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MathGroup Archive 2005

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Re: Re: Reevaluation of conditional arguments when the condition has changed

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61134] Re: [mg61107] Re: Reevaluation of conditional arguments when the condition has changed
  • From: Igor Antonio <igora at wolf-ram.com>
  • Date: Tue, 11 Oct 2005 03:20:36 -0400 (EDT)
  • Organization: Wolfram Research, Inc.
  • References: <200510080649.CAA20991@smc.vnet.net> <diabpq$iim$1@smc.vnet.net> <200510100640.CAA26952@smc.vnet.net>
  • Reply-to: igora at wolf-ram.com
  • Sender: owner-wri-mathgroup at wolfram.com

albert wrote:
> Hi Igor,
>  
> 
>>You shouldn't expect it because you're using the Set function (=).  When
>>you call:
>>
>>expr1 = foo[a b]
>>
>>It evaluates the rhs (giving "foo[a b]") and sets that *value* to the
>>value of
>>expr1.  Note it's *not* the function definition foo[a b], but it's value.
>>Its value just happened to be the "unevaluated value", but it's still its
>>value. When you modify the foo function and reevaluate expr1 (I'm assuming
>>you're simply doing "expr1" in a new cell), you're simply getting that
>>value that was stored in the variable.
>>
>>That is the whole idea behind using Set vs. SetDelayed ( := )
> 
> 
> I don't agree, otherwise the following would be a bug:
> 
> In[1]:= a=b
> 
> Out[1]= b
> 
> In[2]:= b=1
> 
> Out[2]= 1
> 
> In[3]:= a
> 
> Out[3]= 1
> 
> I think the difference between Set and SetDelayed is not the point in what
> David asked and I also guess that he is very aware of that difference. The
> question is whether the observation he made is an indiaction of a bug or
> not. I think it is a bug, even though Carl Woll has posted a workaround and
> that makes it not a severe problem. Maybe it would be a good idea to at
> least mention the use of UpValues as one of the possible "special
> circumstances" in the Update help. On the other hand, the usage of UpValues
> doesn't seem to be so special after all...
> 
> albert

I stand corrected.  Thanks for pointing my incorrect answer. :-)


-- 


Igor C. Antonio
Wolfram Research, Inc.
http://www.wolfram.com

To email me personally, remove the dash.


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