How to accelerate the process of `find` in large for loop ?

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wei zhang
wei zhang el 22 de Jul. de 2020
Comentada: wei zhang el 23 de Jul. de 2020
I have two arrays A,B. I want to find the index of every element of B in A. I'm sure that every element of B must appears in A twice. The code of mini case is as below.
A = [1 2 3 4 5 6 5 4 2 7];
B = [2 4 5];
result = zeros(length(B),2);
for i=1:length(B)
result(i,:) = find(A==B(i));
end
% result should be [2,9;4,8;5,7]
The A and B in practice is very large (8000000). It takes long time in this for loop way. Is there any way to improve the code?
I had tried to use ismember function. But it seems I could not bypass the find function.
[ia,ib] = ismember(A,B);
result = zeros(length(B),2);
for i=1:length(B)
result(i,:) = find(ib==i);
end
I know I could use parallel computing toolbox. I just want a more delicated coding way.

Respuesta aceptada

Stephen23
Stephen23 el 22 de Jul. de 2020
>> [X,Y] = ismember(A,B);
>> [~,Z] = sort(Y(X));
>> V = find(X);
>> R = reshape(V(Z),2,[]).'
R =
2 9
4 8
5 7
  1 comentario
wei zhang
wei zhang el 23 de Jul. de 2020
How could you understand the appilication of these functions, sort ismember reshape unique, so well?! Your answer is so good! Even I know the meaning of the above functions, I still can't use them properly, for several years!

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Más respuestas (1)

Mehmed Saad
Mehmed Saad el 22 de Jul. de 2020
Try the following code
A = [1 2 3 4 5 6 5 4 2 7];
B = [2 4 5];
[r,c] = find(A-B'==0);
[~,ic] = sort(A(c));
c(ic)
  1 comentario
wei zhang
wei zhang el 23 de Jul. de 2020
Because my size of A and B are very large, the matrix A-B' is out of memory very severely, unfortunately.

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