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