Sort year-month column by year AND month

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The as.yearmon() function (and the "yearmon" class) in package zoo is designed for this sort of data:

dat <- c("2009-Sep","2009-Feb","2009-Jan")
require(zoo)
d2 <- as.yearmon(dat, "%Y-%b")
> sort(d2)
[1] "Jan 2009" "Feb 2009" "Sep 2009"
> order(d2)
[1] 3 2 1
> d2[order(d2)]
[1] "Jan 2009" "Feb 2009" "Sep 2009"

You could of course paste0() a day onto each date and coerce to class "Date" via as.Date() but as.yearmon() seems more natural to me:

> as.Date(paste0(dat, "-01"), "%Y-%b-%d")
[1] "2009-09-01" "2009-02-01" "2009-01-01"

Note you can generate that same result by coercing the "yearmon" object to class "as.Date", e.g.:

> as.Date(d2)
[1] "2009-09-01" "2009-02-01" "2009-01-01"

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