorrect. This shows all the ways a table can be indexed: https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html. However, writecell() will be a more robust solution.

savetofile = 'test.csv';
date = datetime(now,'ConvertFrom','datenum');
data = [1 4; 2 5; 3 7];
data = num2cell(data);
headerlines = {'Date:', date;
                ' A', 'B'};
C = [headerlines; data];
writecell(C, savetofile)

[Ted Baker]

Thanks Ameer, the example works, but it is not working in my code unfortunately - I have framed the question wrong. The following better matches my code, and retruns the same error message - "All tables in the bracketed expression must have the same variable name" on line 16. This is a result of me merging my data arrays as in the following:

savetofile = 'test.csv';
date = datetime(now,'ConvertFrom','datenum');
dataa = [1 2 3];
datab = [4 5 7];

dataa = reshape(dataa, 3,1);
datab = reshape(datab, 3,1);

output = cat(2,dataa,datab);
output = array2table(output);

headerlines = {'Date:', date;
                'A', 'B'};
headerlines = array2table(headerlines);
            
C = [headerlines; output];
writecell(C, savetofile)

Would you be able to advise as to what I've done wrong, and how I could fix it?

[Ameer Hamza]

I am not sure why you want to create table in your code. You can just use cell arrays.

savetofile = 'test.csv';
date = datetime(now,'ConvertFrom','datenum');
dataa = [1 2 3];
datab = [4 5 7];

dataa = reshape(dataa, 3,1);
datab = reshape(datab, 3,1);

output = cat(2,dataa,datab);
output = num2cell(output);

headerlines = {'Date:', date;
                'A', 'B'};
            
C = [headerlines; output];
writecell(C, savetofile)

[Ted Baker]

Thank you very much Ameer - I confused the num2cell function.

[Ameer Hamza]

I am glad to be of help!

[Peter Perkins]

Ted, creating a cell array is going to use a lot of memory if your data are even moderately large, and using cell arrays for numeric adata is just a bad habit that you want to stay away from. Nothing wrong with using a table. Write your header to the file, then append to it using writetable. If you have a recent version of MATLAB, use writematrix.

Also, use date = datetime('now') instead of convert from datetime.