Using FIND for common elements WITHOUT a loop (Vectorized?)

Question

Dave 2019-2-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/442854-using-find-for-common-elements-without-a-loop-vectorized

评论： Omar Salah 2020-4-11

采纳的回答： Stephen23

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Hello,

I have an array A 100x1 with different datenum that

I need to find in a big array B 5000000x1 (all datenum in B are in A, just repeated many times)

and then replace w another datenum C 100x1 (same index as A)

I am using find( ) but it is taking too long. Is there a way to do this without a loop?

BB=B %making a copy of B to check after.

for i=1:size(A,1)
loc=find(B==A(i))
BB(loc)=C(i);
end

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Star Strider 2019-2-2

The unique function comes quickly to mind.

The third output from unique may be what you want.

Dave 2019-2-3

编辑：Dave 2019-2-3

thank you, I don't know how to take your comment as the accepted answer.

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Answer 1

Stephen23 2019-2-3

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https://ww2.mathworks.cn/matlabcentral/answers/442854-using-find-for-common-elements-without-a-loop-vectorized#answer_359242

Use ismember and its second output argument:

>> A = randperm(5)
A =
   3   5   4   1   2
>> B = randi(5,1,9)
B =
   5   1   2   5   1   2   4   5   4
>> C = (1:5).^2
C =
    1    4    9   16   25
>> [~,X] = ismember(B,A);
>> BB = C(X)
BB =
    4   16   25    4   16   25    9    4    9

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Stephen23 2019-2-5

"Thanks, is there a way to use that for a matrix instead?"

Probably. Please explain the logic of your new example (which is unrelated to your earlier question when C had the same size as A).

Omar Salah 2020-4-11

Hi Stephen, I really wanted to thank you. I wanted to imporve my really slow code in which I tryz to find a 100k elements in a 320k elements vector. using is memeber the code runs in 5.4832 seconds instead of 1170 seconds when using for and parfor. Vectorization is magical for Big data.

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