From ripley at stats.ox.ac.uk  Wed Sep 27 15:30:51 2006
From: ripley at stats.ox.ac.uk (Prof Brian Ripley)
Date: Wed, 27 Sep 2006 14:30:51 +0100 (BST)
Subject: [Rd] colnames is slow for data.frames with implicit row.names
In-Reply-To: <6phu02tmoyz.fsf@gopher4.fhcrc.org>
References: <6phu02tmoyz.fsf@gopher4.fhcrc.org>
Message-ID: <Pine.LNX.4.64.0609271426220.17192@gannet.stats.ox.ac.uk>

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 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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595




[Rd] colnames is slow for data.frames with implicit row.names
Prof Brian Ripley