“%m %” operator with mapply

问题

I am trying to use mapply to add months to the current dates in columns a and b of my dataframe. Here is the code to create a sample data frame:

library(lubridate)
a <- as.Date(c("2012-01-11","2012-06-30","2012-04-18"))
b <- as.Date(c("2013-04-21","2012-03-22","2012-05-01"))
df <- data.frame(a,b)

I can use mapply("+",df, c(30,30)) to add 30 days to both columns of dates. However, when I try to use the command mapply("%m+%",df, months(1:2)) I get the error message:

Error in .setupMethodsTables(fdef, initialize = TRUE) : no slot of name "group" for this object of class "derivedDefaultMethod"

Is it possible to use mapply with the %m+% operator?

回答1:

To my S4 ignorant eyes, this appears to be an issue with the lubridate package and the way the %m+% method is constructed.

Looking at the source,

It appears that the unexported function .quick_month_add will do what you want

 mapply(lubridate:::.quick_month_add,df,months(1:2), SIMPLIFY = FALSE)
$a
[1] "2012-01-11" "2012-06-30" "2012-04-18"

$b
[1] "2013-04-21" "2012-03-22" "2012-05-01"

note that SIMPLIFY must be set to FALSE otherwise you will get numeric matrix as the Date class is stripped when simplifying to a matrix.

Or, Map(lubridate:::.quick_month_add,df,months(1:2))

来源：https://stackoverflow.com/questions/14819362/m-operator-with-mapply