From: Gabor Grothendieck <ggrothendieck_at_gmail.com>

Date: Fri 14 Jul 2006 - 12:39:08 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 Fri Jul 14 12:54:30 2006

Date: Fri 14 Jul 2006 - 12:39:08 EST

I assume your question is given 3 vectors of the same length: a, b and c how do we loop over pairs of them. In the following each iteration displays one pair:

library(combinat)

DF <- data.frame(a = 1:4, b = 5:8, c = 9:12)
for(idx in as.data.frame(combn(3,2))) print(DF[,idx])

On 7/13/06, Jesse Albert Canchola <jesse.canchola.b@bayer.com> wrote:

> I have a problem where I need to loop over the total combinations of

*> vectors (combined once chosen via combinatorics). Here is a
**> simplification of the problem:
**>
**> STEP 1: Define three vectors a, b, c.
**> STEP 2: Combine all possible pairwise vectors (i.e., 3 choose 2 = 3
**> possible pairs of vectors: ab,ac, bc)
**> NOTE: the actual problem has 8 choose 4, 8 choose 5 and 8 choose 6
**> combinations.
**> STEP 3: Do the same math on each pairwise combination and spit out
**> answers each time
**>
**> ####### BEGIN CODE #######
**> #STEP 1
**> a1 <- c(1,2,3,4,5,6,7,8,9,10,11,12)
**> a <- matrix(a1,2,3,byrow=T)
**> a
**>
**> b1 <- c(13,14,15,16,17,18,19,20,21,22,23,24)
**> b <- matrix(b1,2,3,byrow=T)
**> b
**>
**> c1 <- c(25,26,27,28,29,30,31,32,33,34,35,36)
**> c <- matrix(b1,2,3,byrow=T)
**> c
**>
**> # example: combine the first two vectors "a" and "b"
**> combab <- rbind(a,b)
**>
**> # the a,b combined data from the algorithm later below should look like
**> # something like the following:
**> combab
**>
**> # use the combinatorics "combn" function found in the "combinat" package
**> on CRAN
**> m <- combn(3,2) # three choose two combinations
**> m
**>
**> # the first assignment below should be numeric and then subsequent
**> # assignments as character since the first time you assign a number to
**> # a character in a matrix the rest of the numbers in the matrix are
**> coerced to character
**> m[m==1]='a'; m[m=='2']='b'; m[m=='3']='c'
**> m
**>
**> #STEP 2: combine pairwise vectors into a matrix or frame
**> for (i in dim(m)[1])
**> for (j in dim(m)[2])
**> {
**> combined <-
**> rbind(cat(format(m[i]),"\n"),cat(format(m[j]),"\n")) #cat/format removes
**> the quotes
**> combined
**> }
**> traceback()
**>
**>
**> #STEP 3: {not there yet}
**> ################# END CODE ################
**>
**> The problem is that in STEP 2 (not complete), the results in the rbind are
**> not recognized as the objects they represent (i.e., the "a" without quotes
**> is not recognized as the data object we defined in STEP 1. Perhaps this
**> is a parsing problem. Perhaps there is an alterative way to do this. I
**> looked pretty long and hard in the CRAN libraries but alas, I am stuck.
**> BTW, I picked up R about a month ago (I used primarily SAS, Stata and
**> SPSS).
**>
**> Regards and TIA,
**> Jesse
**>
**>
**>
**>
**>
**>
**> Jesse A. Canchola
**> Biostatistician III
**> Bayer Healthcare
**> 725 Potter St.
**> Berkeley, CA 94710
**> P: 510.705.5855
**> F: 510.705.5718
**> E: Jesse.Canchola.b@Bayer.Com
**>
**>
**>
**>
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