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MathGroup Archive 1994

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Re: same options for 30 functions?

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Re: same options for 30 functions?
  • From: PERKINS TYLER R <perkins at spot.colorado.edu>
  • Date: Sun, 27 Mar 1994 00:15:47 -0700 (MST)

Martin McClain (wmm at chem.wayne.edu) wrote:

>Is there any graceful way to provide a default option for 30 or more
>functions in a package?   I want to let the user change defaults in the
>middle of a run, via SetOptions or some similar operator.
>[...]
>I would like to write
>
>f[a_, b_, g_String:(delayed assignment of $GroupName)]:= whatever

You almost have it!  The trick is to make sure that $Groupname remains a
Symbol when the lhs of your definition is evaluated:

In[1]:= Clear[$GroupName]

In[2]:= f1[a_, b_, g_:$GroupName] := DoSomethingWith[g]

In[3]:= $GroupName = "name #1";

In[4]:= f1[1, 2, "name in arg. list"]

Out[4]= DoSomethingWith[name in arg. list]

In[5]:= f1[1, 2]

Out[5]= DoSomethingWith[name #1]

In[6]:= $GroupName = "name #2";

In[7]:= f1[3, 4]

Out[7]= DoSomethingWith[name #2]


  By the way, why doesn't the following also work?  Why should changing
  the pattern g_ to g_String prevent the third argument below from being
  recognized as optional?

  In[8]:= Clear[$GroupName]

  In[9]:= f2[a_, b_, g_String:$GroupName] := DoSomethingWith[g]

  In[10]:= $GroupName = "name #2";

  In[11]:= f2[1, 2]

  Out[11]= f2[1, 2]       (* Hmm! *)

  Evidently, the answer is that the pattern g_String is checked against the
  default value -- which is a Symbol, not a String -- when f2 is defined.
  There is no match, so there is no default assigned to the third argument.


If $GroupName is already assigned, you could use a new variable to save its
value and then use the above method.  Or you could do the following:

In[12]:= f3[a_, b_, g_:Hold[$GroupName]] := DoSomethingWith[ReleaseHold[g]]

In[13]:= f3[1, 2]

Out[13]= DoSomethingWith[name #2]

In[14]:= f3[1, 2, "name in arg. list"]

Out[14]= DoSomethingWith[name in arg. list]

In[15]:= $GroupName = "name #3";

In[16]:= f3[1, 2]

Out[16]= DoSomethingWith[name #3]


All this could have been accomplished more neatly by means of Default[] and
the _. pattern object:

In[17]:= functionSymbols = {f4, f30};    (* your 30 functions or whatever *)

In[18]:= Clear[$GroupName]

In[19]:= (Default[#] = $GroupName)& /@ functionSymbols;(* Assign defaults *)

In[20]:= f4[a_, b_, g_.] := DoSomethingWith[g]         (* Define the      *)

In[21]:= f30[x_, y_.] := DoSomethingElseWith[y]        (*    30 functions *)

In[22]:= $GroupName = "name #4";                         (* Default value *)

In[23]:= {f4[1, 2], f30[11]}

Out[23]= {DoSomethingWith[name #4], DoSomethingElseWith[name #4]}

In[24]:= {f4[1, 3, "nothing"], f30[11, "nuts"]}

Out[24]= {DoSomethingWith[nothing], DoSomethingElseWith[nuts]}

In[25]:= $GroupName = "name #5";

In[26]:= {f4[1, 2], f30[11]}

Out[26]= {DoSomethingWith[name #5], DoSomethingElseWith[name #5]}


Of course, we could also use Hold[] and ReleaseHold[] as above in case
$GroupName is already assigned.

Hope this helps.

        Tyler Perkins                   perkins at colorado.edu






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