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Re: Piecewise inside a Module or Block, I don't understand this behavior.
*To*: mathgroup at smc.vnet.net
*Subject*: [mg83324] Re: Piecewise inside a Module or Block, I don't understand this behavior.
*From*: Albert <awnl at arcor.net>
*Date*: Sat, 17 Nov 2007 05:29:48 -0500 (EST)
*Organization*: Arcor
*References*: <fhjs9v$4vg$1@smc.vnet.net>
W. Craig Carter wrote:
> Hello,
>
> I have a Piecewise function calculated in a module:
> Here is a simplifed example of something which has a
> behavior that puzzles me.
>
> a[c_, d_] :=
> Module[{e, f}, e = d^2; f = c^2;
> Return[Piecewise[{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}]]];
just a sidenote: Return is not necessary since the last expression
within the module is returned anyway. In fact Return sometimes behaves a
little different than expected, so it is better not used if there is no
reason to...
> a[1,2] (*doen't return what I had anticipated*)
>
> (*However this does*)
> a[c_, d_] :=
> Module[{e, f}, e = d^2; f = c^2;
> Return[Piecewise[{d^2, 0 < x < 1/2}, {c^2, 1/2 < x <= 1}]]];
> a[1,2]
>
> (*The same goes for Block, and putting an explicit Evaluate
> inside the local function*)
>
> (*This behavior seems to be unique to Piecewise*)
> a[c_, d_] :=
> Module[{e, f}, e = d^2; f = c^2;
> Return[MyFunction[{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}]]];
> a[1,2]
>
> I can't find anything about Piecewise, or in Module and
> Block documentation that gives me a hint.
>
> Anyone know what is going on?
>
> Thanks, Craig
>
> PS: I have a work-around:
>
> aAlt[c_, d_] :=
> Module[{e, f}, e = d^2; f = c^2;
> {{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}}];
> Piecewise@aAlt[1,2]
>
> but, I am still curious....
>
Piecewise has attributes HoldAll, so the symbols e and f are not
evaluated to d^2 and c^2. When using Piecewise@aAlt[] you are
effectively doing Piecewise at {{...}} which you can also do directly
within the module:
a[c_, d_] := Module[{e, f},
e = d^2; f = c^2;
Piecewise@{{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}}
];
There are other tricks to achieve what you expected to happen, you just
have to find a way to put the evaluated versions of e and f into the
Piecewise function. A very handy candidate for this is With:
a[c_,d_]:= With[{e=d^2,f=c^2},
Piecewise[{{e,0 < x < 1/2},{f, 1/2 < x <= 1}}]
];
which I think is the cleanest way to achieve what you want.
If all this seems strange to you I suggest you learn more about the
evaluation process within mathematica, good starting points are the
tutorials listed at the bottom of guide/EvaluationControl in the
Documentation-Center. This is an advanced topic but understanding it
will probably be an eye opener and make you gain much more from
mathematica's power...
hth,
albert
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