Help with extracting certain lines from a text file?

I'm analyzing a lot of data and needing to extract and certain lines and put them into a new file. The lines I am trying to extract begin with 'vertex.' I would like to use a loop to loop through the whole file extracting lines that begin with vertex and put them into another new text document. How can I do this?

[Paolo]

Please do not open duplicate questions.

Anyway, try:

 data = fileread('Thinker.txt');
[matches]=regexp(data,'(?=vertex)(.*?)(?=vertex|endloop)','match');

To extract those lines.

[Taylor Gates]

Thank you. In what way does that organize the data so I can find where its matching the text file at?

[Taylor Gates]

I think that only extracted the x column of the vertex. How can we fix that? Sorry for not including format it is like "vertex -0.666852 -4.031210 5.684145"

[Jan]

@Taylor: You have posted the search string "vertex." with a trailing dot. Is this wanted or a typo. Should we guess, what the "x column" is?

Answer 1

[Jan]

Str   = fileread('Thinker.txt');
CStr  = strsplit(Str, '\n');
match = strncmp(CStr, 'vertex ', 7);   % or 'vertex.' ?
CStr  = CStr(match);

"I think that only extracted the x column of the vertex" is not a clear description, but with a bold guess:

% An easy loop method:
for k = 1:numel(CStr)
  s = CStr{k};
  v = find(s == ' ');   % or: strfind(s, ' ')
  CStr{k} = s(1:v(2)-1);
end

Now export the cell string to a file again:

fid = fopen('File.txt', 'w');
if fid == -1, error('Cannot open file!'); end
fprintf(fid, '%s\n', CStr{:});
fclose(fid);

You mentioned, that you want to use a loop. Then:

inFID  = fopen((InFile.txt', 'r');
outFID = fopen((InFile.txt', 'w');
if inFID == -1 || outFID == -1
  error('Cannot open files!');
end

while ~feof(inFID)
  s = fgetl(inFID)
  if ~isempty(s) && strncmp(s, 'vertex ', 7);
    v = strfind(s, ' ');
    fprintf(outFID, '%s\n', s(1:v(2)-1));
    % or this might be faster:
    % fwrite(outFID, s(1:v(2)-1), 'char');
    % fwrite(outFID, 10, 'char');
  end
end

fclose(inFID);
fclose(outFID);