I've had to do something similar, so I wrote a small function to help.
This runs in about 1/4 the time of your code on my machine.
Others may have a more efficient approach.
all.combs <- function(num, from=0, to=num) {
# create a matrix of all possible combinations of num items
# restrict each row to have between "from" and "to" items
res <- vector("list", to-from+1)
for(i in seq(from:to)) {
j <- (from:to)[i]
if(j==0) res[[i]] <- rep(FALSE, num)
comb <- combn(num, j)
res[[i]] <- t(apply(comb, 2, function(x)
!is.na(match(1:num, x))))
}
do.call(rbind, res)
}
all.combs(20, 5, 13)
Jean
wwreith <reith_will...@bah.com> wrote on 07/20/2012 07:45:30 AM:
> General problem: I have 20 projects that can be invested in and I need
to
> decide which combinations meet a certain set of standards. The total
> possible combinations comes out to 2^20. However I know for a fact that
the
> number of projects must be greater than 5 and less than 13. So far the
the
> code below is the best I can come up with for iteratively creating a set
to
> check against my set of standards.
>
> Code
> x<-matrix(0,nrow=1,ncol=20)
> for(i in 1:2^20)
> {
> x[1]<-x[1]+1
> for(j in 1:20)
> {
> if(x[j]>1)
> {
> x[j]=0
> if(j<20)
> {
> x[j+1]=x[j+1]+1
> }
> }
> }
> if(sum(x)>5 && sum(x)<13)
> {
> # insert criteria here.
> }
> }
>
> my code forces me to create all 2^20 x's and then use an if statement to
> decide if x is within my range of projects. Is there a faster way to
> increment x. Any ideas on how to kill the for loop so that it won't
attempt
> to process an x where the sum is greater than 12 or less than 6?
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