Convert column of data from cumulative to relative values

I have what I think is a relatively simple question that I haven't been able to figure out the answer to. I had different users complete the same test multiple times, and have 2 columns of data (A and B) in matrix 'TestData' as below. Column A is a user number and Column B is the cumulative test number (how many TOTAL test have been taken)

Col A  Col B
1  1
1  2
1  3
1  4
1  5
2  6
2  7
2  8
3  9
3  10
3  11
3  12
3  13
3  14

I would like to be able to create another column, column C, that converts the cumulative test number to a number that is relative to a given user. In other words, I want column C to show the test number WITHIN a particular users data, as below.

Col A  Col B  Col C
1  1  1
1  2  2
1  3  3
1  4  4
1  5  5
2  6  1
2  7  2
2  8  3
3  9  1
3  10  2
3  11  3
3  12  4
3  13  5
3  14  6

Any help would be appreciated, I've been stuck on this for awhile and was wondering if anyone has a straightforward way to approach this. Thanks in advance.

JD

Answer 1

[Azzi Abdelmalek]

 test=[1 1 1 1 1 2 2 2 3 3 3 3 3 3
       1 2 3 4 5 6 7 8 9 10 11 12 13 14]' 
idx=diff([0; find([diff(test(:,1)); 1])])
new_test=[test cell2mat(arrayfun(@(x) (1:x)',idx,'un',0))]

[Joshua]

Perfect, I made a few tweaks and it was good to go. Thanks Azzi