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## debug output of eigs?

**From**: |
David Bateman |

**Subject**: |
debug output of eigs? |

**Date**: |
Wed, 18 May 2005 14:20:35 +0200 |

**User-agent**: |
Mozilla Thunderbird 0.8 (X11/20040923) |

Dear All,

`In implementing eigs and trying to make it matlab compatible I've run
``across an issue with the debugging output when opts.disp > 0 (default
``value of 1 in matlab). The problem is that ARPACK returns to the calling
``function several times to ask for operations like A*x to be done, but a
``single iteration as defined by ARPACK is when its internal
``representation of the Ritz values are updated, and this counter is not
``returned and several A*x calls are needed for a single iteration.
``However, it is this internal counter that matlab uses for display such as
`
Iteration 27: a few Ritz values of the 20-by-20 matrix:
5.9659
5.9848
5.9848
5.9962
5.9962

`and so matlab certainly has a modified version of ARPACK that they use
``for this purpose. We have several choices.
`

`1) Include ARPACK into the octave source tree and make the necessary
``modifications to return the iteration counter
` in the same manner as matlab.

`2) Print out each time we come back from ARPACK. This means that the
``iteration count is meaningless and that the Ritz values will not change
``at each nominal iteration.
``3) Try to fool ARPACK into giving information about the iteration
``counter by setting the output value workl[ipntr(5)-1] to a NaN, and
``then test when it isn't a NaN for printing. There are several downsides
``to this. If ARPACK terminates on an iteration, the returned eigenvalues
``will contain this NaN, so the termination needs to check for the NaN,
``and without in depth understanding of the ARPACK code there is no
``guarantee that this value is not in fact used in some cases and so there
``is a some potential of adding a difficult to find bug to eigs
``4) Store a local copy of the Ritz values and consider that there is an
``iteration when these values have changed. This is rather messy, require
``an overhead in terms of testing for equality of the Ritz values and
``storing them, but it will work.
``5) Ignore the whole mess of the matlab debugging arguments and just
``don't display these messages
`

`What is the preferred solution, of people on this list? In any case
``apart of this and a couple of issues with the general eigenvalue
``equation (when B is set) I now have most of eigs working...
`
Cheers
David
--
David Bateman address@hidden

`Motorola Labs - Paris +33 1 69 35 48 04 (Ph)
``Parc Les Algorithmes, Commune de St Aubin +33 1 69 35 77 01 (Fax)
``91193 Gif-Sur-Yvette FRANCE
`

`The information contained in this communication has been classified as:
``
``[x] General Business Information
``[ ] Motorola Internal Use Only
``[ ] Motorola Confidential Proprietary
`

**debug output of eigs?**,
*David Bateman* **<=**