How can I modify the first row(i.e. strings) of an existing csv file without overwriting the rest of data in matlab

Hello ,

I am trying to modify a particular cell("PMW") of an existing csv file to "P (MW)"and keep the rest of the content intact. The first row consists of name tags for the rest rows of the csv file. I have used fopen() in rt+ permission argument to avoid discarding the rest of the contents in the csv file. The code looks like this

Prewrite = strcat('P',{' '},'(MW)');
fid = fopen('xxx.csv');
lineex = fgetl(fid);
lineex = replace(lineex,"PMW",Prewrite);
fclose('all');
fid = fopen('xxx.csv','rt+');
fprintf(fid,'%s\n',lineex);
fclose('all');

However, the resulting csv file has overwritten the first column of the second row; three char characters are mssing for the B1 cell("ROL2" in the screenshot below while it should be SAIROL2).

I am not sure if this is due to the fact that fprintf will overwirte the exisitng data when using \n new line operator. If so how can I fix this issue to maintain its original content?

[Bob Thompson]

I suspect the reason this is happening (I haven't specifically tested it) is because the new lineex is three characters longer than the original, so when you use fprintf it overwrites three characters from the second row. My first thought on how to work around this would be to split apart fid by new line character using regexp, and then replace the first line with the new first line.

[Jialun Zhang]

Hi. Could you be more specific on how to use regexp to split apart fid (p.s I assume you are referring to the csv file)? Do you intend to firstly leave a row space for the new line to be inserted by deleting its original contents?

[Stephen23]

@Jialun Zhang: changing some characters in the middle of a text file without changing the rest of the file data is only possible if you overwrite exactly the same number of characters. It is not possible to insert extra characters (as you are trying to do) without overwriting something. If you want extra characters you will have to rewrite the whole file.

This is because text files are stored simply as a sequence of characters. If you want to insert characters in the middle, the the rest of the characters all have to get shifted along.

[Jialun Zhang]

@Stephen Cobeldick: Ok that makes sense. Thanks a lot for your answer!

Answer 1

[Walter Roberson]

Use MS Windows with Excel installed and xlswrite to the csv file passing in the range to overwrite .

There might also be some approaches using readtable and writetable.

Every approach will rewrite to end of file because as explained the content needs to be shifted to make room. The xlswrite approach takes care of the details for you .

[Jialun Zhang]

Yeah I also thought about using cell array to store the data which might potentially solve this problem. Thanks anyway!