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. > > 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?
Hi. 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 sum(choose(20, 6:12)) 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, ] nrow(x) [1] 888896 Hope this helps. Petr Savicky. ______________________________________________ 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.