The problem that makes write.table() not symmetric

R_write.table_with_dates

#Create a data frame that includes a POSIXlt or POSIXct, write.table() to a file, and then read.table() of that file

> a <- data.frame(col1 = 1:10, col2 = as.POSIXlt(Sys.time()), stringsAsFactors=F)
> write.table(a, "test")
> read.table("test")
Error in scan(file, what, nmax, sep, dec, quote, skip, nlines, na.strings,  : 
  line 1 did not have 4 elements

#Now, do the same with that data as a character instead:

> a <- data.frame(col1 = 1:10, col2 = as.character(Sys.time()), stringsAsFactors=F)
> write.table(a, "test2")
> read.table("test2")
   col1                col2
1     1 2011-03-22 14:37:11
2     2 2011-03-22 14:37:11
3     3 2011-03-22 14:37:11
4     4 2011-03-22 14:37:11
5     5 2011-03-22 14:37:11
6     6 2011-03-22 14:37:11
7     7 2011-03-22 14:37:11
8     8 2011-03-22 14:37:11
9     9 2011-03-22 14:37:11
10   10 2011-03-22 14:37:11


#Patch write.table() to quote POSIXlt and POSIXct when quoting, and this gets fixed

        if (qset) 
            quote <- if (length(x)) 
                which(unlist(lapply(x, function(x) is.character(x) || 
-                  is.factor(x))))
+                  is.factor(x) || is(x, "POSIXlt") || is(x, "POSIXct") )))
            else numeric(0L)



> a <- data.frame(col1 = 1:10, col2 = as.POSIXlt(Sys.time()), stringsAsFactors=F)
> write.table(a, "test3")
> read.table("test3")
   col1                col2
1     1 2011-03-22 14:53:09
2     2 2011-03-22 14:53:09
3     3 2011-03-22 14:53:09
4     4 2011-03-22 14:53:09
5     5 2011-03-22 14:53:09
6     6 2011-03-22 14:53:09
7     7 2011-03-22 14:53:09
8     8 2011-03-22 14:53:09
9     9 2011-03-22 14:53:09
10   10 2011-03-22 14:53:09