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From: Joe Riel <joer@san.rr.com>
Newsgroups: comp.soft-sys.math.maple
Subject: Re: Numerical definition of variables
Date: Tue, 14 Jun 2011 16:24:52 -0700
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Herman Rubin <hrubin@skew.stat.purdue.edu> writes:

> On 2011-06-14, Joe Riel <joer@san.rr.com> wrote:
>> Herman Rubin <hrubin@skew.stat.purdue.edu> writes:
>
>>> On 2011-06-13, rouben@shady.(none) (Rouben Rostamian) <rouben@shady> wrote:
>>>> In article <slrnivcqre.jgv.hrubin@skew.stat.purdue.edu>,
>>>> Herman Rubin  <hrubin@stat.purdue.edu> wrote:
>
>>>>>As an example, suppose I have two equations r and rr, and
>>>>>two variables b and c which occur in them.  Now when I enter
>
>>>>>	fsolve({r,rr},{b,c})
>
>>>>>it will produce the values of b and c.  What I want done is
>>>>>to use this, without too much additional syntax, to DEFINE
>>>>>the values of b and c for subsequent work.
>
>>>> Will this do?
>
>>>>    fsolve({r,rr},{b,c}); assign(%);
>
>>> I tried this, and it gives the error message
>
>>> Error (in assign) invalid arguments
>
>> What was returned by fsolve?  It works for the simple case
>
>> (**) eqs := { a + 3*b = 12,  b - a = 1 }:
>> (**) fsolve(eqs, {a,b});                 
>>                          {a = 2.250000000, b = 3.250000000}
>
>> (**) assign(%);                          
>> (**) a,b;
>>                               2.250000000, 3.250000000
>
>
>> My experience is that directly assigning to the solved variables
>> is usually not the best way to work, at least not if you are
>> want to deal with symbolic expressions.  The problem is that it
>> then makes expressions that use those variables numeric, which
>> frequently prevents further analysis.
>
>> I'll generally assign the solution to a variable, say
>
>>  fsol := fsolve( ... );
>
>> and then, when I need numeric values for expressions, do
>
>>   eval( some_expr, fsol );
>
> What if the next thing you want to do is
>
> fsolve(eq, c); 
>
> where a and b are in eq?  Then a, b, and c are used in
> some other calculations, including plotting.

You could form the union of solutions in another set,
which you then use to evaluate as you go along:

sols := {}:

eqs1 := { a+b=1, a-b=2 }:
eqs2 := { a*b^2-c=1, c+d=a }:

sols := sols union fsolve(eval(eqs1, sols));
                 sols := {a = 1.500000000, b = -0.5000000000}

sols := sols union fsolve(eval(eqs2, sols));
    sols := {a = 1.500000000, b = -0.5000000000
           , c = -0.6250000000, d = 2.125000000}


-- 
Joe Riel