Do leave the posts for anyone else who might google the same question. (I don't think you really could delete the post anyways, perhaps only on one mirror)

You could probably use some combination or rev() and t() to fill by row, but I haven't thought through the geometry all the way yet. 

Michael 

On May 16, 2012, at 8:13 PM, YUProf <casperyc@hotmail.co.uk> wrote:

> Hi Michael,
> 
> I have figured out a 'super' easy way myself and already deleted the post.
> 
> It can be done using: (no package necssary)
> 
> d=c(3,6,2,1,4,5)
> x=matrix(,3,3)
> 
> # by column, 
> x[!lower.tri(x)]=d
> 
> I am still trying very hard to think of a way to fit it by row as I sometimes have to!
> 
> THANKS!
> 
> Chen
> 
> ==============================================================================
> Mr Chen YU
> PhD candidate in Statistics
> School of Mathematics, Statistics and Actuarial Science, University of Kent
> 
> D7/D Woolf College, The Pavilion, Giles Lane, Canterbury, Kent CT2 7BQ
> Mobile: +44(0)7725003559
> ==============================================================================
> 
> > From: michael.weylandt@gmail.com
> > Date: Wed, 16 May 2012 19:41:36 -0400
> > Subject: Re: [R] triangular matrices input/output
> > To: casperyc@hotmail.co.uk
> > CC: r-help@r-project.org
> > 
> > The Matrix package provides good support for many special sorts of
> > matrices, but here it looks like you probably don't need that
> > additional machinery for such small case:
> > 
> > makeUpper <- function(vec, diag = FALSE){
> > n <- (-1 + sqrt(1 + 8*length(vec)))/2
> > stopifnot(isTRUE(all.equal(n, as.integer(n))))
> > 
> > if(!diag) n <- n + 1
> > 
> > mat <- matrix(0, ncol = n, nrow = n)
> > mat[upper.tri(mat, diag)] <- vec
> > mat
> > }
> > 
> > I think does what you want and it's not too hard to generalize to
> > lower triangular.
> > 
> > E.g.,
> > 
> > v <- 1:6
> > makeUpper(v)
> > makeUpper(v, diag = TRUE)
> > 
> > It's not super well tested though so caveat lector.
> > 
> > Michael
> > 
> > On Wed, May 16, 2012 at 5:09 PM, casperyc <casperyc@hotmail.co.uk> wrote:
> > > Hi,
> > >
> > > Is there any package that deals with triangular matrices?
> > >
> > > Say ways of inputting an upper (lower) triangular matrix?
> > >
> > > Or convert a vector of length 6 to an upper (lower) triangular matrix (by
> > > row/column)?
> > >
> > > Thanks!
> > >
> > > -----
> > > ######################
> > > PhD candidate in Statistics
> > > Big R Fan
> > > Big LEGO Fan
> > > Big sTaTs Fan
> > > ######################
> > >
> > > --
> > > View this message in context: http://r.789695.n4.nabble.com/triangular-matrices-input-output-tp4630310.html
> > > Sent from the R help mailing list archive at Nabble.com.
> > >
> > > ______________________________________________
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> > > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained, reproducible code.

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