[R] extendable arrays in R

[Sundar Dorai-Raj]

Brad Thompson wrote:
> In an R program I am working on, I progressively build up several
> vectors of integers.  I never know how long the vectors will eventually
> be, so I can't preallocate them with vector().  If I preallocate all of
> the vectors to their maximum size, I will run out of memory.  I tried
> using c() or append() to build up the vectors, as in the following (tried
> on R 2.1.0 on Linux and R 2.2.1 on MacOS 10):
> 
>   li <- vector()       # also tried list() and pairlist()
>   for (i in 0:40000) {
>     li <- c(li, i)     # also tried with i and li swapped
>     if (i %% 10000 == 0)
>       system('date')
>   }
> 
> The problem is that based on the times this outputs, it is O(n^2),
> matching the straightforward implementation of c() where everything
> passed to c() is copied.
> 
> I tried extending the vector by assigning to length(li) instead of
> using c(), but that also runs in O(n) (so the loop runs in O(n^2)) and
> appears to copy the elements.
> 
> What I am looking for is an array that can dynamically resized in
> amortized constant or log time (like python's list or C++'s
> std::vector).  I could build up a data structure inside R (the same way
> std::vector is built on top of C arrays), but I was hoping someone
> might have some advice on a better way to do this.
> 
> Does R have a resizeable array type that I'm missing?  What do others
> generally do in this case?
> 
> Thank you,
> 
> Brad
> 

Hi, Brad,

You can add length as needed as in:

## create a buffer
li <- vector("numeric", 1000)
t1 <- proc.time()[3]
for (i in 1:999999) {
   ## add more space if needed
   if(i > length(li)) length(li) <- length(li) + 1000
   li[i] <- i
   if(i %% 10000 == 0) {
     t2 <- proc.time()[3]
     cat(sprintf("n = %6d; time = %4.2f\n", i, t2 - t1))
     ## flush.console() ## Windows only, I think
     t1 <- t2
   }
}
## resize 'li'
length(li) <- i

HTH,

--sundar