[R] How to optimize this loop ?
Petr Pikal
petr.pikal at precheza.cz
Thu Jan 18 14:51:00 CET 2007
Hi
discard your loop do not optimise it.
rle is your friend
> my_fun<-function(x) {
+
+ len<-length(x)
+ x1<-rle(x[len]>x[1:len-1])
+ last<-length(x1$values)
+ ifelse(x1$values[last],x1$lengths[last],0)
+ }
> my_fun(my_series)
[1] 0
> my_series <- c(3, 4, 10,14,8,3,4,6,9)
> my_fun(my_series)
[1] 4
>
and vectorise, vectorise, vectorise.
HTH
Petr
On 18 Jan 2007 at 14:11, Nicolas Prune wrote:
Date sent: Thu, 18 Jan 2007 14:11:11 +0100
From: Nicolas Prune <np at alambic.org>
To: r-help at stat.math.ethz.ch
Subject: [R] How to optimize this loop ?
> Dear R Users,
>
> I request your help to optimize a loop.
>
> Given a series of observations, I want to know how many consecutive
> past observations are below the last one.
>
> e.g :
> my_series <- c(3, 4, 10,14,8,3,4,6,9)
>
> As the last number (9) is higher than the four preceding numbers (6,
> 4, 3, 8), this function should return 4.
>
> my_series <- c(3, 4, 10,14,8,3,4,11,9)
> Here, it should return 0, as 9 is immediately preceeded by a higher
> number.
>
> So far, I do this awful loop :
>
> result <- 0
> for (i in 1:length(my_series-1))
> {
> if (my_series[length(my_series)-i]>end(my_series)[1])
> { result <- i-1 ; break }
> }
>
> I thing there's a better way...
>
> my_series > my_series[end][1] returns :
> TRUE TRUE FALSE FALSE TRUE TRUE TRUE TRUE FALSE
> , which seems more appealing (once the last "FALSE" is removed), but
> now, how to know the size of the last consecutive series of "TRUE" ?
>
> Can you see a better way ?
>
> Thanks.
>
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Petr Pikal
petr.pikal at precheza.cz
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