How to replace elements in an array with the indices of the array's sorted, unique values?

This is what I have (below). I am wondering if there is a better way of accomplishing the aforementioned task. Also, is there a name for this process? I don't have a formal computer science education so I'm curious.

        A  = randi(9,5,2);         % Initialize matrix.
        uA  = unique(A);            % Unique values of A.
        B   = A;                        % Output.
        for idx = 1:length(uA)          % Index to unique values, replace with
            B(B == uA(idx)) = idx;      % their sorted indices.
        end

Answer 1

[Stephen23]

All you need is the third output of unique followed by reshape:

>> [~,~,Y] = unique(A);
>> reshape(Y,size(A))

For example:

>> B % from running your code
B =
   4   5
   2   3
   1   4
   4   7
   5   6
>> [~,~,Y] = unique(A);
>> reshape(Y,size(A))
ans =
   4   5
   2   3
   1   4
   4   7
   5   6

[Dominik Mattioli]

Thanks!

[Stephen23]

@Dominik Mattioli: I hope that it helps!

It is always worth reading the documentation for every function that you use: there is a lot of useful info in there!

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

Answer 2

[Walter Roberson]

That code is wrong: you are changing B "in place" and comparing against all of it, so you could end up changing the same location multiple times. For example, suppose one of the unique values was 1/2 and another was 1, and those got index values 1 and 2 respectively. You change the 1/2 entries to 1. But that creates a 1 that is then there to be matched against uA(2) so you would mistake the 1 that was the index with the 1 that was the value.

You should just use

[uA, B] = unique(A);

[Dominik Mattioli]

Very good point, thank you. The other two "Steve"s pointed out the easy solution to my naive question.

Answer 3

[Steven Lord]

Do you want the ic third output from unique, or a version of that output that you've used reshape on to make it the same shape as A?

[Dominik Mattioli]

Thank you!