How to vectorize strfind

6 views (last 30 days)
Paolo Binetti
Paolo Binetti on 22 Dec 2016
Edited: Paolo Binetti on 23 Dec 2016
Is it possible to use strfind in a vectorized way? Suppose I want to get find not just one pattern inside a string, but several of them at the same time, so that the output would not be a vector of indexes, but a matrix: is it possible?
Concretely, take a string 'TRSDGHNENJRRDSENTRFDGDGT'. I want to find 'TR', 'DG', and 'EN'. The output of the function would be a matrix 3 x length(string) where line one are zeros and ones at indexes relative to 'TR', line two for 'DG' and line three for 'EN'. Possible?
The purpose is to avoid a for loop which even with pre-allocation is time-consuming. But I actually don't even know if this vectorization I am thinking of would be quicker.
  1 Comment
Image Analyst
Image Analyst on 23 Dec 2016
Define quick for you. Exactly how long is your for loop solution taking? I can do around five hundred million iterations of a for loop in less than half a second. How many billions of iterations are you doing in your loop, and how long is it actually taking?

Sign in to comment.

Answers (1)

Walter Roberson
Walter Roberson on 22 Dec 2016
In R2016b,
S = 'TRSDGHNENJRRDSENTRFDGDGT';
targets = ['TR'; 'DG'; 'EN'];
output = [targets(:,1) == S(1:end-1) & targets(:,2) == S(2:end), false(size(targets,1),1)];
In earlier versions you would need to use bsxfun()
  2 Comments
Walter Roberson
Walter Roberson on 23 Dec 2016
The bsxfun version would be
output = [bsxfun(@eq, targets(:,1), S(1:end-1)) & bsxfun(@eq, targets(:,2), S(2:end)), false(size(targets,1),1)];
If you do not have bsxfun then you must be using a version before R2007a, and if you are then it is important for us to know that so that we do not keep suggesting features you cannot use.

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!