Re: [Rd] colnames is slow for data.frames with implicit row.names

From: Martin Morgan <mtmorgan_at_fhcrc.org>
Date: Wed 27 Sep 2006 - 13:45:32 GMT

Yes, obviously, with hindsight. In lieu of code change, the colnames documentation could indicate the restricted sense of 'equivalent'.

  For a data frame, 'rownames' and 'colnames' are equivalent to   'row.names' and 'names' respectively.      

It might help to add 'names' to See Also of ?data.frame.

Martin

Prof Brian Ripley <ripley@stats.ox.ac.uk> writes:

> On Wed, 27 Sep 2006, Martin Morgan wrote:
>
>> colnames on a data.frame with implicit row.names
>>
>>> df <- data.frame(x=1:6000000)
>>
>> is slow
>>
>>> system.time(colnames(df))
>> [1] 21.655 0.327 21.987 0.000 0.000
>>> system.time(names(df))
>> [1] 0 0 0 0 0
>>
>> because colnames calls dimnames calls row.names.data.frame calls
>> as.character on the implicit row.names.
>
> So use names() and not colnames():
>
> rownames and colnames for matrices
> row.names and names for data frames.
>
> All colnames assumes is that there is a dimnames method: this could be
> relevant for objects inheriting from "data.frame", but there is a
> price for generality.
>
> --
> Brian D. Ripley, ripley@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 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595

-- 
Martin T. Morgan
Bioconductor / Computational Biology
http://bioconductor.org

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