[R] Problem in selecting rows in R

[Kristi Glover]

Hi R Users,
Some individuals recorded multiple within a time period. But, I want to select the row of last site within each time period for each individual. I spent a substantial time, but no luck in selecting the rows. Would you give me a hint for this one? I have a very large data set, but this is just an example.
Thanks for your help.

I want to get dat2 from dat1.

dat1<-structure(list(sn = 1:16, Species = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 4L, 5L, 5L, 5L, 5L, 5L), .Label = c("Sp1",
"Sp2", "SP3", "Sp4", "Sp5"), class = "factor"), Observed.site = structure(c(1L, 3L, 1L, 2L, 1L, 2L, 4L, 3L, 1L, 2L, 2L, 3L, 5L, 4L, 1L, 3L), .Label = c("SiteA",
"SiteB", "SiteC", "SiteD", "SIteD"), class = "factor"), Time = structure(c(1L, 5L, 6L, 6L, 6L, 2L, 3L, 11L, 8L, 10L, 1L, 1L, 4L, 4L, 7L, 9L), .Label = c("1/15/15",
"1/17/15", "1/29/15", "2/17/15", "2/25/15", "2/27/15", "2/28/15", "3/27/15", "3/5/15", "4/19/15", "7/3/15"), class = "factor"),
    Month = structure(c(3L, 2L, 2L, 2L, 2L, 3L, 2L, 4L, 4L, 1L, 3L, 3L, 2L, 2L, 2L, 4L), .Label = c("April", "Feb", "Jan",
    "March"), class = "factor")), .Names = c("sn", "Species", "Observed.site", "Time", "Month"), class = "data.frame", row.names = c(NA, -16L))
dat1
#---
dat2<-structure(list(sn = c(1L, 5L, 6L, 9L, 10L, 11L, 12L, 15L, 16L), Species = structure(c(1L, 1L, 2L, 3L, 3L, 4L, 5L, 5L, 5L), .Label = c("Sp1",
"Sp2", "SP3", "Sp4", "Sp5"), class = "factor"), Observed.site = structure(c(1L, 1L, 2L, 1L, 2L, 2L, 3L, 1L, 3L), .Label = c("SiteA", "SiteB",
"SiteC"), class = "factor"), Time = structure(c(1L, 3L, 2L, 5L, 7L, 1L, 1L, 4L, 6L), .Label = c("1/15/15", "1/17/15", "2/27/15",
"2/28/15", "3/27/15", "3/5/15", "4/19/15"), class = "factor"), Month = structure(c(3L, 2L, 3L, 4L, 1L, 3L, 3L, 2L, 4L), .Label = c("April",
    "Feb", "Jan", "March"), class = "factor")), .Names = c("sn", "Species", "Observed.site", "Time", "Month"), class = "data.frame", row.names = c(NA, -9L))
dat2

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