How do I store only the lines in a text file that contain a certain string?

I have a text file that I would like to read and I only want to display lines which start with a certain string. For example, the contents of the text file are:

FX = 698 FY = 96325 FZ = 987 MX = 87 MY = 96 MZ = 169 FX = 856 FY = 1234 FZ = 654 MX = 5916 MY = 1648 MZ = 4789

What I want to do is display only the numerical values for the lines in the text file which start with FY. Then I would like to output these values into another text file. How do I do this? Thanks!

Answer 1

[Image Analyst]

This is a fairly robust way to do it:

% Open the file for reading.
inputFullFilename = fullfile(pwd, 'test2.m');
fidInput = fopen(inputFullFilename, 'rt');
% Open the file for reading.
if fidInput == -1
  message = sprintf('Could not open %s for reading', inputFullFilename);
  uiwait(errordlg(message));
  return;
end
% Open the file for writing.
outputFullFilename = fullfile(pwd, 'output.txt');
fidOutput = fopen(outputFullFilename, 'wt');
if fidOutput == -1
  message = sprintf('Could not open %s for writing', outputFullFilename);
  uiwait(errordlg(message));
  return;
end
% Read first line.
thisLine = fgetl(fidInput);
% Define the pattern youi're looking for.
searchPattern = 'FY =';
% Loop over lines, checking for the search pattern.
% Bail out if there are more than 10,000 lines (fail safe to prevent infinite loop)
lineCount = 1; % Fail safe
while ischar(thisLine) && lineCount < 10000
  if length(thisLine) >= length(searchPattern)
    if strcmp(thisLine(1:length(searchPattern)), searchPattern)
      % Found it.  Print it out.
      fprintf('%s\n', thisLine);
      fprintf(fidOutput, '%s\n', thisLine(length(searchPattern)+1:end));
    end
  end
  % Read next line
  thisLine = fgetl(fidInput);
  lineCount = lineCount + 1; % Increment fail safe
end
% Close files.
fclose(fidInput);
fclose(fidOutput);
winopen(outputFullFilename); % Windows only

[Wilson Perez]

Thank you so much! This is exactly what I was looking for.