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Re: a dangerous feature of Module[]?

  • To: mathgroup at
  • Subject: [mg57447] Re: [mg57422] a dangerous feature of Module[]?
  • From: "David Park" <djmp at>
  • Date: Sat, 28 May 2005 05:39:11 -0400 (EDT)
  • Sender: owner-wri-mathgroup at


I agree, this is a trap for many Mathematica users. It's a feature that most
old timers know about. The same thing happens with Set and SetDelayed even
outside a Module. When Mathematica evaluates a statement, or a Module, if
fills in all the value definitions it knows and can. It doesn't notice that
x_ is a pattern on the lhs of a Set statement. Nor did it notice that x was
a locally defined variable in Module. It seems to only take that into
account after the known values are filled in. But it does notice the
difference between Set and SetDelayed.

The basic precaution is don't assign values to variables and then use them
on the right hand side of Set statements, unless you want that value to be
used. (And sometimes you might want the Set values to be used and maybe
that's why Mathematica behaves this way.)

Also, I try to avoid giving values to single character symbols. They are too
valuable as symbols and this is the single greatest cause of problems for
users. It is much better to assign values with data rules or by filling them
into an argument list on specific calls. Stay symbolic and exact as long as
you can. Most times substituting specific values is the last step in a

David Park
djmp at

From: Gennady Stupakov [mailto:stupakov at]
To: mathgroup at

I've spent considerate time recently debugging my Mathematica code until I
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

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)


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