[R] Speed up or alternative to 'For' loop

[MacQueen, Don]

Well, speaking of hasty...

This will also do it, provided that each tree's initial height is less
than the previous tree's final height. In principle, not a safe
assumption, but might be ok depending on where the data came from.

df$delta <- c(NA,diff(df$Height))
df$delta[df$delta < 0] <- NA

-Don



-- 
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062





On 6/10/13 2:51 PM, "MacQueen, Don" <macqueen1 at llnl.gov> wrote:

>How about
>
>for (ir in unique(df$TreeID)) {
>  in.ir <- df$TreeID == ir
>  df$HeightGrowth[in.ir] <- cumsum(df$Height[in.ir])
>}
>
>Seemed fast enough to me.
>
>In R, it is generally good to look for ways to operate on entire vectors
>or arrays, rather than element by element within them. The cumsum()
>function does that in this example.
>
>-Don
>
>
>-- 
>Don MacQueen
>
>Lawrence Livermore National Laboratory
>7000 East Ave., L-627
>Livermore, CA 94550
>925-423-1062
>
>
>
>
>
>On 6/10/13 10:28 AM, "Trevor Walker" <trevordaviswalker at gmail.com> wrote:
>
>>I have a For loop that is quite slow and am wondering if there is a
>>faster
>>option:
>>
>>df <- data.frame(TreeID=rep(1:500,each=20), Age=rep(seq(1,20,1),500))
>>df$Height <- exp(-0.1 + 0.2*df$Age)
>>df$HeightGrowth <- NA   #intialize with NA
>>for (i in 2:nrow(df))
>> {if(df$TreeID[i]==df$TreeID[i-1])
>>  {df$HeightGrowth[i] <- df$Height[i]-df$Height[i-1]
>>  }
>> }
>>
>>Trevor Walker
>>Email: trevordaviswalker at gmail.com
>>
>>	[[alternative HTML version deleted]]
>>
>>______________________________________________
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>
>______________________________________________
>R-help at r-project.org mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.