It apparently works now. Something weird happens, though: the new column sums 1 extra hour and assigns the day according to its own time. To solve it I subtracted 1 hour to this new column:
 niga$night<- as.POSIXct ( ifelse(niga$h>=0 & niga$h<9, niga$a, niga$timepos), origin="1970-01-01") # it works but adds 1 hour (!!!)
niga$isnight=niga$night - as.difftime ( 1, units= "hours" ) # subtraction of 1 hour
Thank you very much for your time!
 
Santi

From: R. Michael Weylandt <michael.weylandt@gmail.com>
>To: Santiago Guallar <sguallar@yahoo.com> 
>Cc: "r-help@r-project.org" <r-help@r-project.org> 
>Sent: Sunday, May 6, 2012 3:13 PM
>Subject: Re: [R] creating a new column assigning values of other columns
>
>It looks like part of your problem is that some of your time/date
>variables are stored as factors rather than actual times / dates. Use
>str() to see which ones and try to convert those. You'll need this
>format string: format = "%d/%m/%y %H:%M:%S" for the ones that are
>currently factors.
>
>As regards getting POSIXct out of ifelse(), yes -- that seems to be an
>infelicity, but I'm  sure its easily remedied. You just need:
>
>as.POSIXct( ifelse( your_code_here) , origin = "1970-01-01")
>
>Don't add a format string since that refers to the format of the input
>(if you're giving a character input), not the output (which is
>standardized in the definition of POSIXct)
>
>Michael
>
>On Sun, May 6, 2012 at 5:13 AM, Santiago Guallar <sguallar@yahoo.com> wrote:
>> Hi Michael,
>> Yes, I tried ifelse() before but this function returns a numeric value. When
>> I try to convert it back to POSIXct I get a <NA>*. If I use if else I get a
>> POSIXct output although it does not return a correct answer (it only returns
>> y$timepos even when the condition "h<9" fails to be met).
>> Attached the outputs you suggested (thanks, by the way, I didn't know
>> dput()). Hope they go through this time.
>>
>> Thank you,
>>
>> Santi
>>
>> *niga$isnight<- as.POSIXct( niga$nit, tz="GMT", format="%Y-%m-%d %H:%M:%S",
>> origin="2007-06-19" )
>>
>>
>>
>> From: R. Michael Weylandt <michael.weylandt@gmail.com>
>> To: Santiago Guallar <sguallar@yahoo.com>
>> Cc: "r-help@r-project.org" <r-help@r-project.org>
>> Sent: Sunday, May 6, 2012 2:18 AM
>> Subject: Re: [R] creating a new column assigning values of other columns
>>
>> Bahhhh -- far too much work to recreate (and I don't think you sent us
>> the file "act.lig"): here's a much better route:
>>
>> Go to the step immediately before you're in trouble and use dput() on
>> your data. R will print out a nice plaintext representation that we
>> can copy and paste and reproduce *exactly* without having to do all
>> that you show below.
>>
>> Incidentally, your warning message suggests you should be using
>> ifelse() instead of if.
>>
>> To compare:
>>
>> x <- seq(-3, 3)
>> abs.x.wrong <- if(x < 0) -x else x # Warning message gives some hint
>> abs.x.right <- ifelse(x < 0, -x, x)
>>
>> Hope this helps,
>>
>> Michael
>>
>> On Sat, May 5, 2012 at 5:09 PM, Santiago Guallar <sguallar@yahoo.com> wrote:
>>> Hello,
>>>
>>> I have to create a new column from the values of other columns of a data
>>> frame. The new column (y$n) is created imposing a condition (using a third
>>> variable y$h) that assigns the values of two time variables (y$b and
>>> y$timepos). Here's the piece of code to get there (using the attached
>>> files):
>>>
>>> xact <- read.table("act.lig", sep = ',',
>>> col.names=c("ok","time","secs","act"))
>>> xlig <- read.table("lig.txt", sep = ',',
>>> col.names=c("ok","time","secs","lig"))
>>> w<- merge(xact, xlig, by = c("time" ,"secs"), all = TRUE, sort=F)
>>> require(reshape)
>>> z <- cbind(w, colsplit(w$time, split=" ", names=c("date", "clock")))
>>> zh<-cbind(z, colsplit(z$clock, split=":", names=c("h","m","s")))
>>> zhd<- cbind(zh, colsplit(zh$date, split="/", names=c("d","mo","y")))
>>> night <- subset(zhd, zh$lig<6 & zhd$h<9 | zh$lig<6 & zhd$h>21)
>>> night$timepos<-as.POSIXct(night$time, tz="GMT", format="%d/%m/%y
>>> %H:%M:%S")
>>> a=night$timepos - as.difftime( 1, units="days" )
>>> nighta<-cbind(night,a)
>>> y<- cbind(nighta, b=as.character(a, tz= "GMT", format= "%Y-%m-%d"))
>>> y$n<-with(y, if (h>=0 & h<9) {b} else {timepos}) ## Missing warnings
>>>
>>>                                                                             In
>>> if (h >= 0 & h < 9) { :
>>>
>>>                                                                             condition
>>> has length > 1 and only the first element will be used
>>>
>>> How can I go around this problem and get the new column?
>>>
>>> Thank you,
>>>
>>> Santi
>>> ______________________________________________
>>> R-help@r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>>
>>
>>
>
>
> 
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