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Re: Bug in Default[] or Bug in My Understanding?

• To: mathgroup at smc.vnet.net
• Subject: [mg89312] Re: Bug in Default[] or Bug in My Understanding?
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Thu, 5 Jun 2008 00:43:34 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <g1m8ea$i2$1@smc.vnet.net>

Salvatore Mangano wrote:

> In[208]:= ClearAll[f] ;
> Default[f,2] = 0;
> 
> In[210]:= f[a_, b_.] := a + b
> 
> In[211]:= f[1]
> Out[211]= 1
> 
> In[212]:= (* Now change Default *)
> Default[f,2] = 1;
> 
> (* Note that Mathematica seems to recognize change *)
> In[213]:= ?f
> Global`f
> f[a_,b_.]:=a+b
> 
> f/:Default[f,2]=1
> 
> 
> (* But not really! Expected 2 *)
> In[214]:= f[1]
> Out[214]= 1
> 
> Even if it is a rule that you can't change the Default after the function is defined it is certainly not nice of Mathematica to allow such a change and advertise the change in the output of Definition[] and Information[].
> 
> I am using 6.0.2.0 on Windows XP.

No bug here. The "more information" section of the online help for 
*Default[]* reads "The necessary values for Default[f] must always be 
defined before _. is used as an argument of f."

The following works as expected:

In[25]:= ClearAll[f];
Default[f, 2] = 0;
f[a_, b_.] := a + b
f[1]

ClearAll[f];
Default[f, 2] = 1;
f[a_, b_.] := a + b
f[1]

Out[28]= 1

Out[32]= 2

Regards,
-- Jean-Marc