[R] Performance note: Preallocating helps? and two questions

[Emmanuel Paradis]

At 06:00 01/11/00 -0800, you wrote:
>hi-
> in r 1.1 on windows 2000
> with length(AU) of 35833
> AUcap30<-0
> for(i in 1:length(AU))AUcap30[i]<-min(30,AU[i]) 
>took over an hour on pentium II 300 mhertz (I esc'ed before it finished)
>but
>AUcap30<-AU 
> for(i in 1:length(AU))AUcap30[i]<-min(30,AU[i]) 
>is very quick (a few seconds)
>
>Is this performance difference common in r (ie is linux the same way)?

I do not think this is a problem with memory. In the first case, you have
length(AUcap30)==1, thus your loop is non-sense. I would have expected an
error message here like "subscript out of range for AUcap30" (is this a
bug? it seems R is trapped in the loop). In the second case, you have:

> length(AU)==length(AUcap30)
[1] TRUE

and things are fine.

Emmanuel Paradis


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