[R] two questions for R beginners

Duncan Murdoch murdoch at stats.uwo.ca
Mon Mar 1 12:37:30 CET 2010

Karl Ove Hufthammer wrote:
> On Mon, 1 Mar 2010 11:02:59 +0100 Karl Ove Hufthammer <karl at huftis.org> 
> wrote:
>>> * What were your biggest misconceptions or
>>> stumbling blocks to getting up and running
>>> with R?
>> Also I found it quite confusing that
> One more thing that still trips me up sometimes. '$' works on data 
> frames but not on matrices (with dimnames/colnames). Even though the two 
> objects *look* exactly the same, '$' on one of them works while '$' on 
> the other gives a *very* confusing error message. Example:
>   d=head(iris[1:4])
>   d2=as.matrix(d)
>   d
>   d2
>   d$Sepal.Width
>   d2$Sepal.Width
> Some functions output matrices where you would expect them to output 
> data frames, and then this problem occurs. (Is there a reason why '$' 
> could/should not be made to 'work' on matrices too?)
The reason for the difference is that data.frames are lists organized 
into columns (so the $ handling comes from the list, where it means 
"extract the component") whereas a matrix is a single vector displayed 
in columns. 

Of course, the problem is that a beginner only knows that they both look 
the same.  But I think the idea of a list is so fundamental to R that it 
needs to be something learned pretty early, so I'd rather not blur the 
distinction between dataframes and matrices.

Duncan Murdoch

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