r - Subset data.frame by date

Question

I have a dataset called EPL2011_12. I would like to make new a dataset by subsetting the original by date. The dates are in the column named Date The dates are in DD-MM-YY format.

I have tried

EPL2011_12FirstHalf <- subset(EPL2011_12, Date > 13-01-12)

and

EPL2011_12FirstHalf <- subset(EPL2011_12, Date > "13-01-12")

but get this error message each time.

Warning message:
In Ops.factor(Date, 13- 1 - 12) : > not meaningful for factors

I guess that means R is treating like text instead of a number and that why it won't work?

Answer 1

Well, it's clearly not a number since it has dashes in it. The error message and the two comments tell you that it is a factor but the commentators are apparently waiting and letting the message sink in. Dirk is suggesting that you do this:

 EPL2011_12$Date2 <- as.Date( as.character(EPL2011_12$Date), "%d-%m-%y")

After that you can do this:

 EPL2011_12FirstHalf <- subset(EPL2011_12, Date2 > as.Date("2012-01-13") )

R date functions assume the format is either "YYYY-MM-DD" or "YYYY/MM/DD". You do need to compare like classes: date to date, or character to character. And if you were comparing character-to-character, then it's only going to be successful if the dates are in the YYYYMMDD format (with identical delimiters if any delimiters are used).