Tarmo Remmel wrote:
Dear List,
Running on a PC (Windows 2000) with 256 MB RAM, Version R1.9.1
This one is quite outdated...
I have a relatively simple problem, which I can solve for relatively small
datasets, but run into difficulties with larger ones. I believe that my
approach is a hack rather than something elegant and I was hoping that
somebody on this list might help me improve my code. Basically, given a
vector of values (e.g., 0,0,1,1), I want to generate all of the unique
arrangements of these values, of which there are 4!/(2!2!) = 6.
0 0 1 1
0 1 0 1
0 1 1 0
1 0 0 1
1 0 1 0
1 1 0 0
Using unique() in conjunction with expand.grid(), and later filtering
impossible results, I can obtain the answer. However, this is slow, and
does not work for large initial vectors and is difficult to filter when
using values beyond 0,1. Is there some mathematically elegant method for
doing this? I'd hope to have initial vectors significantly longer than the
demonstrated 4 values (e.g., thousands).
Nice for length 4, but you will get problems far sooner than for length
1000... please calculate the size before!
For thing as short as 4, you might want to try out permutations() in
package "gtools" (formerly in bundle gregmisc, since yesterday a single
package).
Uwe Ligges
Any help is appreciated and I will gladly SUM afterwards.
Thank you,
Tarmo
__________________________________________
Tarmo Remmel Ph.D.
GUESS Lab, Department of Geography
University of Toronto at Mississauga
Mississauga, Ontario, L5L 1C6
Tel: 905-828-3868
Fax: 905-828-5273
Skype: tarmoremmel
http://eratos.erin.utoronto.ca/remmelt
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