How do you write to a fopen text file without deleting previous text?

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));

Answer 1

[Image Analyst]

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.

[VBBV]

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]

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.

Answer 2

[Stephen23]

Simply use the append option a:

 fileID = fopen('myfile.txt','a+');

That is it.

[Image Analyst]

He never mentioned append, but I think you might be right. If only he had mentioned that at first....

Answer 3

[Muhammad Usman Saleem]

do not use r+ read reason form documentation here

change this line to

fileID = fopen('myfile.txt','w'); % opening for writing only

r+ for both reading and writing. Which change previous content

[Recap]

I tried 'w' and it does the same thing

[Muhammad Usman Saleem]

have you define Number as matrix? and loop over with index variable name num?

if possible to share with me your text file what you want to write in it

[Recap]

Number is a matrix 1x5 containing ('12345'). The way I am entering it into the text file is as follows

[a b] = size(Number);
[c d] = size(Letter);
if b>3
    for num = 1:2
        fprintf(fileID,'%d',Number(num)); %first 2 values
    end

      fprintf(fileID,'-%s-',Letter); %%letter in 3rd position

      for num = 4:b
          fprintf(fileID,'%d',Number(num)); % continue with numbers

      end
  end

[Muhammad Usman Saleem]

what is your matrix Number? how can it contains string? I am supperise to know matrix with string and numeric?

[Recap]

Number is doesnt not contain string. it contain values[1 2 3 4 5] Letter is a string containing 'D'