Question

Why does iterating through a Date or POSIXct object result in numeric? For example:

test = as.Date("2009-01-01")
print( class( test ) )
# [1] "Date"
for ( day in test )
{
    print( class( day ) )
}
# [1] "numeric"

The same thing happens with POSIXct:

test = as.POSIXct("2009-01-01")
print( class( test ) )
# [1] "POSIXct" "POSIXt"
for ( day in test )
{
    print( class( day ) )
}
# [1] "numeric"

Answer 1

?"for" says that seq (the part after in) is “[A]n expression evaluating to a vector (including a list and an expression) or to a pairlist or ‘NULL'”.

So your Date vector is being coerced to numeric because Date objects aren’t strictly vectors:

is.vector(Sys.Date())
# [1] FALSE
is.vector(as.numeric(Sys.Date()))
# [1] TRUE

The same is true for POSIXct vectors:

is.vector(Sys.time())
# [1] FALSE
is.vector(as.numeric(Sys.time()))
# [1] TRUE