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MathGroup Archive 2003

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Re: Detecting unsuccessful computations

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39383] Re: [mg39302] Detecting unsuccessful computations
  • From: Omega Consulting <info at omegaconsultinggroup.com>
  • Date: Thu, 13 Feb 2003 04:53:46 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

At 12:07 AM 2/10/2003, 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             |
>---------------------------------------------------------------------

It is not true that the LinearSolve is reevaluated when you test the Head. 
You can see this in the Timing and that a second message isn't created.

In[1]:=
Timing[sol=LinearSolve[{{1,3},{2,6}},{1,1}]]

 From In[1]:=
LinearSolve::nosol: Linear equation encountered which has no solution.

Out[1]=
{0.02 Second,LinearSolve[{{1,3},{2,6}},{1,1}]}

In[2]:=
Timing[Head[sol]===LinearSolve]

Out[2]=
{0. Second,True}

Even if it was reevaluated, the value is cached so very little time is lost.

In[3]:=
Timing[LinearSolve[{{1,3},{2,6}},{1,1}]]

 From In[3]:=
LinearSolve::nosol: Linear equation encountered which has no solution.

Out[3]=
{0. Second,LinearSolve[{{1,3},{2,6}},{1,1}]}

So testing the head of the result is a fine method for error checking.

--------------------------------------------------------------
Omega Consulting
"The final answer to your Mathematica needs"
http://omegaconsultinggroup.com



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