[R] Speeding up a loop
savicky at cs.cas.cz
Fri Jul 20 16:26:34 CEST 2012
On Fri, Jul 20, 2012 at 05:45:30AM -0700, wwreith wrote:
> 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.
> for(i in 1:2^20)
> for(j in 1:20)
> 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?
The restriction on the sum of the rows between 6 and 12 eliminates the
tails of the distribution, not the main part. So, the final number of
rows is not much smaller than 2^20. More exactly, it is
which is about 0.8477173 * 2^20. On the other hand, all combinations
may be created using expand.grid() faster than using a for loop.
Try the following
g <- as.matrix(expand.grid(rep(list(0:1), times=20)))
s <- rowSums(g)
x <- g[s > 5 & s < 13, ]
Hope this helps.
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