[R-sig-Geo] Incorrect month order in zApply function

[Dominik Schneider]

Sorry I'm not sure how to fix this, but it looks like the months are  in
alphanumeric order, indicating they are being treated as a factor. Might be
worth a bug report?



On Thursday, July 28, 2016, Thiago V. dos Santos via R-sig-Geo <
r-sig-geo at r-project.org> wrote:

> Dear all,
>
> I am using the raster package to calculate monthly averages of climatic
> variables.
>
> Essentially, this is the function I use:
>
> library(raster)
>
> # Create date sequence
> idx <- seq(as.Date("1996/1/1"), as.Date("2010/12/31"), by = "day")
>
> # Create raster stack and assign dates
> r <- raster(ncol=5, nrow=5)
> s <- stack(lapply(1:length(idx), function(x) setValues(r,
> runif(ncell(r)))))
> s <- setZ(s, idx)
>
> # Calculate monthly average
> x <- zApply(s, by=months, mean, name=month.abb[])
>
> names(x)
> [1] "April" "August" "December" "February" "January" "July" "June"
> [8] "March" "May" "November" "October" "September"
> getZ(x)
> [1] "April" "August" "December" "February" "January" "July" "June"
> [8] "March" "May" "November" "October" "September"
>
>
> The problem here is the output of both names(x) and getZ(x). It looks like
> a random month order is returned (even though I provide the labels), which
> makes me confused about the results.
>
>
> By performing the same calculation in a different software and comparing
> the results, I came to realize that the order of months for the results by
> raster should, in fact, be Jan-Feb-Mar-Apr-May-Jun-Jul-Aug-Sep-Oct-Nov-Dec
>
> How can I control the way raster delivers the object names after using
> zApply, in order to sort the months in the "natural" order?
>
> Greetings, -- Thiago V. dos Santos
>
> PhD student
> Land and Atmospheric Science
> University of Minnesota
>
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