Re: perplexed by blank sequence in pattern replacement

*To*: mathgroup at smc.vnet.net*Subject*: [mg68734] Re: perplexed by blank sequence in pattern replacement*From*: bghiggins at ucdavis.edu*Date*: Thu, 17 Aug 2006 04:18:43 -0400 (EDT)*References*: <ebuj5d$6f5$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Blake, The key point to recognize is that Times (like Plus) applies built-in rules before any user defined rules. Also Times[a] =>a Now suppose we had a user defined function f that we wanted to create a list of its arguments then In[86]:= f[a,b,c]/.f[x__]\[RuleDelayed]List[x] Out[86]= {a,b,c} But if we use Times instead of f then we must prevent its arguments from being evaluated. One way is to use Hold In[94]:= Times[Hold[a,b,c]]/.Hold[x__]\[RuleDelayed]List[x] Out[94]= {a,b,c} Another way is In[97]:= Times[Hold[a,b,c]]/.Times[x__]\[RuleDelayed]List[x]//ReleaseHold Out[97]= {a,b,c} In this case the LHS of the rule Times[x__] evaluates to x__ which matches Hold[a,b,c], and thus we need to apply ReleaseHold. Remember Times[Hold[a,b,c]] evaluates to Hold[a,b,c] Hope this helps, Cheers Brian Blake wrote: > Dear MathGroup: > > I have been blithely using blank sequences in pattern matching for some > time. In persuit of a bug in a package of mine, I was quite alarmed to > find that I don't really understand how to use the blank sequence, as > expressed in the following simplified example: > > In[1]:=Replace[a*b*c,Times[mysequence__]:>{mysequence}] > > Out[1]={a b c} > > I expected Out[1]={a,b,c}, from a naieve reading of the full form of > a*b*c > > In[2]:=FullForm[a*b*c] > > Out[2]//FullForm=Times[a,b,c] > > Will someone PLEASE tell me why In[1] does not yield the results I > expected? (I can readily use a work-around, what I am concerned with is > a correct understanding of pattern matching). > > Blake Laing > thesis slave > University of Oklahoma