Replacing one line (or one part of a line) in a text file

Hello. I have a config file that contains 1 specific line that I want to change just the number (in this case from 1000 to e.g. 235)

This is my code so far:

           %Get current Value
           FilePath='C:\Program Files\Teledyne DALSA\Sapera\CamFiles\User\myCamera.ccf'
           Filedata = readlines(FilePath)
           TF = contains(Filedata,"Time Integrate Duration=")
           k = find(TF,1);
           expStr=Filedata(TF);
           ReportMessage(app,expStr);     %My report function that reports to a UITextarea
           oldLine=Filedata(k)
           Filedata(k) ="Time Integrate Duration=235"     %this is the new line I want
           
            [fid, msg] = fopen('FilePath', 'w')
            if fid < 1; 
                error('could not write output file because "%s"', msg);
            end
            fwrite(fid, strjoin(Filedata, '\n'));
            fclose(fid);

But it doesn't seem to be changing the file

I also enclose the config file (text)

[Jason]

Seems I need writelines!

But how can I handle errors with this?

writelines(Filedata,FilePath)

Answer 1

[dpb]

FilePath='C:\Program Files\Teledyne DALSA\Sapera\CamFiles\User';
FileName='myCamera.ccf';
FQName=fullfile(FilePath,FileName);         % separate path, name for easy mods, build fully-qualified name
TargetString="Time Integrate Duration=";    % I'd make this an argument to the callback function, not hardcoded
NewString="Time Integrate Duration=235";    % make data, not code for flexibility
Filedata=readlines(FQName);
TF=contains(Filedata,TargetString);
if ~any(TF), error('Target String Not Found.  Aborting.'); end  % make more user-friendly error handler, but gets message across
expStr=Filedata(TF);
ReportMessage(app,expStr);     %My report function that reports to a UITextarea
Filedata(TF)=NewString;
writematrix(FileData,FQName)                % rewrite the file

You could be a little more general if you were to only replace the numeric value of the target string with a new value; the passing the target and the new value would let you use the routine for any unique target string.

...
NewString=TargetStringNewValue;
Filedata(TF)=NewString;
...

[Jason]

Perfect, thankyou!

[Jason]

Actually, I created a function as you suggested:

function currentStr = ReplaceStringInfile(FilePath,FileName,TargetString,NewString)
FQName=fullfile(FilePath,FileName);         % separate path, name for easy mods, build fully-qualified name
% TargetString="Time Integrate Duration=";    % I'd make this an argument to the callback function, not hardcoded
% NewString="Time Integrate Duration=235";    % make data, not code for flexibility
Filedata=readlines(FQName);
TF=contains(Filedata,TargetString);
k = find(TF,1)
if ~any(TF), error('Target String Not Found.  Aborting.'); end  % make more user-friendly error handler, but gets message across
currentStr=Filedata(TF)
Filedata(TF)=NewString
writematrix(FileData,FQName)    
end

and it falls over here:

Filedata(TF)=NewString
Unable to perform assignment because the left and right sides have a different number of elements.

Error in ReplaceStringInFile (line 12)
Filedata(TF)=NewString

[Jason]

Seems to be because

TargetString="Time Integrate Duration=";
expms=app.ExposuremsDalsaEditField.Value;
NewString=horzcat(TargetString,num2str(expms*1000))

NewString = 

1×2 string array

"Time Integrate Duration="    "50000"

I think I need this:

NewString=TargetString+num2str(expms*1000)

[Jason]

Also needed to replace writematrix with write lines

[Image Analyst]

@Jason I'm confused by your latest comments. Is it solved or not? You accepted an Answer. Or was the original question solved but now you have additional, different questions? Or are we completely done here?

[dpb]

@Image Analyst, I think I understand -- I never recall that there is a writelines() now, it wasn't yet introduced in the version I'm running; for it (R2021b) writematrix is the only choice, but requires using the name/value pair 'FileType','text' unless the extension is one of the few recognized ones.

writelines is documented to write text files so it doesn't appear to care about the extension; @Jason just didn't recognize that that would also have fixed his problem; he didn't "need" to replace writematrix although that is certainly the current preferred use; he could have added the file type and continued on.

Answer 2

[Saurabh]

Hi Jason,

So, the objective is to modify one line in your code or simply you want to re-assign a different value to Time Integrated Duration variable. I have tried to do the same on my end, below you can see the code that I have implemented:

function modify_duration_in_file(file_path, new_duration)
    % Read the file
    lines = {};
    fid = fopen(file_path, 'r');
    tline = fgetl(fid);
    while ischar(tline)
        lines{end+1} = tline;
        tline = fgetl(fid);
    end
    fclose(fid);

    % Find and modify the line
    for i = 1:length(lines)
        if contains(lines{i}, 'Time Integrate Duration=')
            parts = strsplit(lines{i}, '=');
            parts{2} = num2str(new_duration);
            lines{i} = strjoin(parts, '=');
            break;
        end
    end

    % Write the file
    fid = fopen(file_path, 'w');
    for i = 1:length(lines)
        fprintf(fid, '%s\n', lines{i});
    end
    fclose(fid);
end

% Example usage
file_path = 'path/to/your/textfile.txt';
new_duration = 235;
modify_duration_in_file(file_path, new_duration);

Feel free to ask, if you have any doubt in understanding the code.

I hope this was helpful.

[Jason]

Thankyou for this, but I didnt see it until I accepted the previous answer and I do prefer fewer lines