[R] looping using combinatorics

Fri Jul 14 19:49:25 CEST 2006

Many thanks, Gabor.  This is very close to what would be ideal.  You gave 
me an idea as follows:

Rather than combine pairs of data vectors/frames AFTER the "combn" 
function, combine all data before (though I belive this would be less 
efficient) and add an index then use that index to choose your pairs (or 
whatever combinatorics you are using; e.g., 8 choose 4  so all 
combinations of 4 out of 8 for a total of 70 combinations.)

Example data frames with variable names:

Data frame "a" where I add an "index":
id measure index
1  1.1  1
2  1.2  1
3  1.3  1

Data frame "b" where I add an "index":
id measure index
4  2.1  2
5  2.2  2
6  2.3  2

Data frame "c" where I add an index:
id measure index
7  3.1  3
8  3.2  3
9  3.3  3

If we combine all these data at once using rbind, we get:
id measure index
1  1.1  1
2  1.2  1
3  1.3  1
4  2.1  2
5  2.2  2
6  2.3  2
7  3.1  3
8  3.2  3
9  3.3  3

We can then use something similar to your code and the index to choose the 
required pairs as derived from the "combn" function. 
For example, the "combn" function will choose the data pairs
(1,2)
(1,3)
(2,3)

where, for example, the pairs (1,2) will have data from frames "a" and 
"b":
1  1.1  1
2  1.2  1
3  1.3  1
4  2.1  2
5  2.2  2
6  2.3  2

so that we can go down the list subsetting what we need and doing 
operations on each combined pair as we go. 

Is there an easy way in R to do this operation?

For the above, an attempt might be:

########## STAB CODE #########
DF <- rbind(a,b,c)
DF
for(index in as.data.frame(combn(3,2))) print(DF[,index])
######## END STAB CODE ######

but this is choosing 3 choose 2 COLUMNS within the combined file rather 
than 3 choose 2 ROWS.

Best regards and TIAA,
Jesse 

"Gabor Grothendieck" <ggrothendieck at gmail.com> 
07/13/2006 07:39 PM

To
"Jesse Albert Canchola" <jesse.canchola.b at bayer.com>
cc
r-help at stat.math.ethz.ch
Subject
Re: [R] looping using combinatorics

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 at 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 at Bayer.Com
>
>
>
>
> 
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