[R] avoiding a loop

[Leeds, Mark (IED)]

thanks duncan. I think you are right. There are very smart people on
this list but there were very smart people on
The splus list 3 years ago so that's why I presented my question with
caution. 
I'll just keep it as is for now. Thanks again.






-----Original Message-----
From: Duncan Murdoch [mailto:murdoch at stats.uwo.ca] 
Sent: Tuesday, October 24, 2006 3:05 PM
To: Jerome Asselin
Cc: Leeds, Mark (IED); R-help at stat.math.ethz.ch
Subject: Re: [R] avoiding a loop

On 10/24/2006 2:53 PM, Jerome Asselin wrote:
> On Tue, 2006-10-24 at 14:36 -0400, Leeds, Mark (IED) wrote:
>> 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)
>>  
>> }
> 
> You sure can vectorize this. Try this below... I haven't tested, but 
> it should be close to your solution. There's also
> 
> a <- invector[-1]
> outvector <- invector
> wh <- which(a<1)+1
> outvector[wh] <- a[wh] * invector[-length(invector)][wh-1] outvector

I think this misses the fact that if a sequence of invector values are
less than 1, then the multiplication accumulates:  e.g. with invector
equal to rep(0.5, 5) the output should be

 > constructLt(rep(0.5,5))
[1] 0.50000 0.25000 0.12500 0.06250 0.03125

whereas your function gives

 > asselin(rep(0.5,5))
[1] 0.50 0.25 0.25 0.25   NA

A function like filter() might be able to do this, but in general R
doesn't give a way to write recursive formulae in a simple vector form.

In this particular case, I'd just use the for loop; if it turned out to
be too slow, I'd rewrite it in C, in a nearly literal translation.

Duncan Murdoch
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