Re: [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, [EMAIL PROTECTED] 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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of Rolf Turner
Sent: Wednesday, 28 May 2008 1:02 PM
To: Daniel Yang
Cc: r-help@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,                  [EMAIL PROTECTED]
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

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