[R] how to bind lists recursively

[Prof Brian Ripley]

But pre-allocation still helps

a <- vector("list", 10001)
for(i in 0:10000) a[[i+1]] <- i

gives
    user  system elapsed
   0.042   0.001   0.057

on my system, against
    user  system elapsed
   0.743   0.000   1.907

for Bill's 'best' solution.

On Wed, 28 May 2008, Bill.Venables at csiro.au wrote:

> This is somewhat subtle.
>
> Rolf's solution (here corrected to...)
>
> a <- list()
> for(i in 0:10000) a[[i+1]] <- i
>
> is the best of the loop solutions (or at least the best I know of).  The
> apparently similar
>
> a <- list(0)
> for(i in 1:10000) a <- c(a, list(i))
>
> will take a lot longer, although the result is the same.  For example:
>
>> system.time({
>    a <- list()
>    for(i in 0:10000) a[[i+1]] <- i
>    })
>   user  system elapsed
>   0.59    0.00    0.59
>> system.time({
>    a <- list(0)
>    for(i in 1:10000) a <- c(a, list(i))
>    })
>   user  system elapsed
>   6.87    0.00    6.89
>>
>
> That's a factor of about 11 times as long.  The best of the lot is
>
> a <- as.list(0:10000)
>
> of course, but this has problems with generalisation, (which everyone
> suspects is going to be needed here...).
>
> Bill Venables.
>
>
>
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
> On Behalf Of Rolf Turner
> Sent: Wednesday, 28 May 2008 1:02 PM
> To: Daniel Yang
> Cc: r-help at r-project.org
> Subject: Re: [R] how to bind lists recursively
>
>
> On 28/05/2008, at 2:43 PM, Daniel Yang wrote:
>
>> Dear all,
>>
>> I want to create a list that contains 0,1,2,3, ..., 10000 as its
>> elements. I used the following code, which apparently doesn't work
>> very well.
>>
>> a <- 0
>> for(i in 1:10000) {
>>    a <- list(a, i)
>> }
>>
>> The result is not what I wanted. So how to create the bind lists
>> recursively so that the last element would be the newly added one
>> while the previous elements all remain the same?
>
> a <- list()
> for(i in 1:10000) a[[i]] <- i
>
> (The word ``bind'' is inappropriate.)
>
> 	cheers,
>
> 		Rolf Turner

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595