[R] avoiding a loop

[Christos Hatzis]

Try this (essentially the trick is to shift the invector to get the y[i-i]
effect):

constructLt<-function(invector, a=1) {
    invector[invector<a] <- c(0,invector)[invector<a] * invector[invector<a]
    invector 
}

> aa <- c(1,1,0.5,2,3,0.4,4,5)
> aa
[1] 1.0 1.0 0.5 2.0 3.0 0.4 4.0 5.0
> constructLt(aa)
[1] 1.0 1.0 0.5 2.0 3.0 1.2 4.0 5.0

-Christos

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Leeds, Mark (IED)
Sent: Tuesday, October 24, 2006 2:36 PM
To: R-help at stat.math.ethz.ch
Subject: [R] avoiding a loop

I think I asked a similar question 3 years ago to the  Splus list and I
think the answer was no or noone answered so noone should spend more than 5
minutes on this because it could definitely be a waste of time.
 
My question is whether the function below can be rewritten without a for
loop. apply is fine if it can be done that way but i doubt it. I call it a
lot and would prefer to not loop.
 
#-----------------------------------------------------------------------
--------------------------
 
constructLt<-function(invector) {
 
outvector<-invector
 
 for ( i in 2:length(invector) ) {
 if ( invector[i] < 1 ) {
  outvector[i]<-invector[i]*outvector[i-1]
 }
}
 
return(outvector)
 
}

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