[R] Iterate over 1 <= i < j <= n

[David Meyer]

On Sun, 27 Jan 2002, Gabor Grothendieck wrote:

> 
> What is the preferred way to iterate over 1 <= i < j <= n  ?
> 
> The following does not work as it gives an error message 
> when i+1 exceeds n:
> 
>    for(i in 1:n)
>       for(j in seq(i+1,n,1))

for(i in 1:(n-1))
  for (j in (i+1):n)

should work, I think.

-d



> 
> This works but seems like too much work:
> 
>    for(i in 1:n) {
>       j <- i+1
>       while(j<=n) {
>          ...
>          j <- j+1
>       }
>    }
> 
> This is briefer but offends my sense of how to do things:
> 
>    for(i in 1:n)
>       for(j in i:n) {
>          if (i==j) next
>          ...
>       }
> 
> This works too but seems somewhat involved:
> 
>    d <- diag(n)
>    for( idx in seq(d)[upper.tri(d,diag=FALSE)] ) {
>       i <- row(d)[idx]; j <- col(d)[idx]
>       ...
>    }
> 
> What is the preferred way to do this?
> 
> 
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