POSIXct coerced into numeric when filling a data frame

HI,
Try this:
dat1<-do.call(data.frame,x)
dat1<-data.frame(ID=letters[1:4],dat1)
dat1
#  ID               first              second
#1  a 2011-08-27 10:45:00 2011-08-27 11:00:00
#2  b 2011-10-30 15:45:00 2011-10-30 15:30:00
#3  c 2011-10-30 16:00:00 2011-10-30 15:45:00
#4  d 2012-06-22 09:30:00 2012-06-22 10:00:00
  str(dat1)
#'data.frame':    4 obs. of  3 variables:
# $ ID    : Factor w/ 4 levels "a","b","c","d": 1 2 3 4
  #$ first : POSIXct, format: "2011-08-27 10:45:00" "2011-10-30 15:45:00" ...
  #$ second: POSIXct, format: "2011-08-27 11:00:00" "2011-10-30 15:30:00" ...
dat2<-dat1
  dat2$first<-as.Date(dat2$first,format="%Y-%m-%d %HH:MM:%SS")
  dat2$second<-as.Date(dat2$second,format="%Y-%m-%d %HH:MM:%SS")
  dat2
#  ID      first     second
#1  a 2011-08-27 2011-08-27
#2  b 2011-10-30 2011-10-30
#3  c 2011-10-30 2011-10-30
#4  d 2012-06-22 2012-06-22
  str(dat2)
#'data.frame':    4 obs. of  3 variables:
# $ ID    : Factor w/ 4 levels "a","b","c","d": 1 2 3 4
# $ first : Date, format: "2011-08-27" "2011-10-30" ...
# $ second: Date, format: "2011-08-27" "2011-10-30" ...
dat2<-within(dat2,ID<-as.character(ID))
subset(dat2,ID%in% c("a","b"))
#  ID      first     second
#1  a 2011-08-27 2011-08-27
#2  b 2011-10-30 2011-10-30


A.K.




----- Original Message -----
From: Arnaud Duranel <arnaud.duranel.09 at ucl.ac.uk>
To: r-help at r-project.org
Cc:
Sent: Monday, September 24, 2012 4:06 AM
Subject: [R] POSIXct coerced into numeric when filling a data frame

Hello

I have a list of POSIXct objects, from which I want to extract those
values that match a specific date, keeping them in POSIXct format. For a
specific date there is 0-2 matching values.
As an example (the actual list and objects are much longer):

x<-list()
x[["first"]]<-as.POSIXct(c("2011-08-27 10:45:00 GMT", "2011-10-30
15:45:00 GMT", "2011-10-30 16:00:00 GMT", "2012-06-22 09:30:00 GMT"))
x[["second"]]<-as.POSIXct(c("2011-08-27 11:00:00 GMT", "2011-10-30
15:30:00 GMT", "2011-10-30 15:45:00 GMT", "2012-06-22 10:00:00 GMT"))

If I use the following expression for one specific object of the list, I
get the result I expect:

x[[1]][as.Date(x[[1]])=="2011/10/30"][1]

[1] "2011-10-30 15:45:00 GMT"


Now if I write a for loop based on that expression to store the values
of interest in a data frame, the POSIXct values are coerced into numeric:

y<-data.frame()
for (i in 1:length(x)) {
    y[i,"ID"]<-names(x[i])
    y[i,"first"]<-x[[i]][as.Date(x[[i]])=="2011/10/30"][1]
    y[i,"second"]<-x[[i]][as.Date(x[[i]])=="2011/10/30"][2]
}

    ID      first     second
1  a 1319989500 1319990400
2  b 1319988600 1319989500


I am a bit confused about why that is.
I could coerce them back to POSIXct with another line of code, but is
there a better way? And is there a way to avoid using a for loop to
complete this task?
No need to say I am quite new to R, so apologies for any obvious mistake
or oversight.

Many thanks

A. Duranel, UCL Department of Geography


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