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Re: Strange Solve result after previous bad input

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100967] Re: Strange Solve result after previous bad input
  • From: Alois Steindl <Alois.Steindl at tuwien.ac.at>
  • Date: Thu, 18 Jun 2009 20:48:49 -0400 (EDT)
  • References: <h1d4ao$o0q$1@smc.vnet.net>

Alain Cochard schrieb:

> Here, there is '=' instead of '==' in the 2nd eq:
> 
>      In[2]:= Solve[{a==x + y, b=-x + y},{x,y}]
> 
With this input you assigned -x+y to b. In subsequent expressions b is 
replaced by -x+y.
Furthermore the "side effect" of that assignment is, that its result 
-x+b is passed to Solve, so to Mathematica your input looked as if you 
had typed "Solve[{a == x + y, -x + y}, {x, y}]"
 >
>      Solve::eqf: -x + y is not a well-formed equation.
> 
>      Out[2]= Solve[{a == x + y, -x + y}, {x, y}]
> 
> and then, subsequent properly input systems will still fail:
> 
Here b has still the value -x+y, so now you ask for the solution of 
-x+y=-x+y and a==x+y.
>      In[3]:= Solve[{a==x + y, b==-x + y},{x,y}]
> 
>      Solve::svars: Equations may not give solutions for all "solve" variables.
> 
>      Out[3]= {{x -> a - y}}
> 
> 
> Is this normal?  
> 
Yes
> If so, is there a cure (some "resetting" procedure), apart from
> quiting Mathematica and entering back?
> 
The simplest way:
b=.
Also different variants of Clear[] should help.
> Also, is there a way of testing that a previous bad input is indeed
> the cause of that strange output? (Because if it happened a long time
> ago in the Mathematica history, one might no longer remember.)
> 
Similar problems happen to me quite often. If Mathematica seems to 
behave in a strange way, I try to look at the input more closely.
In your case you could have tried to input just
{a==x + y, b==-x + y}

Good luck
Alois




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