MATLAB: Vectorize for-loop that changes overlapping parts of an array

Question

Hi everyone,

I have problem speeding up my code. I have a very large (20-40m datapoints) logical array and at every position where it reads "1", the next N datapoints should be set to "1" a well.

I have the following code that is quite slow since it has a for loop:

N=5000-1;
logarray; %logical array of size 40000000x1 
inx=find(logarray);  %inx can be quite large as well, 1-5m points
for ii=1:length(inx)  
    logarray(inx(ii):(inx(ii)+N))=1;
end

This code works, but is very slow (more than 10 secs). I tried something like

 logarray(find(inx):(find(inx)+100))=1;

but this did not work. I was wondering whether vectorisation would be a solution here? Mind that overlapping parts of the logical array are changed by the loop.

Does anyone have a good suggestion?

Thanks, Maximilian

Answer 1

Easy when using convolution:

logarray = logical([0 1 1 0 0 0 1 0 0 0 1 0 0 0 0 1 0 1 0 0 0 0 1])
N = 2 ;
out = conv2(double(logarray(:)), ones(N+1,1)) ;
out = out(1:numel(logarray))>0 ;
out = reshape(out,size(logarray)) ; 
disp([logarray(:) ; out(:)]) ;