Create an index to table - matlab

Question

I have a table T as below:
T = table({'A';'A';'B';'B';'B';'B';'C';'C';'D';'D'},...
{'xd';'z';'x';'y';'z';'w';'x';'wh';'z';'w'},...
[4;2;4;1;2;5;2;1;1;5], ...
'VariableNames', {'memberId', 'productId','Rating'});
T =
memberId productId Rating
________ _________ ______
'A' 'xd' 4
'A' 'z' 2
'B' 'x' 4
'B' 'y' 1
'B' 'z' 2
'B' 'w' 5
'C' 'x' 2
'C' 'wh' 1
'D' 'z' 1
'D' 'w' 5
I need to index it by memberId and productId so the result is:
A: {'xd' 'z'}
B: {'x' 'y' 'z' 'w'}
C: {'x' 'wh'}
.......

Answer 1

You can use categorical arrays and a structure to do this:
% convert to categorical arrays
T.memberId = categorical(T.memberId);
T.productId = categorical(T.productId);
% cross-tabulate memberId vs. productId
cross_T = crosstab(T.memberId,T.productId);
% a function to return the productId for all 1 in row
productId = categories(T.productId).';
row = #(x) productId(logical(cross_T(x,:)));
% preform on all rows
rBy_c = arrayfun(row,1:size(cross_T,1),'UniformOutput',false).';
% convert to structure for readability
s = cell2struct(rBy_c,categories(T.memberId))
To get the output (s):
A: {'xd' 'z'}
B: {'w' 'x' 'y' 'z'}
C: {'wh' 'x'}
D: {'w' 'z'}