Anonymous user
Talk:QR decomposition: Difference between revisions
→Questionnable intent
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Is this enough? Or should they implement the algorithm given in the description?
I think they should show the algorithm implemented not just the function called!
== Questionnable intent ==
The task is misleading at best. Some answers successfully compute Householder projections,
but, like the task description, they fail to understand that a QR decomposition is never
computed this way, as this would be too much time- and space- consumming.
Only the vector ''u'' of array ''I - s uu''' is ever stored, together with ''s'' or something equivalent to ''s''.
And both ''Q'' and ''R'' are stored in ''A'' in the process, with only a supplementary vector.
LINPACK and LAPACK are of course no exception, though they handle the question in a slightly
different way [http://stackoverflow.com/questions/3031215/mystified-by-qr-q-what-is-an-orthonormal-matrix-in-compact-form].
And when solving a system given an already computed QR in packed form, there are also ways to do
it in a clever way, not computing the ''Q'' and ''R'' matrices effectively.
Most answers only show bad ways to do the job, and they are just showing programming language
in (bad) action.
While I understand that Rosetta Code is not the place for state-of-the-art algorithms, it should be at least
mentionned that they are indeed very poor in this case. And anyway, a reasonnable QR algorithm is not more difficult
to implement, it just requires some work.
[[User:Arbautjc|Arbautjc]] ([[User talk:Arbautjc|talk]]) 11:05, 30 April 2015 (UTC)
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