[Coin-ipopt] slacks vs. ||C||, and presolve issue

[Jonathan Berry]

Happy holidays to the ipopt project!

Thanks again for sharing it.


In the course of debugging an application, I've come across
two issues:

- I use ampl's   display <constraint>.slack command for all of
  my constraints, and there are no negative slacks.  I interpret
  this to mean that I have successfully initialized all of my
  variables to represent a feasible solution.  However, the
  value of ||C|| at iteration 0 is about 1e+03.  This value is
   affected by the setting of ISCALE, but never starts at 0.

- In debugging this, I've fixed most variables to their initial
   values in ampl, and let IPOPT try to determine the unfixed
   ones.  Ampl thinks that there are 7 variables after the presolve;
   ipopt thinks that there are 23.  I tried running KNITRO on
   NEOS using the same ampl model and script, and knitro thought
   that there were 7 variables after the presolve.  Back in the IPOPT
   world, I can't match the X(..) values in IPOPT.OUT to the ampl
   variables in xxx.col.  Has anybody dealt with this before?

-Jon