MATLAB: Not sure how to ask this, but here’s the scenario:

combinationspermutations

Ok, i'm not sure what this transformation is called, but here's what i want to do.
Given a matrix of categorical variables:
  x = ['a', 'b';
   'a', 'c';
   'a', 'b';
   'b', 'c';
   'c', 'a';
   'a', 'd' ]
Generate a matrix that has all of the combination counts:
  a  b  c  d
        -       -       -       -
a |   0  2  1  1
b |   0  0  1  0
c |   1  0  0  0
d |  0  0  0  0
Any thoughts?

Best Answer

%%Data
x = {'a', 'b';
   'a', 'c';
   'a', 'b';
   'b', 'c';
   'c', 'a';
   'a', 'd'};
%%Engine
uv = unique(x(:));
[~,idx] = ismember(x,uv);
m = accumarray(idx,1,numel(uv)+[0 0]);
%%Formatting
T = array2table(m,'VariableNames',uv,'RowNames',uv)
Results:
T = 
       a    b    c    d
       _    _    _    _
  a    0    2    1    1
  b    0    0    1    0
  c    1    0    0    0
  d    0    0    0    0
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