The key is that "Reduce" function -- it takes in a list of elements
and combines them iteratively, e.g., `+` is only defined for two
elements (at a time) but we can do something like
Reduce(`+`, list(1,2,3)) = Reduce(`+`, list(1+2,3)) = Reduce(`+`,
list(3,3)) = 3 + 3 = 6
Michael
On Tue, Mar 20, 2012 at 8:51 AM, Paul Miller <pjmiller...@yahoo.com> wrote:
> Hi Brian,
>
> This works very well. Still trying to develop some skill with R. So can't say
> I understand your function completely as yet, but will work on it. I had
> thought that your function might only work for two columns (because of the
> "function(x,y)" part), but the example below suggests it will work for any
> number of columns.
>
> Appreciate your showing this to me.
>
> Thanks,
>
> Paul
>
>
> Demog <- data.frame(PFS = as.Date(c("2006-07-22", NA, "2007-12-16",
> "2008-01-19", "2009-05-05", "2006-04-29", "2006-06-18", NA)),
> DOD = as.Date(c("2006-07-23", "2008-07-09",
> "2007-12-16", "2008-01-19", "2009-05-05", "2006-04-29", "2006-06-18", NA)),
> LKDA = as.Date(c(NA, NA, NA, NA, NA, NA, NA,
> "2008-03-25")))
>
> coalesce <- function(...) {
> dots <- list(...)
> ret <- Reduce(function (x,y) ifelse(!is.na(x),x,y), dots)
> class(ret) <- class(dots[[1]])
> ret
> }
>
> Demog$Test <- with(Demog, coalesce(PFS, DOD, LKDA))
>
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