Re: evaluation-- one or many levels, your thoughts?
- To: mathgroup at smc.vnet.net
- Subject: [mg117064] Re: evaluation-- one or many levels, your thoughts?
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Wed, 9 Mar 2011 06:56:47 -0500 (EST)
Working with x=x+1, Newton iterations, etc. is a job for Nest or
FixedPoint.
Bobby
On Tue, 08 Mar 2011 04:34:57 -0600, David Bailey
<dave at removedbailey.co.uk> wrote:
> On 07/03/2011 10:47, Richard Fateman wrote:
>> On 3/5/2011 3:07 AM, David Bailey wrote:
>> ..
>>
>>
>
>> e.g.
>> x1=x2
>> x2= Sum[a[i]*v^i,{i,1,1000}]
>>
>> x3=x1 is, or should be, fast whether it really assigns literally x2
>> or the Sum.
> It is too fast to measure - I have just tested it! Mathematica seems to
> be VERY good at tracking which parts of expressions need evaluating. I
> have often wondered about this, and I wish Wolfram would devote a blog
> article to lifting the hood a little about this!
>
>>
>> 2. You are mimicking the execution of a program in a language like
>> FORTRAN, but are doing everything symbolically so as to find bugs,
>> and relate the computed values to expected mathematical formulas.
>> The fact that x=x+1 produces, for x, a value of "x+1" means that
>> x was not properly initialized and you have found a bug in the FORTRAN
>> program.
>>
> If I wanted to do this, I'd probably replace X by a sequence of
> variables representing the successive values, so we would end up with
> something like
> X3=X2+1
>
> I'd certainly not represent the operation x=x+1 by that Mathematica
> expression!
>
> Before you object that this would generate an impossible number of
> variables, it is worth pointing out that such a simulation would
> probably also generate some horrendously large expressions, if performed
> for many cycles.
>
> It is interesting that you need to resort to such an obscure example!
>
> I think complete evaluation is desirable because there is no clear
> distinction between programming variables and algebraic variables.
> If you write
>
> x=f[i,j,l,m] (where f stands for a complicated function)
>
> you clearly want f to evaluate if i is a loop variable, but not if none
> of the variables have changed since the expression has been created - in
> other words, Mathematica would need to track variable changes, whether
> it performed exhaustive evaluation or not!
>
> More generally, I think any CAS system has to contain compromises,
> particularly when it is also meant to a wider role as a numerical
> calculator. When I first encountered Mathematica, the exhaustive
> evaluation paradigm surprised me, but it is clearly implemented in an
> efficient way, so I am happy with it. Furthermore, there is absolutely
> no way in which you could change this now, so any discussion seems a
> little pointless.
>
> David Bailey
> http://www.dbaileyconsultancy.co.uk
>
>
--
DrMajorBob at yahoo.com