Dates with lapply and sapply

Question

I have imported a CSV file with numerous dates and have use the as.Date function to convert the date. However, when I use the the mapply function to find the earlier of two dates, I either end up with a list with dates or a numeric vector. How can I get a vector with dates?

POP.Start.final <- mapply(min, combinedOM$Cons.Start.Date.y, 
                               combinedOM$OS.Start.Date.y, 
                                MoreArgs = list(na.rm=T),SIMPLIFY=T)

This returns a numeric vector, changing to SIMPLIFY=F returns a list of dates, but I want a vector of dates.

Answer 1

In this specific case, I believe pmin does the trick:

POP.Start.final <- pmin(combinedOM$Cons.Start.Date.y,
  combinedOM$OS.Start.Date.y,
  na.rm=TRUE)

In the general case, SIMPLIFY=TRUE (the default) uses the utility function simplify2array to convert lists to vectors of atomic mode via as.vector. Because dates are internally stored as numeric, SIMPLIFY=TRUE will convert the list of dates to a vector of mode numeric and remove the Date class. You can set SIMPLIFY=FALSE to keep the Date class and then use do.call with c to convert the list to a vector.

POP.Start.final <- do.call(c,mapply(min, combinedOM$Cons.Start.Date.y, 
                               combinedOM$OS.Start.Date.y, 
                                MoreArgs = list(na.rm=TRUE),SIMPLIFY=FALSE))

Some reproducible code:

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"))
pmin(a,b)
#[1] "2012-01-11" "2012-03-22" "2012-04-18"
do.call(c,mapply(min,a,b,MoreArgs=list(na.rm=TRUE),SIMPLIFY=FALSE))
#[1] "2012-01-11" "2012-03-22" "2012-04-18"

As an aside, using T and F for TRUE and FALSE is slightly worrying because T and F can be reassigned while TRUE and FALSE cannot be reassigned.