Dear R users,

I have this data frame,
           y samp
8  0.03060419    X
18 0.06120838    Y
10 0.23588374    X
3  0.32809965    X
1  0.36007100    X
7  0.36730571    X
20 0.47176748    Y
13 0.65619929    Y
11 0.72014201    Y
17 0.73461142    Y
6  0.76221313    X
2  0.77005691    X
4  0.92477243    X
9  0.93837591    X
5  0.98883581    X
16 1.52442626    Y
12 1.54011381    Y
14 1.84954487    Y
19 1.87675183    Y
15 1.97767162    Y

and I am trying to find the number of X's that occur before ith Y occurs.
For example, there is 1 X before the first Y, so I get 1. There are 4 X's
before the second Y, so I get 4, there is no X between second and third Y,
so I get 0 and so on. Any hint to at least help me to start this will be
appreciated. Thanks a lot!

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