Replace loop with more efficient procedure to make it faster

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Lukas Netzer 2021-8-19

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评论： Peter Perkins 2021-9-2

采纳的回答： Fangjun Jiang

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So I'm running this loop:

for n = 1:size(table1)
    for m = 1:size(table2)
        if table1.WP1(n) == table2.WP1(m) && table1.WP2(n) == table2.WP2(m)
            table2.STOPS(n) = table2.STOPS(m);
        end
    end
end

Which takes really long to process - is there a faster way to do it?

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Fangjun Jiang 2021-8-19

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Would this give you a clue?

t1=1:3;
t2=(1:4)';
flag=(t1==t2)

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dpb 2021-8-19

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Ah...by transposing you invoked the silent array expansion option -- and so then the two logicals are not vectors but arrays and, then in fact, are the same result.

So then, the Q? is, do you want the many-to-one relationship or a 1:1?

What is the expected size of the result -- possibly variable or just one element from the second for each n record in the first?

table2.STOPS(n) = table2.STOPS(m);

in the above double loop results in there being one value in the LHS but it will be the last matching location where WP1 and WP2 match (if there can be more than one).

Alternatively, the cell array solution could keep however many matches there are for each n.

You've not precisely defined which solution is the wanted one -- the first code will run and give a specific solution; the alternative can also run but may provide a different solution if there can be more than a single match.

If the data are such that only one match is known to be possible or only the first is wanted, then the double loop could be made to run faster by exiting the inner loop one a match is found.

But, we don't know which is the actual solution wanted...

Fangjun Jiang 2021-8-20

The code I provided is for multiple matching. All can be done in one shot without loops.

If you know there is only one match, the double-for loop could be faster. Your need to use "break" (instead of "continue") to break the loop.

But again, it "could" be faster. If the matching one is the last iteration in the double-for loop, then the "break" has no effect.

Peter Perkins 2021-9-2

The double loop is unlikely to be faster for any case. If there is at most one match in table2 for each row of table1, then perhaps ismember would work. But unless these tables are pretty big, the "row == col" strategy is likely to be the fastest.

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