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

Answer 1

[Jan]

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.

[Fangjun Jiang]

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."

Answer 2

[Fangjun Jiang]

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.

[X]

Fangjun, Sorry for the confusion. When I used 'r', it reads the file but it does not write it (this is expected). However, when I used 'r+', it does not either read or write the file. I already tried 'rt+', but it did not make a difference. Thank you fro your reply.

[Fangjun Jiang]

I wonder if you have the file opened but not closed due to debugging. Can you try fclose('all') and then try again. BTW, it is 'r+t', not 'rt+'.

[Walter Roberson]

BTW, 'rt+' is fine

http://www.mathworks.com/help/techdoc/ref/fopen.html

To open files in text mode, attach the letter 't' to the

permission, such as 'rt' or 'wt+'

[Fangjun Jiang]

All right, good to know! The example provided in help fopen shows 'w+t'

[Jan]

@Jiang: Walter's link contains a 'wt+' example also.