Trying to replace for loop with faster code

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Lukas Netzer 2021-8-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1439134-trying-to-replace-for-loop-with-faster-code

评论： Lukas Netzer 2021-8-23

I have the following code in a rather large script, which unfortunately is VERY slow:

for x = 1:size(table1)
    for y = 1:size(Loc_all)
    if ismember(table1.Loc(x),Loc_all(y)) == 1
        table1.dur(x) = t_avg(y);
        break
    end
    end
end

It definitely works, but again the duration time is not "workable".

So I tried the following:

f = ismember(table1.Loc,Loc_all) == 1;
[IndexM, IndexN]=find(f);
table1.dur(IndexM) = t_avg(IndexN);

It works for IndexM which defines the location in table1.Loc - but it wont work for IndexN - all I get here is 1's. What am I missing?

As you can see t_avg and Loc_all have the same size, as do table1.Loc and table1.dur - I'm expecting IndexN to give the Location in Loc_all where a match happens.

Loc_all only has unique values. table1.Loc is a large table of values which may or may not be in Loc_all.

Any hints are very much appreciated!

Thank you!

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DGM 2021-8-23

It would help to provide some succinct example data to demonstrate what exactly you're dealing with.

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Answer 1

Voss 2021-8-23

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The second output argument of the find function is column indices. In order to get indices in Loc_all where a match happens, you can use the second output argument of ismember:

[f, IndexN] = ismember(table1.Loc,Loc_all); % f is the same as your f; IndexN is what you expect, except it has zeros where no match occurred (i.e., where f is false)
IndexN = IndexN(f); % keep only the IndexN where a match occurred
IndexM = find(f); % convert logical index f to integer index IndexM; IndexM is the same as your IndexM