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.
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