R help - Mar 2006 - How to avoid for or while loops when one index depends on another

Dear R Community,

I'm trying to exploit the elegance of R by doing the following
pseudocode routine without a WHILE or FOR loop in R:

for i = 1 to length-1
  for j = (i+1) to length
    print a[i], a[j]

That is, I want i and j to be the indices of a half-matrix
1 2, 1 3, 1 4, ..., 1 length,
      2 3, 2 4, ..., 2 length,
           3 4, ..., 3 length

1. Can this be done with the 'whole object' approach (Introduction to R 
v2.1.1 section 9.2.2) and not while loops?

2. (Extra credit) Is your solution likely to be more efficient than a loop?

3. (Extra credit) How could once do this with FOR as opposed to WHILE in 
R? Clearly if you attempt "j in i+1:length" you're in trouble when
i
exceeds length.

Thanks for your help with this excellent open-source resource,
Dan

-- 
Daniel Goldstein, Ph.D.
Assistant Professor of Marketing
London Business School s234
Regent's Park, Sussex Place
London NW1 4SA, United Kingdom
p: +44 (0) 20 7706 6796
f: +44 (0) 20 7724 1145
www.dangoldstein.com

> mat <- matrix(rnorm(10000),100)
 > system.time({    res1 <- NULL ;
                   for (i in 1:(ncol(mat)-1)) for (j in (i+1):ncol(mat)) 
res1 <- rbind(res1, c(i, j))})
[1] 1.51 0.01 1.53   NA   NA
 > system.time(res2 <- which(upper.tri(mat), T))
[1] 0.02 0.00 0.02   NA   NA
 > all.equal(res1,res2[order(res2[,"row"]),])
[1] TRUE


Daniel Goldstein a ?crit :
>Dear R Community,
>
>I'm trying to exploit the elegance of R by doing the following
>pseudocode routine without a WHILE or FOR loop in R:
>
>for i = 1 to length-1
>  for j = (i+1) to length
>    print a[i], a[j]
>
>That is, I want i and j to be the indices of a half-matrix
>1 2, 1 3, 1 4, ..., 1 length,
>      2 3, 2 4, ..., 2 length,
>           3 4, ..., 3 length
>
>1. Can this be done with the 'whole object' approach (Introduction
to R
>v2.1.1 section 9.2.2) and not while loops?
>
>2. (Extra credit) Is your solution likely to be more efficient than a loop?
>
>3. (Extra credit) How could once do this with FOR as opposed to WHILE in 
>R? Clearly if you attempt "j in i+1:length" you're in trouble
when i
>exceeds length.
>
>Thanks for your help with this excellent open-source resource,
>Dan
>
>  
>

Here are a few alternatives to try.  I doubt they are particularly
efficient but if you want to compare them see ?system.time

n <- 4

dd <- expand.grid(i = 1:n, j = 1:n)[upper.tri(diag(n), diag = FALSE),]
for(r in 1:nrow(dd)) with(dd[r,], cat(i, j, "\n"))

library(gtools)
dd <- structure(as.data.frame(combinations(n,2)), .Names = c("i",
"j"))
for(r in 1:nrow(dd)) with(dd[r,], cat(i, j, "\n"))

mat <- matrix(seq(n*n), n)
for(idx in seq(mat)[upper.tri(mat)]) {
	i <- row(mat)[idx]
	j <- col(mat)[idx]
	cat( i, j, "\n")
}

for(i in seq(n)) for(j in seq(n)[-seq(i)]) cat(i, j, "\n")



On 3/24/06, Daniel Goldstein <dgoldstein at london.edu>
wrote:
> Dear R Community,
>
> I'm trying to exploit the elegance of R by doing the following
> pseudocode routine without a WHILE or FOR loop in R:
>
> for i = 1 to length-1
>  for j = (i+1) to length
>    print a[i], a[j]
>
> That is, I want i and j to be the indices of a half-matrix
> 1 2, 1 3, 1 4, ..., 1 length,
>      2 3, 2 4, ..., 2 length,
>           3 4, ..., 3 length
>
> 1. Can this be done with the 'whole object' approach (Introduction
to R
> v2.1.1 section 9.2.2) and not while loops?
>
> 2. (Extra credit) Is your solution likely to be more efficient than a loop?
>
> 3. (Extra credit) How could once do this with FOR as opposed to WHILE in
> R? Clearly if you attempt "j in i+1:length" you're in trouble
when i
> exceeds length.
>
> Thanks for your help with this excellent open-source resource,
> Dan
>
> --
> Daniel Goldstein, Ph.D.
> Assistant Professor of Marketing
> London Business School s234
> Regent's Park, Sussex Place
> London NW1 4SA, United Kingdom
> p: +44 (0) 20 7706 6796
> f: +44 (0) 20 7724 1145
> www.dangoldstein.com
>
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