Very slow function, excessive overhead?
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I've got a function that looks at a n x 2 array (1st column time vector, 2nd column flag vector) and it's running incredibly slow. As you can see, line 17 takes almost all of the runtime. I've read that this could be related to overhead, but I don't actually know what that means or how to improve it. Any ideas on what could be causing this?
采纳的回答
Walter Roberson
2023-3-27
You are not assigning to flag within your loop, so you should use
flagrows = size(flag,1);
while i < flagrows
end
The end for a loop does get "charged" the cost of the loop overhead, which in this case is doing more computations than required.
2 个评论
Jesse
2023-3-27
Walter Roberson
2023-3-28
you used length(flag(:, 1)) which starts by extracting the first column and then taking the length of the column. That sequence involves copying data and then checking the size. That is a lot more work then just checking the size of the original data.
更多回答（1 个）
The inner while loop is not necessary: if flag(i,2)-flag(i-1,2) == 1, then of course flag(i,2)-flag(i-1,2) ~== -1. Thus the incrementation i = i+1 will always happen. This is strange since you increment i again after you quit the if-clause.
Preallocating "events" as zeros(size(flag,1),3) and finally cutting it by events = events(1:j-1,:) might speed up the function also.
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