[R] z[j,]$a v. z$a[j]

[ripley@stats.ox.ac.uk]

On Tue, 16 Jul 2002, Bartz, Kevin wrote:

> I noticed that R takes much longer to pull an element from a data frame when
> I select the row first (z[j,]$a) than when I select the column first
> (z$a[j]).
>
> > data.frame(a = 1:10, b = letters[1:10], c = LETTERS[1:10]) -> z
> > z
>     a b c
> 1   1 a A
> 2   2 b B
> 3   3 c C
> 4   4 d D
> 5   5 e E
> 6   6 f F
> 7   7 g G
> 8   8 h H
> 9   9 i I
> 10 10 j J
> > system.time(for (i in 1:1000) for (j in 1:10) z$a[j])
> [1] 0.96 0.02 0.99 0.00 0.00
> > system.time(for (i in 1:1000) for (j in 1:10) z[j,]$a)
> [1] 46.20  6.04 52.55  0.00  0.00
>
> Why is this?

Because a row of a data frame is still a data frame, a much more complex
object than a column of a data frame.

-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._