[R] vectorize subscript ranges?

[David Winsemius]

On Jun 15, 2010, at 1:12 AM, James McCreight wrote:

> Hi R'ers-
>
> I'll believe this can be done and that I'm just not clever enough.
>
> I'm trying to do this in 2D, but i think we have the same problem in  
> 1D.
>
> #Assume you have some 1D time series
> d<-rnorm(100)
>
> #Say you want to get the average over intervals of different lengths
> #like from time 10 to time 24 and time 51 to time 86
> starts <- c(10,51)
> ends <- c(24,86)
> results <-c(NA,NA)
>
> #then you'd typically loop
> for (i in 1:2) { results[i] <- mean(d[starts[i]:ends[i]) }
>
> can it be done without looping?

Depends on what you mean by "without looping":

 > mapply(function(x,y,z){mean(z[x:y])} ,x=starts, y=ends,  
MoreArgs=list(d))
[1] -0.219429514 -0.008565724

Probably no more, and quite possibly less, efficient than a for-loop.

>
> Thanks in advance.
>
> James
>
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>
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