Improving Efficiency of Find Algorithm

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Matt C
Matt C 2021-8-19
回答： Cris LaPierre 2021-8-19
Hello,
I am aware that logical indexing is much faster than the usage of the find function, in specific instances. I'm wondering if there is a way to improve the following algorithm - I'm not quite sure how to use indexing (if possible) in this situation.
What I have is a matrix of ascending values, though some of those values may be repeated (specifically, I have millions of ascending timestamps with many repeated). I am then seeking the start and end indices of a window that is between time X and Y.
Here is an example of the algorithm that I currently have implemented:
myDataTimestamps = [10 20 30 30 30 40 50 60 60 60 70 70 80 90];
window_start_time = 30;
window_end_time = 80;
start_index = find(myDataTimestamps >= window_start_time,1,'first');
end_index = find(myDataTimestamps <= window_end_time,1,'last');
Is there a way to improve the speed of this code and still return the same start_index of 3 and end_index of 13?
Much appreciated!

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回答（2 个）

Cris LaPierre
Cris LaPierre 2021-8-19
Ran in:
This approach may only work for this simple case, but here's a way to do it using max/min.
myDataTimestamps = [10 20 30 30 30 40 50 60 60 60 70 70 80 90];
window_start_time = 30;
window_end_time = 60;
% find start/end index
ind = 1:length(myDataTimestamps);
wind = myDataTimestamps==window_start_time | myDataTimestamps==window_end_time;
start_index = min(ind(wind))
start_index = 3
end_index = max(ind(wind))
end_index = 10
  1 个评论
Matt C
Matt C 2021-8-19
编辑：Matt C 2021-8-19
I haven't been able to do a comparison, but that wasn't the massive improvement that I was hoping for. I had cancelled the script early without grabbing a total runtimes for comparison, but it looked like the proposed algorithm was going to take just as long (if not longer) than the find function. I implemented the recommendation as:
myDataTimestamps = [10 20 30 30 30 40 50 60 60 60 70 70 80 90];
window_start_time = 30;
window_end_time = 60;
% find start/end index
ind = 1:length(myDataTimestamps);
start_index = min(ind(myDataTimestamps>=window_start_time));
end_index = max(ind(myDataTimestamps<=window_end_time));
Have I blown anything in my above implementation? Note that my processor loading was ~50%, and only ~1 GB of my 24 GB of RAM was being used.
Edit: I can confirm that it took much longer using the min/max method. My code took ~45 minutes to fully execute using 'find', whereas it had only completed ~25% after about 2 hours using the min/max method.

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Cris LaPierre
Cris LaPierre 2021-8-19
I wonder if this is a scenario where using tall data may help. See this page.

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