[Rd] Suggest adding a 'pivot' argument to qr.R

peter dalgaard pdalgd at gmail.com
Tue Sep 11 18:03:53 CEST 2012

On Sep 11, 2012, at 16:02 , Warnes, Gregory wrote:

> On 9/7/12 2:42 PM, "peter dalgaard" <pdalgd at gmail.com> wrote:
>> 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.
> As I understand Tim's proposal, the pivot argument defaults to FALSE, so
> the new behavior would only be activated at the user's request.

Sure. I'm just saying that I see little use for the un-pivoted qr.R because, generically, the first thing you want to do with qr.R is to invert it, which is easier when it is triangular.

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

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