If you really want to code like a C++ coder in R, then create your own
object and extend it when necessary:

# take a variation of this; preallocate and then extend when you read a
limit
x <- numeric(2)
for (i in 1:100){
    if (i > length(x)){
        # double the length (or whatever you want)
        length(x) <- length(x) * 2
    }
    x[i] <- i
}

On Thu, Dec 10, 2009 at 11:30 AM, Peng Yu <pengyu.ut@gmail.com> wrote:

> I have a situation that I can not predict the final result's dimension.
>
> In C++, I believe that the class valarray could preallocate some
> memory than it is actually needed (maybe 2 times more). The runtime
> for a C++ equivalent (using append) to the R code would still be C*n,
> where C is a constant and n is the length of the vector. However, if
> it just allocate enough memory, the run time will be C*n^2.
>
> Based on your reply, I suspect that R doesn't allocate some memory
> than it is currently needed, right?
>
> On Fri, Dec 11, 2009 at 11:22 AM, Henrik Bengtsson <hb@stat.berkeley.edu>
> wrote:
> > Related...
> >
> > Rule of thumb:
> > Pre-allocate your object of the *correct* data type, if you know the
> > final dimensions.
> >
> > /Henrik
> >
> > On Thu, Dec 10, 2009 at 8:26 AM, Peng Yu <pengyu.ut@gmail.com> wrote:
> >> I'm wondering where I can find the detailed descriptions on R memory
> >> management. Understanding this could help me understand the runtime of
> >> R program. For example, depending on how memory is allocated (either
> >> allocate a chuck of memory that is more than necessary for the current
> >> use, or allocate the memory that is just enough for the current use),
> >> the performance of the following program could be very different.
> >> Could somebody let me know some good references?
> >>
> >> unsorted_index=NULL
> >> for(i in 1:100) {
> >>  unsorted_index=c(unsorted_index, i)
> >> }
> >> unsorted_index
> >>
> >> ______________________________________________
> >> R-help@r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

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