MATLAB: How to find MIN value with same number in the other column

unique

Hi, This one is really tricky.
I have a matrix like below:
A=[
   1,1;
   1,2;
   1,3;
   2,4;
   2,5;
   2,6;
   3,7;
   3,8;
   3,9;
   2,10;
   2,11;
   2,12;
   1,13;
   1,14;
   1,15;]
I want to fine Min of each (1,2,3,2,1) groups. Therefore, the answer I want to get is, [
 1,1;
 2,4;
 3,7;
 2,10;
 1,13]
I tried using unique function but I couldn't solve problem because there are 2 separate groups of 1 and 2.
Please help me and thank you very much!

Best Answer

Here is one way of doing it:
A=[1,1;
   1,2;
   1,3;
   2,4;
   2,5;
   2,6;
   3,7;
   3,8;
   3,9;
   2,10;
   2,11;
   2,12;
   1,13;
   1,14;
   1,15;]
runlengths = diff([0; find(diff(A(:, 1))); size(A, 1)]);  %find length of each run of consecutive numbers
rowids = repelem((1:numel(runlengths))', runlengths);  %assign unique ids to each run
[A(cumsum(runlengths), 1), accumarray(rowids, A(:, 2), [], @min)]  %use accumarray to find the minimum per rowid
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