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Re: Protect a variable against being used as an iterator (related to the HoldAll - Evaluate problem)

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  • Subject: [mg116181] Re: Protect a variable against being used as an iterator (related to the HoldAll - Evaluate problem)
  • From: Guido Walter Pettinari <coccoinomane at gmail.com>
  • Date: Fri, 4 Feb 2011 01:39:52 -0500 (EST)

Thank you very much for the answer!  This completely solves my issue.
I did not realize it was so simple, now I am little bit ashamed :-)

Best regards,

Guido

On Feb 3, 10:28 am, Albert Retey <a... at gmx-topmail.de> wrote:
> Am 02.02.2011 12:13, schrieb Guido Walter Pettinari:
>
>
>
>
>
>
>
>
>
> > Dear Mathematica group,
>
> > I would like to ask you a simple question that, if answered, would
> > make my life much easier :-)
>
> > Does anybody know how to trigger a warning/error message whenever a
> > particular symbol is used as an iterator? By iterator, I mean the
> > second argument of functions like Plot, Table, Sum, i.e. the "i" in
> > Table [ i^2,  {i, 10} ]
>
> > I am looking for this feature since I get errors/unmeaning results
> > whenever I use as an iterator a variable already defined in some other
> > part of the code (which I may have forgot of).
> > E.g., this happens with NDSolve.  Take the following example:
>
> > tmin = 0;
> > tmax = 1;
> > sol[a_] := NDSolve [ {y'[t] == a y[t], y[0] == 1},  y,  {t, tmin,
> > tmax} ]
> > exp[a_, t_] := y[t] /. sol[a]
>
> > The following Plot command does not work:
> > Plot[exp[2, t], {t, tmin, tmax}]
> > while changing the iterator name works:
> > Plot[exp[2, x], {x, tmin, tmax}]
>
> > I know that (i) the first Plot command does not work because Plot
> > (like Table and Sum) has the HoldAll attribute, and (ii) using the
> > Evaluate function on exp[2, t] solves the problem.
>
> > However, say that I publish a package that uses a variable in the same
> > way I use "t" in the above example.  How does the user of the package
> > know that she should not use that variable to iterate?  It would be
> > nice if she gets a warning message whenever she tries to do so.
>
> > I guess that a workaround would be to use a Unique[] symbol, either as
> > an iterator or as, say, the NDSolve independent variable.... do you
> > think this is doable?
>
> I'm not sure whether I oversee a complication in your question, but I
> think proper localization of the variables used in your functions would
> be the cleanest way to solve this: it wouldn't ever be necessary to warn
> the user to not use a particular symbol. This is just one way to avoid
> the problem altogether:
>
> tmin = 0;
> tmax = 1;
> ClearAll[sol];
> sol[a_, tt_] := Block[{y, t},
>   y[tt] /.
>    First[NDSolve[{y'[t] == a y[t], y[0] == 1}, y, {t, tmin, tmax}]]
>   ]
>
> Plot[sol[2, t], {t, tmin, tmax}]
>
> But I think an even better approach would be to just return a function
> (or InterpolatingFunction) with no argument and to not redo the NDSolve
> for every of the t you want to plot, like so:
>
> tmin = 0;
> tmax = 1;
> sol[a_] := sol[a] = Module[{y,t},
>   y /. First[NDSolve[{y'[t] == a y[t], y[0] == 1}, y, {t, tmin, tmax}]]
>   ]
>
> you could make a plot then like this:
>
> Plot[sol[2][t], {t, tmin, tmax}]
>
> but you could also do stuff like this:
>
> sol4 = sol[4];
>
> Plot[sol4[t],{t,tmin,tmax}]
> Integrate[sol4[t],{t,tmin,tmax}]
>
> note that that will all work with only one call to NDSolve...
>
> hth,
>
> albert


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