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Re: a dangerous feature of Module[]?

  • To: mathgroup at
  • Subject: [mg57497] Re: a dangerous feature of Module[]?
  • From: Gennady Stupakov <stupakov at>
  • Date: Sun, 29 May 2005 21:00:16 -0400 (EDT)
  • References: <d76ol2$7si$>
  • Reply-to: Gennady Stupakov <stupakov at>
  • Sender: owner-wri-mathgroup at

Thanks to everybody who replied to my post.

Experts are aware of this feature of Module, and I was referred to a 
recent discussion of a related issue in

I checked the description of scoping in Mathematica book (section 2.6) to 
find out that most examples there use Set, rather then them SetDelayed, to 
illustrate the mechanism of local variables in Module. The book totally 
ignores all the subtleties of the scoping issues. That is frustrating -- 
after more then 10 years programming in Mathematica I realised that many of my 
codes might have serious bugs.


On Fri, 27 May 2005, Gennady Stupakov wrote:

> I've spent considerate time recently debugging my Mathematica code until I figured
> out that the problem is in a strange behaviour of the Module[] function.
> Here is a simple example that demonstrate this feature.
> I have a gloval variable x:
> In[1]:= x=3
> Out[1]= 3
> Now I define a module with local varibles x and f, call g, and get the
> expected result. The value of global variable x does not interfere with
> the local x, as expected.
> In[2]:= g:=Module[{x,f},f[x_]:=x;f[5]]
> In[3]:= g
> Out[3]= 5
> However, if I use Set (=) instead of SetDelayed (:=) in the definition of
> my function f inside the module, the result will be different:
> In[4]:= h:=Module[{x,f},f[x_]=x;f[5]]
> In[5]:= h
> Out[5]= 3
> In this case the Module ignores the fact that x is supposed to be a local
> variable, and for some reason uses the value of the global x inside the
> module.
> Can somebody explain me if this is an expected behaviour of the Module[]?
> I am using version 5.1 on a PC:
> In[7]:= $Version
> Out[7]= 5.1 for Microsoft Windows (October 25, 2004)
> Thanks.
> Gennady.

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