[R] Counting occurrences in a moving window

[mdvaan]

Hi list, based on the following data.frame I would like to create a variable
that indicates the number of occurrences of A in the 3 years prior to the
current year:

DF = data.frame(read.table(textConnection("  A  B
8025  1995
8026  1995
8029  1995
8026  1996
8025  1997
8026  1997
8025  1997
8027  1997
8026  1999
8027  1999
8028  1995
8029  1998
8025  1997
8027  1997
8026  1999
8027  1999
8028  1995
8029  1998"),head=TRUE,stringsAsFactors=FALSE))

becomes:

A            B      C
8025  1995  0  
8026  1995  0
8029  1995  0
8026  1996  1
8025  1997  1
8026  1997  2
8025  1997  1
8027  1997  0
8026  1999  2
8027  1999  2
8028  1995  0
8029  1998  1
8025  1997  1
8027  1997  0
8026  1999  2
8027  1999  2 
8028  1995  0
8029  2000  1

So 8026 in 1997 = 2 because 8026 can be found in 1995 and 1996 which are
both within the appropriate window (1996 - 1994).

Any ideas? I looked at the rollapply vignette, but couldn't figure out how
to apply it to my data.

Thanks a lot!




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