MATLAB: How to extract unique values in one column per groups of rows in other column

unique values

I have several excel worksheets with over 6000 rows of data in excel that go across multiple columns. In column 1 I have the number corresponding to an image (goes up sequentially) and in column 2 I have the parent cell number that goes up to some maximum number which varies image to image and can be in duplicates. I need to extract the number of unique values in column 2 per image but I'm not sure how to script it in matlab.
Matrix Example to illustrate problem:
1       2
1       2
1       3
1       5
1       23
2       1
3       1
3       2
3       2
3       2
4       3
4       20
4       13
4       17
4       31
4       17
etc.
and I need an output that resembles
1      4
2      1
3      2
4      5
etc.

Best Answer

Assuming sequential integers starting from one in the first column:
 >> accumarray(M(:,1),M(:,2),[],@(n)numel(unique(n)))
 ans =
     4
     1
     2
     5
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