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Bartz, Kevin

2002-Jul-16 17:31 UTC

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

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?
> R.version         _                   
platform sparc-sun-solaris2.6
arch     sparc               
os       solaris2.6          
system   sparc, solaris2.6   
status                       
major    1                   
minor    5.1                 
year     2002                
month    06                  
day      17                  
language R                 

Thanks,

Kevin
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ripley@stats.ox.ac.uk

2002-Jul-16 17:46 UTC

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

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

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R help - Jul 2002 - z[j,]$a v. z$a[j]

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

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