Question

I have the following code:

[~,ind]=max(Defender.Q,[],6);

Defender.Q is a HUGE multidimensional matrix.

When there are multiple maximums in the 6th dimension of Defender.Q, the max function
is giving me the index of the first of these multiple maximums. I want to get an index
that is randomized between multiple maximums. Any ideas? Thanks for your help!

Answer 1

Ok, this is a bit involved, but you can get the indices of all the maxima, and then randomly pick one using randi and accumarray:

%# (1) Find the maxima

%# if you are interested in the global maximum
%# that may occur multiple times along dimension 6
[maxVal,maxIdx] = max(Defender.Q(:));

%# ALTERNATIVELY

%# if you are interested in local maxima along dimension 6
maxVal = max(Defender.Q,[],6);
maxIdx = find(bsxfun(@eq,Defender.Q,maxVal));

%# (2) pick random maximum for each 5D subarray

%# this assumes that there is no dimension #7 etc
%# In case there is, you need to add a column of ones
%# and then d7 etc to second input of accumarray

%# find row, col, etc subscripts of the maxima
[d1,d2,d3,d4,d5,d6] = ind2sub(size(Defender.Q),maxIdx);

%# create a 5-d array, containing one random index
%# from the maxima along dimension 6, or NaN
randIdx = accumarray([d1,d2,d3,d4,d5],d6,[],@(x)x(randi(length(x))),NaN);