How to find number of values which are repeating in a column?

Hi, I have matrix of 4535 x 23. In first column I have id's which are repeating. I would like to know that how many times a specific id is repeating in first column of matrix. For example: visually I can see 75 is repeating 3 three times so I want output in a matrix which should be like matrix having first column as id and second column corresponding number repetition of values. (like: 75, 3). I tried histogram but unable to receive required information. Could you please help in wiring script?

[Walter Roberson]

http://www.mathworks.co.uk/matlabcentral/answers/43073-a-guide-to-tags

Answer 1

[Peter O]

Take a look at the histcounts function: [N,edges] = histcounts(X,edges) sorts X into bins with the bin edges specified by the vector, edges. The value X(i) is in the kth bin if edges(k) ≤ X(i) < edges(k+1). The last bin also includes the right bin edge, so that it contains X(i) if edges(end-1) ≤ X(i) ≤ edges(end).

So then:

    X = A(:,1);
    Vals = unique(X); % Already sorted for you!
    [N, HighEdge]  = histcounts(X, [Vals+0.5]); 
Assuming your IDs are all integers this spits them into the bin underneath. Alternatively you can do something like 

    Edges = [(min(Vals)-1):1:max(Vals)] + 0.5;

to get counts for each ID.

[Yasir Ali]

Thanks Peter for your answer. Once I used unique command and got sorted value in column 1, now I want how many times that value was repeated in given column and that number should appear in column 2? (say 75 repeated 3 so matrix would 75,3). thanks

[Peter O]

Yes, looking back that solution was a little hasty. Sorry about that. Try this instead. Again, it's assuming your input is all integers.

    Vals = unique(X);
    V2 = [Vals(1)-0.5, Vals+0.5];
    N = histcounts(X,V2);
    C = [Vals' N'];

C will have the ID's in columm #1 and the repeat counts in Column #2.

Here's a test case input:

    X = [233 11 18 91 01 91 18 17 10002 -43];

and expected output:

C =

         -43           1
           1           1
          11           1
          17           1
          18           2
          91           2
         233           1
       10002           1

[Peter O]

And for completeness, you can also use unique by itself, in a two-liner. This variant handles cases where you're looking for total counts and your input doesn't have to be stricly integer. IMO, it's the more elegant way:

    [Vals,~,ic] = unique(X);
    N = arrayfun(@(x) sum(ic==x),1:max(ic));
    C = [Vals', N'];