On Fri, 26 Mar 2010, elgorgonzola wrote:
Hi,
i am working on a problem where i need to compute the products of all
possible combinations of size m of the elements of a vector. I know that
this can be achieved using the function combn(), e.g.:
vector <- 1:6
combn(x = vector, m = 3, FUN = function(y) prod(y))
In my case the vector has 2000 elements and i need to compute the values
specified above for m = 32. Using combn() i encounter problems for m >= 4.
Is there any alternative to combn() that works more efficiently? Also my
vector contains many duplicates, so there are actually only about 300
distinct values in the vector. This reduces the number of possible
combinations significantly. Is there any way to use this fact to reduce the
computational cost?
It doesn't matter what the computational cost is.
Even if the problem was just to enumerate the choose(300,32) products of
unique elements you would need more than 10^33 Gigabytes of memory to
store the result.
Unless there is a lot more structure in your problem than you have
described or unless some form of approximation is acceptable, it is not a
problem you can hope to solve.
HTH,
Chuck
Thanks in advance,
El
--
View this message in context:
http://n4.nabble.com/More-efficient-alternative-to-combn-tp1692117p1692117.html
Sent from the R help mailing list archive at Nabble.com.
______________________________________________
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
and provide commented, minimal, self-contained, reproducible code.
Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
______________________________________________
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
and provide commented, minimal, self-contained, reproducible code.
