Re: Rule and Module not working together

*To*: mathgroup at smc.vnet.net*Subject*: [mg77915] Re: [mg77864] Rule and Module not working together*From*: DrMajorBob <drmajorbob at bigfoot.com>*Date*: Tue, 19 Jun 2007 06:58:40 -0400 (EDT)*References*: <6059828.1182167540984.JavaMail.root@m35>*Reply-to*: drmajorbob at bigfoot.com

A and B in the Module have nothing to do with A and B on the pattern side of the rule (the scope is different), and you'll often need RuleDelayed (":>") rather than Rule ("->"): Clear[myfunc] myfunc[x_] := x /. Log[a_] + Log[b_] :> Log[a*b] b = 666; myfunc[Log[f] + Log[g]] Log[f g] myfunc[Log[4] + Log[5]] Log[20] Bobby On Mon, 18 Jun 2007 06:07:06 -0500, Steven Siew <stevensiew2 at gmail.com> wrote: > On Mathematica 5.2, I tried to write a function which applies the rule > Log[A_] + Log[B_] -> Log[A * B] but somehow it does not work as I > expected. > > Instead of Log[f] + Log[g] becoming Log[f * g], I get the result of > Log[666 * f] even though I have marked both A and B as private > variables. > > Can anyone help? I'm sure I made a big mistake somewhere in my > understanding of Mathematica. > > > > In[1]:= AppendTo[$Echo,"stdout"] > Out[1]= {stdout} > > In[2]:= (* Write your mathematica code below *) > > In[3]:= If[Length[Names["Global`*"]] > 0, Remove["Global`*"]] > > In[4]:= Off[General::spell, General::spell1] > > In[5]:= myfunc[x_] := Module[{s, A, B}, > s = {Log[A_] + Log[B_] -> Log[A*B]}; Return[x /. s]] > > In[6]:= ? myfunc > Global`myfunc > > myfunc[x_] := Module[{s, A, B}, s = {Log[A_] + Log[B_] -> Log[A*B]}; > Return[x /. s]] > > In[7]:= B = 666; > > In[8]:= myfunc[Log[f] + Log[g]] > > Out[8]= Log[666 f] > > In[9]:= myfunc[Log[4] + Log[5]] > > Out[9]= Log[2664] > > In[10]:= (* End of mathematica code *) > > In[11]:= Quit[]; > > > -- DrMajorBob at bigfoot.com