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

[Prof Brian Ripley]

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)
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