[R] melt in reshape2 destroying dates?

[arun]

In the first case:
 mdat$value<-.POSIXct(mdat$value,tz="EST")
 mdat$value[1:3]
#[1] "2012-01-01 01:00:00 EST" "2012-01-01 02:00:00 EST"
#[3] "2012-01-01 03:00:00 EST"
 mydata$dd1[1:3]
#[1] "2012-01-01 01:00:00 EST" "2012-01-01 02:00:00 EST"
#[3] "2012-01-01 03:00:00 EST"
A.K.



----- Original Message -----
From: John Kane <jrkrideau at inbox.com>
To: r-help at r-project.org
Cc: 
Sent: Monday, May 13, 2013 1:36 PM
Subject: [R] melt in reshape2 destroying dates?

When I melt a data frame with some dates I am getting some strange results. I seem to lose the date format with POSIXct and get a row of zeros with POSIXlt

Any suggestions as to what I am messing up? Code and data below.
Thanks,
John Kane
Kingston ON Canada

mydata  <-  structure(list(dd1 = structure(c(1325397600L, 1325401200L, 1325404800L, 
1325408400L, 1325412000L, 1325415600L, 1325419200L, 1325422800L, 
1325426400L, 1325430000L, 1325433600L, 1325437200L, 1325440800L, 
1325444400L, 1325448000L, 1325451600L, 1325455200L, 1325458800L, 
1325462400L, 1325466000L), class = c("POSIXct", "POSIXt"), tzone = ""), 
    dd2 = structure(c(1325426400, 1325412000, 1325426400, 1325422800, 
    1325422800, 1325426400, 1325433600, 1325437200, 1325455200, 
    1325458800, 1325451600, 1325458800, 1325466000, 1325466000, 
    1325476800, 1325476800, 1325480400, 1325466000, 1325480400, 
    1325491200), class = c("POSIXct", "POSIXt"), tzone = ""), 
    emp = structure(c(1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 
    1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L), class = "factor", .Label = c("part-time", 
    "self-employed")), period = 1:20), .Names = c("dd1", "dd2", 
"emp", "period"), row.names = c(NA, -20L), class = "data.frame")

library(lubridate)
library(reshape2)

  str(mydata)
  mdat  <-  melt(mydata, id=c("emp","period"))
  str(mdat)
  
  # alternatively
  
  mydata$dd1  <-  as.POSIXlt(mydata$dd1)
  mydata$dd2  <-  as.POSIXlt(mydata$dd2)
  str(mydata)
  mdat  <-  melt(mydata, id=c("emp","period"))
  str(mdat)

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