[R] subsetting and Dates

[David Winsemius]

On May 23, 2013, at 7:06 PM, Denis Chabot wrote:

> Thank you for the 2 methods to make the columns class Date, but I would really like to know why these variables were not in Date class with my code. Do you know?

I suspect that the problem lies in the dispatch to `[<-.<class>` or `$<-`. When the first argument is 'logical', then the first argument is not of class Date and so not dispatched to `[<-.Date` but rather to .Primitive("[<-"),  there being no `$<-.logical or `[<-.logical` . 

"Arguably", as it were, someone should write S4 methods for `[<-` and `$<-` that would dispatch to the expected method on a signature where the second argument is a Date or POSIXct class. We might also then want methods for the other two "[/$" indexing classes, "numeric" and "character".

-- 

David.
> 
> Denis
> 
> 
> Le 2013-05-23 à 21:44, arun <smartpink111 at yahoo.com> a écrit :
> 
>> You could convert those columns to "Date" class by:
>> 
>> 
>> Data[,c(4,6)]<-lapply(Data[,c(4,6)],as.Date,origin="1970-01-01")
>> #or
>> Data[,c(4,6)]<-lapply(Data[,c(4,6)],function(x) structure(x,class="Date"))
>> 
>> 
>> #  dat1  dat2      Dat1a      Dat1b      Dat2a      Dat2b
>> #1  41327 41327 2013-02-22 2013-02-22 2013-02-22 2013-02-22
>> #2  41334 41334 2013-03-01 2013-03-01 2013-03-01 2013-03-01
>> #3  41341 41341 2013-03-08 2013-03-08 2013-03-08 2013-03-08
>> #4  41348 41348 2013-03-15 2013-03-15 2013-03-15 2013-03-15
>> #5  41355    NA 2013-03-22 2013-03-22       <NA>       <NA>
>> #6  41362 41362 2013-03-29 2013-03-29 2013-03-29 2013-03-29
>> #7  41369 41369 2013-04-05 2013-04-05 2013-04-05 2013-04-05
>> #8  41376 41376 2013-04-12 2013-04-12 2013-04-12 2013-04-12
>> #9  41383    NA 2013-04-19 2013-04-19       <NA>       <NA>
>> #10 41390 41390 2013-04-26 2013-04-26 2013-04-26 2013-04-26
>> #11 41397 41397 2013-05-03 2013-05-03 2013-05-03 2013-05-03
>> A.K.
>> 
>> ----- Original Message -----
>> From: Denis Chabot <chabot.denis at gmail.com>
>> To: R-help at r-project.org
>> Cc: 
>> Sent: Thursday, May 23, 2013 5:35 PM
>> Subject: [R] subsetting and Dates
>> 
>> Hi,
>> 
>> I am trying to understand why creating Date variables does not work if I subset to avoid NAs. 
>> 
>> I had problems creating these Date variables in my code and I thought that the presence of NAs was the cause. So I used a condition to avoid NAs.
>> 
>> It turns out that NAs are not a problem and I do not need to subset, but I'd like to understand why subsetting causes the problem.
>> The strange numbers I start with are what I get when I read an Excel sheet with the function read.xls() from package gdata.  
>> 
>> dat1 = c(41327, 41334, 41341, 41348, 41355, 41362, 41369, 41376, 41383, 41390, 41397)
>> dat2 = dat1
>> dat2[c(5,9)]=NA
>> Data = data.frame(dat1,dat2)
>> 
>> keep1 = !is.na(Data$dat1)
>> keep2 = !is.na(Data$dat2)
>> 
>> 
>> Data$Dat1a = as.Date(Data[,"dat1"], origin="1899-12-30") 
>> Data$Dat1b[keep1] = as.Date(Data[keep1,"dat1"], origin="1899-12-30") 
>> Data$Dat2a = as.Date(Data[,"dat2"], origin="1899-12-30") 
>> Data$Dat2b[keep2] = as.Date(Data[keep2,"dat2"], origin="1899-12-30") 
>> 
>> Data
>>    dat1  dat2      Dat1a Dat1b      Dat2a Dat2b
>> 1  41327 41327 2013-02-22 15758 2013-02-22 15758
>> 2  41334 41334 2013-03-01 15765 2013-03-01 15765
>> 3  41341 41341 2013-03-08 15772 2013-03-08 15772
>> 4  41348 41348 2013-03-15 15779 2013-03-15 15779
>> 5  41355    NA 2013-03-22 15786       <NA>    NA
>> 6  41362 41362 2013-03-29 15793 2013-03-29 15793
>> 7  41369 41369 2013-04-05 15800 2013-04-05 15800
>> 8  41376 41376 2013-04-12 15807 2013-04-12 15807
>> 9  41383    NA 2013-04-19 15814       <NA>    NA
>> 10 41390 41390 2013-04-26 15821 2013-04-26 15821
>> 11 41397 41397 2013-05-03 15828 2013-05-03 15828
>> 
>> So variables Dat1b and Dat2b are not converted to Date class.
>> 
>> 
>> sessionInfo()
>> R version 2.15.2 (2012-10-26)
>> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
>> 
>> locale:
>> [1] fr_CA.UTF-8/fr_CA.UTF-8/fr_CA.UTF-8/C/fr_CA.UTF-8/fr_CA.UTF-8
>> 
>> attached base packages:
>> [1] stats     graphics  grDevices utils     datasets  methods   base    
>> 
>> other attached packages:
>> [1] gdata_2.12.0
>> 
>> loaded via a namespace (and not attached):
>> [1] gtools_2.7.0
>> 
>> Thanks in advance,
>> 
>> Denis


David Winsemius
Alameda, CA, USA