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MathGroup Archive 1998

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Re: Null in Output Solved?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13804] Re: Null in Output Solved?
  • From: "Clemens Frey" <Clemens.Frey at uni-bayreuth.de>
  • Date: Fri, 28 Aug 1998 04:18:26 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

-----Urspr|ngliche Nachricht-----
Von: Tom Goetze <TomG at UltimateTech.com> An: Tom Goetze
<TomG at UltimateTech.com>; Clemens Frey <Clemens.Frey at uni-bayreuth.de>
Datum: Montag, 24. August 1998 23:15 Betreff: Null in Output Solved?


>I previously posted a question about Null in the output from a package.
>Clemens Frey suggested a trap to make this happen of the form:
>
>f[x_] := Module[
>{},
>If[False, NeverExecuted]
>Sum[i,{i,-1,2}]
>]
>
>This outputs '2 Null' when loaded from a package and the call to f is made.
>His reasoning is that since the output for 'False' in the 'If' statement is
>not
>defined it results in the 'Null', and the line break is interpreted as a
>multiplication
>(as Mathematica so nicely warns).
>
>At first I didn't think this was too useful, but indeed it probably was.
>I'm not sure
>of why my new version works, but here is the story.
>
snippe'd'snip

Tom:

I think the case you describe is of the following abstract form. First
you define a function

f[x_] := Module[{},Anything;]

in your complexConstraint-function. The semicolon after 'Anything' makes
that f doesnt return anything, namely Null. You can see this by
evaluating

FullForm[  f[Something]   ]

Then you let Mathematica compute something like

Plus @@ (f[#]& /@ {1,2})

which indeed produces Null+Null being simplified to:

2 Null

meaning in FullForm:

Times[2,Null]

Hope this helped
Clemens



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