How to use rbind to merge data tables with POSIXct and Date column classes?

Question

I would like to merge two data tables, but one of them has column ddate with POSIXct class and another one has column ddate with Date class.

I would expect result data table to has POSIXct class, but the actual result depends on sequence of arguments passed to rbind:

zz1 <- "or,d,ddate,rdate,changes,class,price,fdate,company,number,minutes,added,source
VA1,VA2,2014-05-24,,0,0,2124,2014-05-22 15:50:16,,,,2014-05-22 12:20:03,ss
VA1,VA2,2014-05-26,,0,0,2124,2014-05-22 15:03:44,,,,2014-05-22 12:20:03,s1
VA1,VA2,2014-06-05,,0,0,2124,2014-05-22 15:48:24,,,,2014-05-22 12:20:03,s1
VA1,VA2,2014-06-09,,0,0,2124,2014-05-22 15:37:35,,,,2014-05-22 12:20:03,s2
VA1,VA2,2014-06-16,,0,0,2124,2014-05-22 14:17:33,,,,2014-05-22 12:20:03,ss"

zz2 <- "or,d,ddate,rdate,changes,class,price,fdate,company,number,minutes,added,source
VA3,VA4,2014-05-24 12:23:00,,0,0,2124,2014-05-22 15:50:16,,,,2014-05-22 12:20:03,ss
VA1,VA2,2014-05-26 14:00:01,,0,0,2124,2014-05-22 15:03:44,,,,2014-05-22 12:20:03,s1
VA5,VA6,2014-06-05 18:00:04,,0,0,2124,2014-05-22 15:48:24,,,,2014-05-22 12:20:03,s1
VA7,VA8,2014-06-09 18:00:07,,0,0,2124,2014-05-22 15:37:35,,,,2014-05-22 12:20:03,s2
VA9,VA0,2014-06-16 19:00:20,,0,0,2124,2014-05-22 14:17:33,,,,2014-05-22 12:20:03,ss"

columnClasses <- c("factor", "factor", "Date", "factor", "integer", "factor", "integer", "factor", "factor", "factor", "integer", "factor", "factor")
dt1 <- read.table(text=zz1, header = TRUE, sep = ",", comment.char = "", quote = "", na.strings = c(""), colClasses = columnClasses)

columnClasses <- c("factor", "factor", "POSIXct", "factor", "integer", "factor", "integer", "factor", "factor", "factor", "integer", "factor", "factor")
dt2 <- read.table(text=zz2, header = TRUE, sep = ",", comment.char = "", quote = "", na.strings = c(""), colClasses = columnClasses)

dt <- rbind(dt1, dt2)
class(dt$ddate) # [1] "Date"

dt <- rbind(dt2, dt1)
class(dt$ddate) # [1] "POSIXct" "POSIXt"

What should I do to always have POSIXct? Since I can not control the sequence when I get the data from CSV files like below:

tables <- lapply(files, readCSVFile)
dt <- rbindlist(tables, fill = TRUE)

Answer 1

I think you could treat each file when you load with read.csv so that the ddate variable is converted to POSIXct. Then the merging can be better controlled. This can be accomplished by creating your own function to be used by lapply.

Example

ddate.convert = function(x){tmp <- readcsv(x); tmp$ddate <- as.POSIXct(tmp$ddate); tmp}
tables <- lapply(files, FUN=ddate.convert)
dt <- rbindlist(tables, fill = TRUE)