"Reith, William [USA]" <reith_william@bah.com> wrote on
07/20/2012
09:52:02 AM:
> Would this matrix eat up memory making the rest of my program
> slower? Each x needs to be multiplied by a matrix and the results
> checked against a set of thresholds. Doing them one at a time takes
> at least 24 hours right now.
>
> Optimizing a program is not my thing.
>
> Sent from my Verizon Wireless 4GLTE smartphone
It's not mine either.
Better to ask the group.
I'm ccing R-help on this message.
Jean
> ----- Reply message -----
> From: "Jean V Adams" <jvadams@usgs.gov>
> To: "Reith, William [USA]" <reith_william@bah.com>
> Cc: "r-help@r-project.org" <r-help@r-project.org>
> Subject: [External] Re: [R] Speeding up a loop
> Date: Fri, Jul 20, 2012 10:05 am
>
>
> 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_william@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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