Apply which.min on POSIXct

I am trying to find the row-wise minimum from a data frame of POSIXct. But using apply with which.min does not work:

df <- data.frame(date1 = as.POSIXct(c("2016-01-01", "2016-02-01")),
                 date2 = as.POSIXct(c("2015-10-01", "2016-06-01")))

apply(df, 1, which.min) # not OK
# integer(0)
# Warning messages:
# 1: In FUN(newX[, i], ...) : NAs introduced by coercion
# 2: In FUN(newX[, i], ...) : NAs introduced by coercion

apply(df, 1, function(x) which(x == min(x))) # OK
# [1] 2 1

sapply(1:nrow(df), function(i) which.min(df[i,])) # OK
# date2 date1 
# 2     1

Can anybody explain this behaviour?

Solution

Look at this:

str(apply(df, 1, identity))
# chr [1:2, 1:2] "2016-01-01" "2015-10-01" "2016-02-01" "2016-06-01"
# - attr(*, "dimnames")=List of 2
#  ..$ : chr [1:2] "date1" "date2"
#  ..$ : NULL

Applying identity turns the POSIXct values into characters! This happens because apply coerces its input into a matrix and as.matrix.data.frame uses format on POSIXct values because a matrix can only hold atomic values and S3 objects like POSIXct are not atomic. Thus the resulting matrix is a character matrix. And you can't calculate the minimum of characters. sapply iterates over its input and doesn't turn the data.frame into a matrix.