how to read last few rows/lines from a text file ?

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missy QIANHUI
missy QIANHUI on 27 Oct 2011
Edited: Walter Roberson on 20 Jun 2016
how to read last few rows/lines from a text file ? thanks

Answers (3)

Bjorn Gustavsson
Bjorn Gustavsson on 27 Oct 2011
Edited: Walter Roberson on 20 Jun 2016
If you can use something like this much is won:
[q,w] = system(['tail -n ',num2str(nr_of_lines_to_read),' ',filename]);
Then it can be as simple as this:
w = str2num(w);
...or more complicated depending on the contents of your file.
HTH.
  3 Comments
K E
K E on 20 Jun 2016
tail command doesn't work on Windows machines. Cygwin may be a workaround.

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Walter Roberson
Walter Roberson on 27 Oct 2011
Unless you can set a strict upper limit on the number of characters per line, the only way is to read a line at a time, remembering each new line and throwing away the oldest line (if you already have enough lines in the buffer.)
L = fgetl(fid);
if ~ischar(L); break; end %end of file
if length(LS) >= N %we have an extra line buffered
LS(1) = []; %so delete it
end
LS{end+1} = L; %and remember this new current last line
There are other ways of proceeding but they generally end up reading in the entire file first and then throwing away everything extra.
If you can put a strict limit on the number of characters per line, then there is a short-cut you can use: if the maximum line length is M and you want the last N lines, fseek() backwards (M+2)*N bytes from the end of the file and then start the above reading process. The +2 is to count carriage return and linefeeds.
I should specify that this shortcut of fseek() only works if your characters are single byte, no extended characters like greek or malaysian. If your file is UTF-8 or UTF-16 or UTF-32 encoded, then it is possible to make adjustments to the backwards seek count, but you could end up in the middle of an encoded character and you have to account for that.

Amardeep
Amardeep on 27 Oct 2011
You could used a fscan eof to get to the end of file then fgetl step up a line fgetl and repeat until you have enough lines from the end of the file.

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