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

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Dominik Mattioli 2017-10-23

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评论： Stephen23 2017-10-23

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

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Answer 1

Stephen23 2017-10-23

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编辑：Stephen23 2017-10-23

在 MATLAB Online 中打开

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

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Dominik Mattioli 2017-10-23

Thanks!

Stephen23 2017-10-23

@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

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Answer 2

Walter Roberson 2017-10-23

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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);