Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2003
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2003

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Detecting unsuccessful computations

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39337] Re: [mg39302] Detecting unsuccessful computations
  • From: Selwyn Hollis <selwynh at earthlink.net>
  • Date: Tue, 11 Feb 2003 04:47:28 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Uri,

The following illustrates a technique I've used. (I'm sure I nicked it 
from somewhere, but I can't remember where.)

    progname::"err "= "error message";
    soln = x /. Solve[eqn, x];
    If[soln===x, Message[ progname::"err"]; Return[$Failed]];

---
Selwyn Hollis



On Monday, February 10, 2003, at 01:07  AM, Uri Zwick wrote:

> Hi,
>
> Mathematica functions such as LinearSolve, LinearProgramming,
> FindMinimum and many others, sometimes cannot perform the
> requested task. (The linear system of equations has no solution, the
> linear program has no feasible points, etc.)
>
> When that happens, Mathematica usually prints some error messages
> and then returns the expression it tried to evaluate. This happens,
> for example, with LinearSolve[{{1, 2}, {3, 4}, {5, 6}}, {-1, 2, -3}].
>
> This may be reasonable when only one such calculation is performed.
> But, suppose we write a program that has to solve many linear
> programs, some of them without a feasible solution. When there is
> no solution, we want to take an appropriate action.
>
> What is the recommended way of calling a function like
> LinearProgramming and getting an indication that something
> went wrong, WITHOUT getting any error message printed?
>
> It is possible to catch errors using 'Check'. But, as far as I
> understand it only works if error messages are printed.
> It will not work if the relevant error messages are turned off
> using 'Off'.
>
> It is possible to turn off the error messages and check the head
> of the expression returned to see if it the name of the function 
> called.
> For example:
>
> Off[LinearSolve::"nosol"] ;
>
> If[ SameQ[ Head[sol = LinearSolve[...]] , LinearSolve] ,
>     (* The calculation failed. *) ,
>     (* sol is a solution. *)  ]
>
> (Note that we have to put sol = LinearSolve[...] inside
> If[ SameQ[ Head[...] ... . If we wrote
>
> sol = LinearSolve[...] ;
> If[ SameQ[Head[sol],LinearSolve] , ... , ... ]
>
> then in case of failure, the failed calculation will be
> attempted again!)
>
> Is there a cleaner solution?
>
> Uri
>
> ---------------------------------------------------------------------
> |  Prof. Uri Zwick             |  http://www.cs.tau.ac.il/~zwick    |
> |  Dept. of Computer Science   |  zwick at post.tau.ac.il              |
> |  Tel Aviv University         |                                    |
> |  Tel Aviv 69978              |  PHONE: +972 3 6409610             |
> |  ISRAEL                      |  FAX:   +972 3 6409357             |
> ---------------------------------------------------------------------
>
>



  • Prev by Date: Re: Re: Re: finding periodicity in a set
  • Next by Date: Re: Simplify[Abs[x],x<0]]
  • Previous by thread: Re: Detecting unsuccessful computations
  • Next by thread: Re: Detecting unsuccessful computations