How do you write to a fopen text file without deleting previous text?
110 views (last 30 days)
Show older comments
I'm writing the results of my program to a text file. But every time I run the program and open the text file using fopen, it overwrites everything that was previously written in that file. What can I do to prevent it overwriting the previous text?
Im opening the file using
fileID = fopen('myfile.txt','r+');
and writing to it
fprintf(fileID,'%d',Number(num));
0 Comments
Accepted Answer
Image Analyst
on 2 Apr 2016
Open two separate files. Then write the output file. Afterwards, you can delete the input file and rename the output file if you want.
fInput = fopen(inputFileName, 'rt');
fOutput = fopen(outputFileName, 'wt');
% Now read from input file and write to output file
thisLine = fgetl(fInput);
fprintf(fOutput........
% All done. Close both files.
fclose(fInput);
fclose(fOutput);
% Now delete and/or rename any files you want.
2 Comments
VBBV
on 12 Oct 2021
If I want to replace a specific text string in a file that's filled with both numeric and text data. Will the above solution work ? I want to write to new file with all the data in old file but the specfic text string replaced.
Image Analyst
on 12 Oct 2021
You could do
% Open the file for reading in text mode.
fInput = fopen(inputFileName, 'rt');
% Open the file for writing in text mode.
fOutput = fopen(outputFileName, 'wt');
% Now read from input file and write to output file
% Read the first line of the file.
textLine = fgetl(fileID);
lineCounter = 1;
strPattern = 'This is the line I want to replace'; % Adapt as needed.
while ischar(textLine)
% Print out what line we're operating on.
fprintf('%s\n', textLine);
% Read the next line.
textLine = fgetl(fileID);
lineCounter = lineCounter + 1;
if strcmpi(textLine, strPattern)
% We found the string we're looking for.
% Replace it with what we want.
textLine = 'This is my new Line.'; % Adapt as needed.
end
fprintf(fOutput, '%s', textLine);
end
% All done. Close both files.
fclose(fInput);
Or you could more compactly use fileread() and strrep().
inputString = fileread(inputFileName);
newString = strrep(inputString, oldPattern, newPattern)
% Open the file for writing in text mode.
fOutput = fopen(outputFileName, 'wt');
fprintf(fOutput, '%s', newString);
fclose(fOutput);
I haven't tested either - they're just off the top of my head.
More Answers (2)
Stephen23
on 2 Apr 2016
Simply use the append option a:
fileID = fopen('myfile.txt','a+');
That is it.
1 Comment
Image Analyst
on 2 Apr 2016
He never mentioned append, but I think you might be right. If only he had mentioned that at first....
Muhammad Usman Saleem
on 2 Apr 2016
Edited: Muhammad Usman Saleem
on 2 Apr 2016
change this line to
fileID = fopen('myfile.txt','w'); % opening for writing only
r+ for both reading and writing. Which change previous content
5 Comments
Muhammad Usman Saleem
on 2 Apr 2016
Edited: Muhammad Usman Saleem
on 2 Apr 2016
what is your matrix Number? how can it contains string? I am supperise to know matrix with string and numeric?
See Also
Categories
Find more on Text Files in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!