[R] Odp: optimizing speed of calculation (recursive product)

[Berend Hasselman]

Petr Pikal wrote:
> 
> Hi
> 
> r-help-bounces at r-project.org napsal dne 03.09.2008 16:39:07:
> ....
>> In many cases I've noticed that using apply, sapply etc can help 
>> speeding up processes, but in this case I don't see how I could do so.
>> 
>> a <- runif(10000000,0.5,1.6)
>> C <- 2
>> M <- 10000000
>> system.time( for (i in 1:(M-1)) {C <- C* c(a[i],a[i+1])} )
> 
> Maybe simple math? You want last two members of 2*cumprod(a).
> 
> So
> 
>> system.time(2*cumprod(a)[9999999:10000000])
>    user  system elapsed 
>    1.97    0.04    2.00 
>>
> 
> shall be a little bit quicker then for cycle. But it is valid only for the 
> above calculation.
> 

I think

2*c(cumprod(a)[M-1],cumprod(a[-1])[M-1])

is the answer.

Berend
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