R devel - Sep 2012 - Suggest adding a 'pivot' argument to qr.R

I suggest adding a 'pivot' argument to qr.R, to obtain columns in the
same order as the original x, so that
  a <- qr(x)
  qr.Q(a) %*% qr.R(a, pivot=TRUE)
returns x.

--------------------------------------------------
#  File src/library/base/R/qr.R

qr.R <- function(qr, complete = FALSE, pivot = FALSE)
{
  # Args:
  #   qr: a QR decomposition, produced by qr()
  #   complete: logical, if TRUE then return all columns
  #   pivot: logical, if TRUE then columns of the result are permuted to
  #        be in the same order as the original x.
  if (!is.qr(qr))
    stop("argument is not a QR decomposition")
  R <- qr$qr
  if (!complete)
    R <- R[seq.int(min(dim(R))), , drop = FALSE]
  R[row(R) > col(R)] <- 0
  if(pivot)
    R <- R[, order(qr$pivot)]
  R
}


--------------------------------------------------
% File src/library/base/man/qraux.Rd

qr.R(qr, complete = FALSE, pivot = FALSE)

\item{pivot}{logical expression of length 1.  If \code{TRUE},
  then columns of the result are permuted to be in the same order
  as the original \code{x}.}

zapsmall(qr.R(a))               # columns are int b1 c1 c2 b2 c3
zapsmall(qr.R(a, pivot = TRUE)) # columns are int b1 b2 c1 c2 c3

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On Sep 7, 2012, at 17:16 , Tim Hesterberg wrote:
> I suggest adding a 'pivot' argument to qr.R, to obtain columns in
the
> same order as the original x, so that
>  a <- qr(x)
>  qr.Q(a) %*% qr.R(a, pivot=TRUE)
> returns x.
That would come spiraling down in flames the first time someone tried to use
backsolve on it, wouldn't it? I mean, a major point of QR is that R is
triangular; doesn't make much sense to permute the columns without retaining
the pivoting permutation.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com