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MathGroup Archive 1995

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Re: problem with Mathematica

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg493] Re: problem with Mathematica
  • From: withoff (David Withoff)
  • Date: Mon, 27 Feb 1995 10:52:03 -0600

  I have a problem with Mathematica. I have looked for  
  this problem in MathGroup, but I haven't been successful yet. Maybe  
  you could point me some useful reference or put this problem on  
  MathGroup or explain the reason for following behavior:

  In[1] :=  ClearAll[a, o, doit, done]

  Out[1] =  o = a[2];

  In[2] :=  a[_] := done /; doit 

  In[5] :=  doit = False;

  If `doit' is set to `False' it works as I would expected only the  
  first time.

  In[6] :=  Trace[o]

  Out[6] =  {o, a[2], {{doit, False}, RuleCondition[done, False],
     Fail}, a[2]}

  The second time the delayed rule is not evaluated. Why?  

  In[7] :=  Trace[o]

  Out[7] =  {o, a[2]}
 
      Best Regards,
                                    Pavel Krtous

-------------------------------------------

An explanation of this behavior can be found in the documentation
for the Update function in the Mathematica book (page 310 and page 892).

Expressions are evaluated only when something inside of the expression
has changed.  If you make a change to something that does not appear
anywhere in the expression, then you need to use Update.

In[1]:= o = a[2];

In[2]:= a[_] := done /; doit

In[3]:= doit = False;

In[4]:= Trace[o]

Out[4]= {o, a[2], {{doit, False}, RuleCondition[done, False], Fail}, a[2]}

In[5]:= Trace[o]

Out[5]= {o, a[2]}

In[6]:= Update[a]

In[7]:= Trace[o]

Out[7]= {o, a[2], {{doit, False}, RuleCondition[done, False], Fail}, a[2]}

Dave Withoff
Research and Development
Wolfram Research


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