How to get the highest values on a matrix depending on the values of another matrix

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Victor Francisco Pajuelo Madrigal
编辑： Victor Francisco Pajuelo Madrigal 2015-9-11
I want to sort out the values of a matrix (A) depending on another independent matrix (B), and create a third matrix with the results (C). See the example:
A = [1 2 1; 4 1 3; 3 8 2]
B = [1 2 3; 4 5 6; 7 8 9]
C = [5 9 5; 4 5 7; 7 8 9]
Therefore, for the same elements on A, I want to find the highest elements on B. For instance, for values of 1, I want to loop through B until finding the highest value related, that in the example is 5. Then I want to pass all the values to a new matrix C.
I have tried with generating my own functions or using loop, but I found no solution after 4 days of trying this. Would appreciate some explanation if I am missing something or not understanding the properties of matrices in Matlab, I am quite used to Python and R, but in Matlab I am a newbie.
Thanks!

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采纳的回答

Stephen23
Stephen23 2015-9-11
编辑：Stephen23 2015-9-11
In just three lines, without any loops:
>> A = [1 2 1; 4 1 3; 3 8 2];
>> B = [1 2 3; 4 5 6; 7 8 9];
>> [~,~,X] = unique(A(:));
>> D = accumarray(X,B(:),[],@max);
>> C = reshape(D(X),size(A))
C =
5 9 5
4 5 7
7 8 9

更多回答（1 个）

Guillaume
Guillaume 2015-9-11
A = [1 2 1; 4 1 3; 3 8 2];
B = [1 2 3; 4 5 6; 7 8 9];
assert(isequal(size(A), size(B)), 'matrix A and B have different size');
C = zeros(size(A)); %destination matrix
for ua = unique(A)' %iterate over the unique values of A
C(A == ua) = max(B(A == ua)); %for those value of A that are ua, update C with the max of B
end
  1 个评论
Victor Francisco Pajuelo Madrigal
Victor Francisco Pajuelo Madrigal 2015-9-11
编辑：Victor Francisco Pajuelo Madrigal 2015-9-11
Thanks for the reply! The comments to the code were great to know what was going on! It worked for a small matrix but my dataset is very large (working with satellite images) and it takes a lot of CPU processing.

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