[R] how to count number of occurrences

[David Winsemius]

On Nov 2, 2011, at 12:54 PM, Sl K wrote:

> 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

  dat$nXs <- cumsum(dat$samp=="X")
  dat$nYs <- cumsum(dat$samp=="Y")
  dat
#
             y samp nXs nYs
8  0.03060419    X   1   0
18 0.06120838    Y   1   1
10 0.23588374    X   2   1
3  0.32809965    X   3   1
1  0.36007100    X   4   1
7  0.36730571    X   5   1
20 0.47176748    Y   5   2
13 0.65619929    Y   5   3
11 0.72014201    Y   5   4
17 0.73461142    Y   5   5
6  0.76221313    X   6   5
2  0.77005691    X   7   5
4  0.92477243    X   8   5
9  0.93837591    X   9   5
5  0.98883581    X  10   5
16 1.52442626    Y  10   6
12 1.54011381    Y  10   7
14 1.84954487    Y  10   8
19 1.87675183    Y  10   9
15 1.97767162    Y  10  10

I find that there are 5 X's before the second Y.

 > nXbefore_mthY <- function(m) dat[which(dat$nYs==m), "nXs"]
 > nXbefore_mthY(2)
[1] 5

>
> 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!
>
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT