Find match for a pair of rows in cell arrays

I have 2 cell arrays of equal length, e.g.

>> alfa = {'a'; 'b'; 'c'; 'b'; 'a'; 'c'}

alfa =

    'a'
    'b'
    'c'
    'b'
    'a'
    'c'

>> beta = {'b'; 'd'; 'a'; 'd'; 'a'; 'a'}

beta =

    'b'
    'd'
    'a'
    'd'
    'a'
    'a'

and I want to find where the pairs are repeated, which pair is it and the index where the repeated pairs are located.

So, in this case the index 2 and 4 correspond to the repeated pair 'b' and 'd', and the index 3 and 6 correspond to the repeated pair 'c' and 'a'. It is important that I know which pair is it and all the indexes where this pair is repeated.

Thank you.

[Adam]

What is the significance of having two cell arrays? I don't see what the relevance of the 2nd one is other than just as another example. Is it supposed to be used in some way?

Answer 1

[dpb]

Adam's on the right track, just didn't quite recognize the problem description--

>> ab=[char(alfa) char(beta)];  % mush the two arrays together
>> [u,ia,ib]=unique(ab,'rows'); % get the combinations that are extant
>> u(histc(ib,unique(ib))>1,:)  % find the ones with more than one occurrence
ans =
bd
ca
>>

The alternate returns from histc and unique will provide the positions.

Answer 2

[Adam]

[C, ia, ic] = unique( alfa );

should give you this information. The C vector gives the characters and the ic vector tells you where they occur so repeated indices show the locations of the same letter.

[C, ia, ic] = unique( alfa )

C =

    'a'
    'b'
    'c'

ia =

     1
     2
     3

ic =

     1
     2
     3
     2
     1
     3

[Stefano Alois]

Sorry, probably the question is not clear. Imagine if instead of 2 cell arrays you have a 6x2 cell array: >> gamma = [alfa beta]

gamma =

    'a'    'b'
    'b'    'd'
    'c'    'a'
    'b'    'd'
    'a'    'a'
    'c'    'a'

Now if you observe it in lines 2 and 4 you have the repeated sequence 'b' and 'd', as well as in lines 3 and 6 there is the repeated sequence 'c' and 'a'. The solution you provided only looks at one array finding the same strings in the same array, but it's not what I need.

[Adam]

So what do you have? 2 arrays or 1 array of 6x2 size?

[dpb]

He stated he had two cell arrays only you must consider them together by row--see my response. Simplest way is as done there to convert to a 1D character vector as the 'rows' option doesn't work for cellstr arrays, unfortunately. (Why not is anybody's guess... :( )