Dear all,
While showing some commands of R to my students, I came across the
following behaviour of * which surprised me.
> set.seed(20) # to make it reproducible
# create some objects
> z <- matrix(rnorm(6), ncol=2) # 3x2 matrix
> x1 <- rnorm(3) # vector of length 3
> y1 <- rnorm(4) # vector of length 4
> y2 <- matrix(y1, ncol=1) # 4x1 matrix
> z*x1 # works fine, as expected
> x1*y1 # warning message, as expected
[1] 1.60535635 0.01749800 0.06943188 1.81510895
Warning message:
longer object length
is not a multiple of shorter object length in: x1 * y1
> z*y2 # also expected
Error in z * y2 : non-conformable arrays
> z*y1 # no warning, no error?
[,1] [,2]
[1,] -0.64591924 0.83703776
[2,] 0.01179780 0.24808611
[3,] -0.26850220 -0.01146923
> x1*y2 # no warning, no error?
[,1]
[1,] 1.60535635
[2,] 0.01749800
[3,] 0.06943188
[4,] 1.81510895
I was expecting warning messages in the last examples.
But after consulting the help page (?*) and the 'R Language
definition', I am not sure what I should have expected. :)
The help page says that "[t]hese binary operators perform arithmetic
on vector objects" and "[o]bjects such as arrays [...] can be operated
on this way provided they are conformable". The R Language definition
states "a vector is not the same as a one-dimensional array since the
latter has a dim attribute of length one, whereas the former has no
dim attribute".
So technically, I guess, the last two examples are operations between
a vector object and an array object. And the help page doesn't seem
to say anything about this situation.
I have a vague memory that there as been discussion on this subject in
the (distant?) past, but unfortunately I don't remember what behaviour
was deemed to be sensible. I also couldn't find any recent posts
about this on the bug archives.
Thus, after careful consideration I decided to fill out this bug
report since I came to the conclusion that for these examples either
a) both operands should have been treated as a vector objects and a
warning message should have been issued; or
b) the documentation should make it clear that the warning is not
issued if one operates on a vector object and an array object.
My apologies if this was already reported and I missed it in the bugs
archives.
Cheers,
Berwin
--please do not edit the information below--
Version:
platform = i686-pc-linux-gnu
arch = i686
os = linux-gnu
system = i686, linux-gnu
status =
major = 1
minor = 8.1
year = 2003
month = 11
day = 21
language = R
Search Path:
.GlobalEnv, package:methods, package:ctest, package:mva, package:modreg, package:nls,
package:ts, Autoloads, package:base
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