Copy a txt file and change a line by a other

Dear I have a problem to copy from a txt file to another one, and change a specific line i have found the copy file option

file_A = 'Ok_A.txt';
file_B = 'Ok_B.txt';
copyfile(file_A,file_B)

But i cannot go further

Answer 1

[Walter Roberson]

copyfile() is not appropriate for this. You need to fopen() one file for input and the other for output, and then copy the input to the output until you get to the line that is to be changed. Output the changed version of the line rather than the original version of the line. Then continue copying input to output until the end of file. Then fclose() the two files.

There is no way to make a change in the "middle" of a text file -- not unless the new text is exactly the same size as the original text. This is not a restriction imposed by MATLAB: it is a restriction on all file systems that have been implemented by vendors over the last 30-ish years.

[Lila wagou]

Dear Walter, I'm not experimented in Matlab, but i think to do a copy of the original file, open the copy, localise the specific line and change the string. Any other way please

[Walter Roberson]

You cannot change in the middle of a file unless the replacement is exactly the same size as what you are changing.

Example:

line_to_change = 14;
new_output = 'Henry the IXth I am';

fid_in = fopen('OK_A.txt', 'r');
fid_out = fopen('OK_B.txt', w');
for K = 1 : line_to_change - 1
  %copy lines unchanged
  this_line = fgets(fid_in);
  fwrite(fid_out, this_line);
end

this_line = fgets(fid_in);  %read line that will be changed
fprintf(fid_out, '%s\n', new_output);  %write changed version

while true
  %copy lines unchanged
  this_line = fgets(fid_in);  %try getting a line
  if ~ischar(this_line); break; end   %end of file?
  fwrite(fid_out, this_line);  %no, copy it
end

fclose(fid_in);
fclose(fid_out);

[Lila wagou]

Dear Walter, thank you lot. Please, a small change in the 4 line

fid_out = fopen('OK_B.txt', 'w');
Thank

[Walter Roberson]

fid_in = fopen('Ok_A.txt', 'r');
fid_out = fopen('Ok_B.txt', 'w');

You can change the strings to whatever you need.