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Re: Interaction of Remove and Global variables in a Module

  • To: mathgroup at smc.vnet.net
  • Subject: [mg120193] Re: Interaction of Remove and Global variables in a Module
  • From: Gianluca Gorni <gianluca.gorni at uniud.it>
  • Date: Tue, 12 Jul 2011 06:59:57 -0400 (EDT)
  • References: <201107091132.HAA13513@smc.vnet.net>

Suppose I define a recursive sequence

a[0] = 1; a[n_] := a[n] = 2 a[n - 1]

If I evaluate a[2], then its value will be kept in memory.
When I need to clear and reuse the symbol a, I use Clear[a].

Suppose now that I do it more traditional-looking:

Subscript[a, 0] = 1; Subscript[a, n_] :=
 Subscript[a, n] = 2 Subscript[a, n - 1];

If I call Subscript[a, 2], its value will be remembered.
What can I do to clear the values? Clear[a] and ClearAll[a]
do not work. Remove[a] does work.

Is there a better alternative?

Best regards,
Gianluca

On 09/lug/2011, at 13.32, DrMajorBob wrote:

> So far, I haven't heard a purpose for Remove that ClearAll does not serve 
> just as well... without the side-effect of "spoiling" other symbols.
>
> Bobby
>
> On Fri, 08 Jul 2011 03:53:55 -0500, Leonid Shifrin <lshifr at gmail.com> 
> wrote:
>
>> Brian,
>>
>> The problem you encountered is actually more subtle. Remember that when 
>> you
>> use Remove (as compared to Clear or ClearAll), the symbol is completely 
>> removed
>> from the system. This is a pretty disruptive operation. Now, what should 
>> the system do if the symbol you are removing is referenced by some other 
>> symbols?
>> Keeping it there unchanged would mean that the symbol has not been 
>> really removed
>> from the system. The solution used in Mathematica is to change a 
>> reference to a
>> symbol
>> (say "a") to Removed[a]. In practice, this means that, even when you
>> re-introduce
>> the symbol, those definitions that were involving it are still "spoiled" 
>> and
>> can not
>> be used.  IMO, this is a very sensible design, but this is what leads to 
>> the
>> behavior
>> that puzzled you. Have a look:
>>
>> f[b_]:=Module[{t},a[t_]=b*t^2;]
>> Remove[a];
>> ?f
>>
>> Global`f
>> f[b_]:=Module[{t},Removed[a][t_]=b t^2;]
>>
>> What you have to do in your approach is to re-run the definition for 
>> "f", to
>> be able to
>> use it. This is however pretty error-prone. You can cure this by calling
>> Clear or ClearAll
>> instead of Remove, but even this approach I don't consider a good 
>> practice.
>> Even though
>> you use "a"  as a function, calling Clear or Remove on it means that in a
>> way, you use
>> it as a global variable. In this post:
>>
>> =
http://stackoverflow.com/questions/6236458/plot-using-with-versus-plot-using-block-mathematica/6236808#6236808
>>
>> there is a lengthy discussion why making implicit dependencies on global
>> variables is a
>> bad practice.
>>
>> Here are a few ways out. What you seem to want is to generate a function
>> with embedded
>> parameters (a closure). One way is to generate a pure function and return
>> it:
>>
>> In[31]:= Clear[f];
>> f[b_]:=Function[t,b*t^2];
>> a = f[3];
>> a[t]
>> a=f[4];
>> a[t]
>> Remove[a];
>> a = f[5];
>> a[t]
>>
>> Out[34]= 3 t^2
>> Out[36]= 4 t^2
>> Out[39]= 5 t^2
>>
>> Another way is to explicitly pass to "f"  the symbol to which you want to
>> assign
>> the definition:
>>
>> In[40]:= Clear[ff,a];
>> ff[fname_Symbol,b_]:=Module[{t},fname[t_]=b*t^2;];
>> ff[a,3];
>> a[t]
>> ff[a,4];
>> a[t]
>> Remove[a];
>> ff[a,5];
>> a[t]
>>
>> Out[43]= 3 t^2
>> Out[45]= 4 t^2
>> Out[48]= 5 t^2
>>
>> By making the function name an explicit parameter to "ff", you make the
>> problematic
>> situation above impossible to happen.
>>
>> HTH
>>
>> Regards,
>> Leonid
>>
>>
>>
>>
>>
>> On Thu, Jul 7, 2011 at 3:28 PM, blamm64 <blamm64 at charter.net> wrote:
>>
>>> This is what I get for querying SetDelayed
>>>
>>> In[1]:= ?SetDelayed
>>> lhs:=rhs assigns rhs to be the delayed value of lhs. rhs is maintained
>>> in an unevaluated form. When lhs appears, it is replaced by rhs,
>>> evaluated afresh each time.  >>
>>>
>>> Note particularly the above reads AFRESH EACH time.  It appears then
>>> the following is inconsistent behavior based on the above description:
>>>
>>> In[2]:= f[b_]:=Module[{t},a[t_]=b*t^2;]
>>> In[3]:= a[t]
>>> Out[3]= a[t]
>>> In[4]:= f[3]
>>> In[5]:= a[t]//InputForm
>>> Out[5]//InputForm=
>>> 3*t^2
>>> In[6]:= f[5]
>>> In[7]:= a[t]//InputForm
>>> Out[7]//InputForm=
>>> 5*t^2
>>> In[8]:= Remove[a]
>>> In[9]:= f[4]
>>> In[10]:= a[t]//InputForm
>>> Out[10]//InputForm=
>>> a[t]
>>>
>>> Apparently AFRESH is not an accurate description of how SetDelayed
>>> operates in this case, or I am missing something about this particular
>>> interaction of Module, Remove, and global variables inside Modules.
>>>
>>> However, if I go back, after executing the last line above (<a> has
>>> been Removed), and place the cursor in the input line where <f> is
>>> defined and hit Enter, which I thought would be identical to just
>>> evaluating <f> AFRESH again, and then execute the <f[4]> line again,
>>> then the global <a> definition is re-constituted.
>>>
>>> The documentation for Remove reads the name is no longer recognized by
>>> Mathematica.  My understanding is that if the same name is defined
>>> AFRESH, it will once again be recognized.
>>>
>>> So if anyone would let me know what I am missing, regarding why the
>>> definition of <a> is not created AFRESH each time <f> is evaluated, I
>>> would appreciate it.
>>>
>>> Please don't construe the definition of <f> as my way of
>>> 'parameterizing' a function definition, I just use that definition to
>>> convey the apparent inconsistency.
>>>
>>> -Brian L.
>>>
>>>
>
>
> --
> DrMajorBob at yahoo.com
>



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