x <- 1:2; dim(x) <- 2? A vector or not?

What you have is a one-dimensional array: they crop up in R most often 
from table() in my experience.

f <- table(rpois(100, 4))
str(f)

'table' int [, 1:10] 2 6 18 21 13 16 13 4 3 4
  - attr(*, "dimnames")=List of 1
   ..$ : chr [1:10] "0" "1" "2" "3" ...

and yes, f is an atmoic vector and yes, str()'s notation is confusing 
here but if it did [1:10] you would not know it was an array.  I 
recall discussing this with Martin Maechler (str's author) last 
century, and I've just checked that R 2.0.0 did the same.

The place in which one-dimensional arrays differ from normal vectors 
is how names are handled: notice that my example has dimnames not 
names, and ?names says

      For a one-dimensional array the 'names' attribute really is
      'dimnames[[1]]'.

I think these days we have enough internal glue in place that an end 
user would not notice the difference (but those working at C level 
with R objects may need to know).

Ran into the follow intermediate case in an external package (w/
recent R v2.8.1 patched and R v2.9.0 devel):

x <- 1:2
dim(x) <- 2
dim(x)

[1] 2

x

[1] 1 2

str(x)

int [, 1:2] 1 2

nrow(x)

[1] 2

ncol(x)

[1] NA

is.vector(x)

[1] FALSE

is.matrix(x)

[1] FALSE

is.array(x)

[1] TRUE

x[1]

[1] 1

x[,1]

Error in x[, 1] : incorrect number of dimensions

x[1,]

Error in x[1, ] : incorrect number of dimensions

Is str() treating single-dimension arrays incorrectly?

What does it mean to have a single dimension this way?  Should it
equal a vector?  I am aware of "is.vector returns FALSE if x has any
attributes except names".

/Henrik

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
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