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Re: about scoping in modules

  • To: mathgroup at smc.vnet.net
  • Subject: [mg85869] Re: about scoping in modules
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Tue, 26 Feb 2008 07:44:07 -0500 (EST)
  • References: <fpkvqk$hrr$1@smc.vnet.net> <fpmeur$8em$1@smc.vnet.net> <fpop16$1la$1@smc.vnet.net>

Oliver Ruebenkoenig wrote:
> On Fri, 22 Feb 2008, David Bailey wrote:
> 
>> P_ter wrote:
>>> Hello,
>>> in the next two modules I would like that the inner module uses a variable from the outer module.
>>> mfb[oneObj_] := Module[{ttmp = oneObj},AppendTo[ global2mfb, ttmp]]
>>> mfa[e_] := Module[{global2mfb = e, t = 3},mfb[t]  ]
>>> mfa[{2,3}] gives a message that global2mfb is not a variable with a value, so it can not be changed.
>>> It means I do not understand the concept of module in Mathematica. My questions are:
>>> 1. why does it go wrong?
>>> 2. how can I realize what I want?
>>> with friendly greetings,
>>> P_ter
>>>
>> Well, despite its name, global2mfb is local to the Module in which it
>> appears - so using it elsewhere refers to a global variable of the same
>> name with no value - hence the diagnostic. This is the purpose of Module
>> - to stop one use of a variable clashing with another.
>>
>> Take the assignment global2mfb=e outside the list of local
>> initialisations, and I think you will get what you want.
>>
> 
> almost,
> 
> AppendTo needs a reference not an explicit List
> 
> such as
> 
> x={1,2}
> AppendTo[x,1]
> 
> AppendTo[{1,2},3] can not work since:
> Attributes[ AppendTo]
> 
> you could not access the result.
> 
> Oliver
> 
> Oliver Ruebenkoenig, <ruebenko AT uni-freiburg.de>
> 
Sorry, I don't know what you mean. Here is the original code modified as 
I suggested:

In[86]:= mfb[oneObj_]:=Module[{ttmp=oneObj},AppendTo[global2mfb,ttmp]];
mfa[e_]:=Module[{t=3},
global2mfb=e;
mfb[t]]
In[88]:= mfa[{2,3}]
Out[88]= {2,3,3}

AppendTo is operating on a variable (or reference, if you prefer)!

David Bailey
http://www.dbaileyconsultancy.co.uk


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