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

2019 2 2
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更新时间：2020 4 11
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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
Star Strider 2019-2-2
The unique function comes quickly to mind.
The third output from unique may be what you want.
Dave
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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Stephen23
Stephen23 2019-2-3

0 个投票

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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Dave
Dave 2019-2-4
Thanks, is there a way to use that for a matrix instead?
%A is 4x1
A =[1, 6, 5, 5]'
%B is much larger, 6x1
B =[5 5 1 1 1 6 ]'
%A2 is
A2=[101, 501, 601;
102, 502, 602;
103, 503, 603]
% BBB is same dim as B, but using elements from A2
%ANSWER SHOULD BE:
BBB=[501 502 101 102 103 601]'
Stephen23
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
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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