[R] keeping dim() for array

[Gavin Simpson]

On Tue, 2006-11-21 at 12:20 +0000, Federico Calboli wrote:
> Hi All,
> 
> I noticed the following:
> 
> pip = array(1:6, dim = c(3,2))
> dim(pip)
> [1] 3 2
> 
> pup = pip[1,]
> dim(pup)
> NULL
> 
> I bet there is a *good* reason why one row of an array is *dimensionless*, but 
> it's highly inconvenient for my purpose, i.e. to use apply() after an array goes 
> through a number of logical steps and is redimensioned, sometimes to one single row.

When you subset a dimensioned object in R, the default is for the empty
dimension to be dropped - in your case converting your 1, row matrix
into a vector of the same length.

I do not know /why/ this is so, but I find it quite useful when writing
code - although I've been bitten by this enough times now to remember to
add drop = FALSE.

> 
> How do I keep dim(pup) to 1 2?

pup <- pip[1, , drop = FALSE]

will stop the empty dimension being dropped:

> pip = array(1:6, dim = c(3,2))
> pup <- pip[1, , drop = FALSE]
> pup
     [,1] [,2]
[1,]    1    4
> dim(pup)
[1] 1 2

> 
> Since pup is at the end of a number of if(), it does not necessarily end up as 
> one row only, so t(as.matrix(pup)) is the *wrong* answer.
> 
> Best,
> 
> Federico

HTH

G
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