Removing Rows From a Matrix by Label Quickly

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Joseph on 25 Jun 2013
I currently have some code that is destined to: import data as a matrix from a text file, read the data and delete any rows in the matrix that do not start with "H", delete the H label from the remaining rows, print this matrix to a text file. My code is as follows:
FID = fopen('Test_Data_2.txt','rt');
while ~feof(FID)
if l(1:1)=='H'
m = [m;str2num(l(2:end))];
FID = fclose(FID);
dlmwrite('md_msd.out', m, 'delimiter', '\t', ...
'precision', 6)
This code works great for small data sets but I am going to have to use it for sets of 100000 rows or more. I need a way to speed up the process as my current code takes far too long. Is there any way I can make this code faster?
Joseph on 2 Jul 2013
Thanks for the help! I finally got the function to run in around 12 seconds for one of the larger data sets. Thank you!

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Answers (1)

Walter Roberson
Walter Roberson on 26 Jun 2013
Use other tools when it makes sense to do so. For example, in Linux or OS-X from their shells:
sed -e '/^$|^[^H]/d', -e 's/^H//p' < Test_Data_2.txt > md_msd.out
Or perl
perl -e '/^H/ && s/^H// && print' < Test_Data_2.txt > md_msd.out
You can invoke perl from within MATLAB using the perl() command.
Joseph on 29 Jun 2013
OK, I understand. So to call perl to help execute the code above would I simply place:
perl -e '/^H/ && s/^H// && print' < Test_Data_2.txt > md_msd.out
in my MATLAB function? Or do I need to wire perl script that preforms the same operations as the above code?

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