From: Berton Gunter <gunter.berton_at_gene.com>

Date: Sat 07 May 2005 - 05:48:59 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat May 07 06:10:52 2005

Date: Sat 07 May 2005 - 05:48:59 EST

If I understand you correctly, here's one way based on expand.grid().

I is just an index set, and so all you really need to do is generate your 2^k rows from the part of the matrix you're using in the right places via replacement:

e.g. newX<-matrix(0, ncol=ncol(X),nrow=2^length(I)) newX[,I]<-expand.grid(as.list(as.data.frame(X[,I])))

N.B. I tried to do the this without the explicit as.list() cast, but got an error message. I would have thought that expand.grid should have recognized that a data.frame IS a list without the cast.

- Bert Gunter Genentech Non-Clinical Statistics South San Francisco, CA

"The business of the statistician is to catalyze the scientific learning process." - George E. P. Box

> -----Original Message-----

*> From: r-help-bounces@stat.math.ethz.ch
**> [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of
**> davidr@rhotrading.com
**> Sent: Friday, May 06, 2005 12:06 PM
**> To: r-help@stat.math.ethz.ch
**> Subject: [R] Choices from a matrix
**>
**> Could someone please suggest a more clever solution to the
**> following problem than my loop below?
**>
**> Given X a 2xN matrix X, and I a k-subset of N,
**> Generate the (2^k)xN matrix Y with columns not in I all zero
**> and the other columns with all choices of an entry from the
**> first or second row of X.
**>
**> For example, with
**> X <- matrix(1:8, nrow=2)
**> I <- c(1,3)
**>
**> X is
**> 1 3 5 7
**> 2 4 6 8
**>
**> and Y should be
**> 1 0 5 0
**> 2 0 5 0
**> 1 0 6 0
**> 2 0 6 0
**>
**> The order of the rows is unimportant.
**> ---
**> I solved this using a loop over the rows of Y after forming
**> some preliminary matrices. I think it could be improved.
**>
**> N <- NCOL(X)
**> k <- length(I)
**> G <- as.matrix(expand.grid(rep(list(c(1,2)),k)))
**> Y <- matrix(0,nc=N,nr=NROW(G))
**>
**> for(i in 1:NROW(G)){
**> ind <- rep(1,N)
**> ind[I] <- G[i,]
**> Y[i,] <- X[array(c(ind,1:N),dim=c(N,2))]
**> }
**> Y[,-I] <- 0
**>
**>
**>
**>
**> David L. Reiner
**>
**> Rho Trading
**> 440 S. LaSalle St -- Suite 620
**> Chicago IL 60605
**>
**> 312-362-4963 (voice)
**> 312-362-4941 (fax)
**>
**>
**> ______________________________________________
**> R-help@stat.math.ethz.ch mailing list
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**> PLEASE do read the posting guide!
**> http://www.R-project.org/posting-guide.html
*

>

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat May 07 06:10:52 2005

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