[R] Speeding up a loop

[wwreith]

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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