Hi, I am running a small program, that first sortrows of a matrix by the some column and then compare the order with another column, if there is a flip order, then delete that row, if not continue.
The code is below:
------------- %M of size[3^N,N+2]. M has a very large size. M=sortrows(M,N+1); %finish sorting
i=2; %no need to start from first one, obvious. j=0; stop=M(3^N,N+1); while M(i,N+1)<stop % loop has not finish till last row if M(i,N+2)>=M(i+1,N+2)% M(i,N+2) is the maximum number so far M(i+1,:)=; %delete i+1 row j=j+1; else i=i+1; end end
------------- The program runs ok, and I tried some tests, however when N is 12 or 13, the program can run more than 3 hours. I wonder why is so? sortrows takes no more than 1 minute, why compare procedure(3^N steps) takes so much time? Is delete a row is time-consuming or something else?
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The problem is that you are resizing the matrix each time the if criteria is met. This requires a memory copy and is very slow for large matrices.
Instead, use a for-loop and build up a vector of rows to remove. Then remove them all at the same time. Simple example:
N = 20; idx = false(N,1); M = magic(N); for ii = 1:N if max(M(ii,:))>min(M(ii,:))*N/2 idx(ii) = true; %This row will be removed. end end M(idx,:) = ; %remove all of the rows here!
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