R help - Nov 1999 - dimnames

Hi,

Maybe this is a simple question:

Using S-Plus, and having a matrix m with row- and col-names, I have the
following problem:

When I import into a function the one-column matrix m[,i], I am no
longer able to get the
column name. Is there a way to get the column-name?

(Although attributes(m) displays row- and col-names, attributes(m[,i])
does no longer show
the col-name.)

Thank  you for your help
Christian Stratowa

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On Sun, 14 Nov 1999 cstrato at EUnet.at wrote:
> Maybe this is a simple question:
> 
> Using S-Plus, and having a matrix m with row- and col-names, I have the
> following problem:
> 
> When I import into a function the one-column matrix m[,i], I am no
> longer able to get the
> column name. Is there a way to get the column-name?
> 
> (Although attributes(m) displays row- and col-names, attributes(m[,i])
> does no longer show
> the col-name.)
If i is a single number, m[, i] is not a matrix but a vector. You need
m[,i, drop=F] to preserve it as a one-column matrix.

You can use dimnames() to see the column names, or colnames() in R.

I am not sure why you sent an S-PLUS question to R-help: would not S-news
be more appropriate?  (Because of the presence of colnames() in R, the
answer might be subtly different.)

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Christian Stratowa asks:
> Hi,
> 
> Maybe this is a simple question:
> 
> Using S-Plus, and having a matrix m with row- and col-names, I
> have the following problem:
> 
> When I import into a function the one-column matrix m[,i], I am
> no longer able to get the column name.
Yes.  This is a phenomenon of R and S that we might call a
"helpful inconsistency".  If you select a single row or column of
a matrix in this way the object you get is not a matrix at all
but a vector (which does not have rows or columns).  Often this
is what you want so the convention is helpful, but it is an
inconsistency that sometimes causes problems as it has with you.
(It has nothing to do with passing on the object to functions, by
the way: it always happens.)
> Is there a way to get the column-name?
You can select the column in a way that does give you a single
column matrix (complete with column name) by using m[,i,drop=F]
instead of m[,i].  

Odd as it may look, this is often what you need to do in
programming when you are relying on some selected set of columns
to be a matrix and not something simpler.
> (Although attributes(m) displays row- and col-names,
> attributes(m[,i]) does no longer show the col-name.)
That's because they have been "dropped".  If you turn off
dropping you will preserve the attribute.

I can't help feeling that this convention, helpful as it might be
in interactive work, is a bit of a mistake.  It second-guesses
what you mean rather than what you actually program.  In that
respect it is for me an uncomfortable reminder of the sort of
thing that goes on in much MicroSoft software...

Bill Venables.

-- 
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