[R] calculate time from dates

[Rui Barradas]

Hello,

It's better if you keep it on the list, the odds of getting more and 
better answers is greater.

As for your question, I've made up a dat2 with an extra column. Try the 
following, it's independent of the number of columns.



dat2 <- data.frame(ID = dat1$ID,
	month = mondf("01/01/2008", dat1$date, format = "%m/%d/%Y") + 1,
	other = rnorm(6))

dat2

sp <- split(dat2, list(dat2$ID, dat2$month))
result <- do.call(rbind, lapply(sp, tail, 1))
rownames(result) <- seq_len(nrow(result))
result


Hope this helps,

Rui Barradas


Em 12-07-2013 09:04, Gallon Li escreveu:
> Dear Rui, I think your solution is the best. however, to keep only the
> last one if month diff are equal part: because my data also have other
> variables besides ID and month, your code doesn't work. where should i
> modify it? in another word, my "dat2" contains more than 2 columns.i
> tried to modify [1:2] to [1:p] where p is the total number of columns in
> dat2 but it still doesn't work.
>
> Best, Gallon
>
>
> On Thu, Jul 11, 2013 at 7:00 PM, Rui Barradas <ruipbarradas at sapo.pt
> <mailto:ruipbarradas at sapo.pt>> wrote:
>
>     Hello,
>
>     The functions in stackoverflow need a date 'format' argument.
>
>
>
>     # Functions from
>     #
>     http://stackoverflow.com/__questions/1995933/number-of-__months-between-two-dates
>     <http://stackoverflow.com/questions/1995933/number-of-months-between-two-dates>
>     # with a 'format' argument added
>     #
>     # turn a date into a 'monthnumber' relative to an origin
>     monnb <- function(d, format = "%Y-%m-%d") {
>              lt <- as.POSIXlt(as.Date(d, origin="1900-01-01", format =
>     format))
>              lt$year*12 + lt$mon
>     }
>     # compute a month difference as a difference between two monnb's
>     mondf <- function(d1, d2, format = "%Y-%m-%d") {
>              monnb(d2, format = format) - monnb(d1, format = format)
>     }
>
>
>     dat1 <- read.table(text = "
>
>     ID date
>     1 4/12/2008
>     1 4/13/2008
>     1 5/11/2008
>     2 3/21/2009
>     2 4/22/2009
>     2 8/05/2009
>     ", header = TRUE)
>
>     dat2 <- data.frame(ID = dat1$ID, month = mondf("01/01/2008",
>     dat1$date, format = "%m/%d/%Y") + 1)
>
>     # Now keep just the last one if month diffs are equal
>     result <- with(dat2, aggregate(month, list(ID, month), FUN = tail,
>     1))[1:2]
>     names(result) <- names(dat2)
>     result
>
>
>     Hope this helps,
>
>     Rui Barradas
>
>     Em 11-07-2013 11:03, andrija djurovic escreveu:
>
>         Hi.
>
>         See
>         http://stackoverflow.com/__questions/1995933/number-of-__months-between-two-dates
>         <http://stackoverflow.com/questions/1995933/number-of-months-between-two-dates>
>
>         Andrija
>
>
>         On Thu, Jul 11, 2013 at 11:56 AM, Gallon Li <gallon.li at gmail.com
>         <mailto:gallon.li at gmail.com>> wrote:
>
>             My data are from 2008 to 2010, with repeated measures for
>             same subjects. I
>             wish to compute number of months since january 2008.
>
>             The data are like the following:
>
>             ID date
>             1 4/12/2008
>             1 4/13/2008
>             1 5/11/2008
>             2 3/21/2009
>             2 4/22/2009
>             2 8/05/2009
>             ...
>
>             the date column are in the format "%m/%d/%y". i wish to obtain
>
>             ID month
>             1 4
>             1 4
>             1 5
>             2 15
>             2 16
>             2 20
>             ...
>
>             also, for the same ID with two identical month, I only want
>             to keep the
>             last one. can any expert help with this question?
>
>                       [[alternative HTML version deleted]]
>
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>
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>
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>         <http://www.R-project.org/posting-guide.html>
>         and provide commented, minimal, self-contained, reproducible code.
>
>