how to replace a specific line in a text file with user data?

clear

Messege = 'Hello';

fileID = fopen('SampleFile.txt', 'rt');

textLine = fgetl(fileID);

lineCounter = 1;

while ischar(textLine)

fprintf('%s\n', textLine)

if startsWith(textLine, 'second line')

textLine = Messege % i want to replace this line with messege

disp('Done')

break

end

textLine = fgetl(fileID);

lineCounter = lineCounter + 1;

end

fclose(fileID);

my sample file contains three line data only "first line" "second line" "third line"... i want to replace my hello messge with line starts with string "second line".. code is running fine and display msg 'done' but gives error on messge line

>> Unable to perform assignmentbecause brace indexing is not supported for variable of this type <<

[Les Beckham]

When I run your code I see no errors. It is not clear what you are really trying to do. Do you wish to change the contents of the file on disk?

Messege = 'Hello';
fileID = fopen('SampleFile.txt', 'rt');
textLine = fgetl(fileID);
lineCounter = 1;
while ischar(textLine)
    fprintf('%s\n', textLine)
    if startsWith(textLine, 'second line')
        textLine = Messege    % i want to replace this line with messege
        disp('Done')
        break
    end
    textLine = fgetl(fileID);
    lineCounter = lineCounter + 1;
end

first line
second line

textLine = 'Hello'

Done

fclose(fileID);

[taimour sadiq]

yes i wish to change the contents of the file on disk... Voss Code Works Fine

Answer 1

[Voss]

input_file = 'SampleFile.txt';
output_file = 'SampleFile_modified.txt';

dbtype(input_file)

1     first line
2     second line
3     third line

Messege = 'Hello';
old_messege = 'second line';

L = readlines(input_file);
L(startsWith(L,old_messege)) = Messege;
writelines(L,output_file);

dbtype(output_file)

1     first line
2     Hello
3     third line

[taimour sadiq]

Dear Voss your answer is very precise and also fulfills the requirement... some issue on my side is writelines works only in R2022 and above.. as i mentioned R2021a if any guidence other than "writelines" will be highly appreciated.

[Walter Roberson]

input_file = 'SampleFile.txt';
output_file = 'SampleFile_modified.txt';

dbtype(input_file)

1     first line
2     second line
3     third line

Messege = 'Hello';
old_messege = 'second line';

L = string(regexp(fileread(input_file), '\r?\n', 'split'));
L(startsWith(L,old_messege)) = Messege;

[fid, msg] = fopen(output_file, 'w');
if fid < 0
    error('failed to open output file "%s" because "%s"', output_file, msg);
end

fwrite(fid, strjoin(L, newline));
fclose(fid)

ans = 0

dbtype(output_file)

1     first line
2     Hello
3     third line

[taimour sadiq]

Works Fine.. Thanks