Error in Reading and Writing a text file

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X 2011-7-11

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https://ww2.mathworks.cn/matlabcentral/answers/11351-error-in-reading-and-writing-a-text-file

I have a problem with my program when i read and write the file. The following code is what I have so far. It works when I use only textscan and fopen with 'r' instead of 'r+'. The error is Invalid file identifier, Use fopen to generate a valid file identifier. Can somebody see a mistake? Thank you in advance

fid = fopen('3.csv', 'r+'); %Opens the file
NData = textscan(fid, '%s %s %d % ... %d', 'delimiter' ,',',         'headerlines',2); 
T_DB = NData {32};  %Gets Dry Bullb Temperature
T_DP = NData {35}; %Gets Dew Point Temperature
RH = NData {38}; %Gets Relative Humitidy
Temp_data = [T_DB T_DP RH]; 
frewind(fid)
fprintf(fid, '%f %f %f', Temp_data);
fclose(fid); %Closes the file

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Answer 1

Jan 2011-7-11

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https://ww2.mathworks.cn/matlabcentral/answers/11351-error-in-reading-and-writing-a-text-file#answer_15490

While "fopen(FileName, 'r')" searchs the file in all folders of the Matlab path, "fopen(FileName, 'r+')" looks in the current folder only. To solve this, change into the folder, which contains the file (see CD) or specify the file by a full path (see FULLFILE).

As a side-effect the fopen with 'r+' is about 2.3 times faster than with 'r' even if the file is specified with a full path.

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Fangjun Jiang 2011-7-11

This could be the reason! Now I notice this sentence in help fopen. "FILENAME can be a MATLABPATH relative partial pathname only if the file

is opened for reading."

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Answer 2

Fangjun Jiang 2011-7-11

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https://ww2.mathworks.cn/matlabcentral/answers/11351-error-in-reading-and-writing-a-text-file#answer_15478

Since you are reading and writing the same file, it is surprise to hear that 'r' (read only) works but 'r+' (read and write but not to create new) doesn't. Are you sure it is not the opposite?

Since you are read and write text file, I wonder if 'r+t' would help.