[R] lexicographic sort of ordered lists

[Prof Brian Ripley]

On Fri, 18 Jul 2003, Duncan Murdoch wrote:

> On Fri, 18 Jul 2003 15:56:31 -0400, "J. P. Morgan" <jpmorgan at vt.edu>
> wrote :
> 
> >Does anyone know how to execute the following sort problem in R? Matrix X
> >has positive integer entries, and each column has been sorted in ascending
> >order. The problem is now to order the columns lexicographically. For
> >instance if the matrix X is
>  
> ...
> 
> >but I need a method that will work regardless of k=number of rows of X. That
> >is, the program must be able to accept any integer-valued matrix X for which
> >each column is sorted, then permute columns accordingly. 
> 
> You could do it in a loop, if you had a "stable" order() function.  I
> don't know if the standard order() is stable; here's one that is
> stable:

It is: why is that not on the help page, I wonder (nor for sort)?

> 
> stableorder <- function(x, ...) order(x, ...,1:length(x))
> 
> Then the idea is to loop from the last row to the first, sorting
> columns in a stable way:
> 
> lexico <- function(X) {
>   for (i in nrow(X):1) {
>     X <- X[, stableorder(X[i,])]
>   }
>   X
> }
> 
> (I just tried this with the regular order(), and got the same result,
> so order() might be stable, but I wouldn't trust it to be...)
> 
> Duncan Murdoch
> 
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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