Maximum of a subset of array (MATLAB)

Question

Suppose in MATLAB I have a real matrix A which is n x m and a binary matrix B of the same size. The latter matrix defines the optimization set (all indices for which the element of B equals one): over this set I would like to find the maximal element of A. How can I do this?

The first idea I had is that I consider C = A.*B and look for the maximal element of C. This works fine for all matrices A which have at least one positive element, however it does not work for matrices with all negative elements.

Floris · Accepted Answer · 2014-02-10 04:32:22Z

2

You can do

C = A(B==1);

to give you an array of just the values of A corresponding to a value of 1 in B. And

max( C )

will give you the maximum value of A where B is 1

With this method you don't run into a problem when all values of A are negative as the zeros don't appear in C.

Obviously you can condense this to

desiredValue = max(A(B(:)==1));

I am using the colon operator to make sure that the result of A(B(:)==1) is a column vector - if B is all ones I am not sure if Matlab would return a vector or a nxm matrix (and I can't confirm right now).

update to get the index of the value, you can do:

f = find(B==1);
[m mi] = max(A(f));
maxIndex = f(mi);

And to get that back to the 2D elements:

[i j] = ind2sub(size(A), maxIndex);

edited Feb 10, 2014 at 4:32

answered Feb 10, 2014 at 4:24

Floris

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1 Comment

SBF Over a year ago

Thanks, but I am interested in the element, not in the value per se. Would it be possible to update your solution to that?

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