[R] Avoiding Loops When Iterating Over Statement That Updates Its Input

[Uwe Ligges]

On 26.05.2010 08:52, Alan Lue wrote:
> Come to think of it, we can't save the output of each invocation and
> concatenate it later, since we need the output as input for the next
> iteration.


Yes, but you can do it a bit cleverer than before by initializing to the 
fill length as in:

r.seq <- numeric(nrow(d))
r.seq[1] <- 2 * (1 / d$Dt[1] - 1)
for (i in 2:nrow(d)) {
   r.seq[i] <- uniroot(bdt.deviation, interval = c(0, 1),
                 D.T = d$Dt[i], r.prior = r.seq[i-1])$root
}

Uwe Ligges



> Alan
>
>
> On Tue, May 25, 2010 at 11:43 PM, Alan Lue<alan.lue at gmail.com>  wrote:
>> Since `for' loops are slow in R, and since `apply' functions are
>> faster, I was wondering whether there were a way to use an apply
>> function—or to otherwise avoid using a loop—when iterating over a
>> statement that updates its input.
>>
>> For example, here's some such code:
>>
>> r.seq<- 2 * (1 / d$Dt[1] - 1)
>> for (i in 2:nrow(d)) {
>>   rf<- uniroot(bdt.deviation, interval=c(0, 1), D.T=d$Dt[i], r.prior=r.seq)
>>   r.seq<- append(r.seq, rf$root)
>> }
>>
>> The call to `uniroot()' both updates `r.seq' and reads it as input.
>> We could save the output of each invocation of `uniroot()' and
>> concatenate it later, but is there a better way to write this (i.e.,
>> to execute more quickly) while updating `r.seq' in each iteration?
>>
>> Alan
>>
>
>
>