[Rd] matplot.Date & matplot.POSIXct

[Edzer Pebesma]

Whether something "is" a matrix depends on whether you listen to
is.matrix(x) or to inherits(x, "matrix"):

x = Sys.Date () + 1:16
y = as.POSIXct (x)
dim(y) = c(4,4)
is.matrix(y)
# [1] TRUE
inherits(y, "matrix")
# [1] FALSE

f = factor(letters[1:9])
dim(f) = c(3,3)
is.matrix(f)
# [1] TRUE
inherits(f, "matrix")
# [1] FALSE

I'm using (abusing?) this in the stars package, where it is convenient
to have n-D arrays of type factor, Date or POSIXct, mostly because the [
method works:

(f2 <- f[1:2,1:2])
#      [,1] [,2]
# [1,] a    d
# [2,] b    e
# Levels: a b c d e f g h i
is.matrix(f2)
# [1] TRUE


On 1/28/20 9:09 AM, Abby Spurdle wrote:
>>> Maybe I'm missing something really obvious here, but I was unable to
>>> create a matrix out of POSIXct object(s).
>>> Perhaps that deserves a separate discussion...?
>>    Can you provide an example?
> 
> ------
> #date and time objects
> x = Sys.Date () + 1:16
> y = as.POSIXct (x)
> 
> #matrices
> str (matrix (x, 4, 4) )
> str (matrix (y, 4, 4) )
> ------
> 
> Creating a matrix from a Date or POSIXct object, results in a numeric
> matrix, not a date/time matrix.
> 
> I think that date/time matrices could be useful.
> It's possible that this has been discussed before.
> But if not, it may be good to discuss it.
> 
> And returning to your original post...
> 
> I re-read the documentation for the matplot function.
> And I feel that it's ambiguous.
> 
> The description says:
> "Plot the columns of one matrix against the columns of another."
> i.e. The matplot function is for *matrices*.
> 
> However, then it says:
> "x,y vectors or matrices of data for plotting. The number of rows should match."
> 
> I'm guessing the current intention is that standard vectors (without a
> dim attribute) would be coerce-ible to single-column matrices,
> implying that using this function with date and time objects, is
> contrary to the way it's currently designed to work.
> 
> But...
> 
> After reading your examples and re-reading the documentation, your
> main suggestion that matplot should support Date and POSIXct objects,
> is still *probably* a good one. I note that function is not generic,
> and modifications to it would not necessarily be trivial.
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics
Heisenbergstrasse 2, 48151 Muenster, Germany
Phone: +49 251 8333081