How do you add a line to the beginning of a .txt file?

I have some .txt files that contain rectangular matrices, and I would like to add one line to the beginning of each .txt file that contains a single number. E.g., I have a .txt file that contains:

1 2 3 4

5 6 7 8

9 10 11 12

And I would like to create a new .txt file that looks exactly the same, except with a new first row added to it, and with a single number in that row. E.g.:

20

1 2 3 4

5 6 7 8

9 10 11 12

Help? I think this has something to do with fprintf, but I don't know how to use it.

Answer 1

[Walter Roberson]

It is difficult to add text inside a text file. The easiest thing to do is to create a new text file and output everything to it, copying from the original file.

   infid = fopen('YourFile.txt', 'rt');
  outfid = fopen('YourNewFile.txt', 'wt');
  fprintf( outfid, '20\n' );  %the text you are adding at the beginning
  while true
    thisline = fgetl(infid);    %read line from input file
    if ~ischar(thisline); break; end    %reached end of file
    fprintf( outfid, '%s\n', thisline );   %write the line to the output file
  end
  fclose(infid);
  fclose(outfid);