Vectorize for-loop that changes overlapping parts of an array

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Maximilian Rüppell
评论： Roger Stafford 2016-4-20
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

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采纳的回答

Jos (10584)
Jos (10584) 2016-4-20
编辑：Jos (10584) 2016-4-20
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(:)]) ;

更多回答（1 个）

Roger Stafford
Roger Stafford 2016-4-20
编辑：Roger Stafford 2016-4-20
It isn't perhaps that you have a for-loop that is slowing you down so much as that your loop is repeating the writing of ones so much in overlapping intervals. You might try the following which first combines overlapping intervals before doing the writing of ones.
N=5000-1;
f1 = find(logarray);
f2 = min(f1+N,length(logarray));
f3 = zeros(size(f1));
f4 = zeros(size(f2));
ie = 1;
f3(ie) = f1(ie);
f4(ie) = f2(ie);
for ib = 1:size(f1,1)-1
if f1(ib+1) > f4(ie)+1
ie = ie+1;
f3(ie) = f1(ib+1);
end
f4(ie) = f2(ib+1);
end
f3 = f3(1:ie); % The intervals defined by f3 and f4 don't overlap
f4 = f4(1:ie);
for ib = 1:ie
logarray(f3(ib):f4(ib)) = 1;
end
  1 个评论
Roger Stafford
Roger Stafford 2016-4-20
I thought of another way you could try which uses 'accumarray'. Assume 'logarray' is a column vector.
N = 1000;
f = find(logarray);
t = ones(size(f));
logarray = min(cumsum(accumarray([f;f+N+1],[t;-t],...
[length(logarray)+N+1,1])),1);
logarray(end) = [];

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