Re: [R] Indexing column of a matrix with infix $

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From: Roger D. Peng (rpeng@jhsph.edu)
Date: Sun 09 May 2004 - 02:23:18 EST


Message-id: <409D0976.7090603@jhsph.edu>

You can access a column of a dataframe using $ but not a matrix.
    That's just how it is. You can access a column of a matrix
via its column name using, for example

> m <- matrix(1:12, 4, 3)
> colnames(m) <- c("a", "b", "c")
> m[, "b"]
[1] 5 6 7 8

-roger

chris1 wrote:
> I'm using 1.9.0 on Windoze 2k and I created a numeric matrix and used
> colnames() to give it some column names, but if I try to select a
> column using matrixname$validname I get a null return but if I use
> matrixname[,4] it works fine. Looking at the help I think this is
> because a matrix is not a recursive structure and I think it's saying
> I shouldn't be surprised nor attempt this.
>
> I'm in the process of transferring all my stats work to R having used
> a mixture of SPSS, S and SAS (and snippets of other things) over the
> years so sorry if I'm being dumb but this surprised me and I'd love a
> kindly explanation as I thought indexing a matrix in this way was
> something I did regularly (I think the last time was actually using it
> on a dataframe though) and I also think it would be useful and produce
> more readable code if I could.
>
> Can some kindly soul explain?
>
> TIA,
>
> Chris
>
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