[R] Nested for loops

[David Winsemius]

On Jul 14, 2009, at 2:25 AM, Michael Knudsen wrote:

> On Tue, Jul 14, 2009 at 8:20 AM, Michael  
> Knudsen<micknudsen at gmail.com> wrote:
>
>> What do you mean? It looks a like a very general solution to me.
>
> Just got an email suggesting using the functions col and row. For  
> example
>
> temp = matrix(c(1:36),nrow=6)
> which(col(temp)>row(temp))

>
> This gives the indices (in the matrix viewed as a vector) of the
> above-diagonal entries.
>

If you want the entries it would then be:
temp[ col(temp) > row(temp) ]

But more simply:

 > temp[ upper.tri(temp) ]
  [1]  7 13 14 19 20 21 25 26 27 28 31 32 33 34 35
 >

If you want the row and column numbers then:
 > row(temp)[ col(temp) > row(temp)]
  [1] 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5
 > col(temp)[ col(temp) > row(temp) ]
  [1] 2 3 3 4 4 4 5 5 5 5 6 6 6 6 6

Any solution that uses col and row will create two additional matrices  
of the same size as the original at least for the duration of the  
operation.

For the sub-and super-diagonals:

http://finzi.psych.upenn.edu/Rhelp08/2009-March/191379.html
-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT