Re: Bug in Pattern Matching with Condition?
- To: mathgroup at smc.vnet.net
- Subject: [mg97362] Re: Bug in Pattern Matching with Condition?
- From: cesar <caguerra at gmail.com>
- Date: Thu, 12 Mar 2009 02:15:03 -0500 (EST)
- References: <email@example.com> <firstname.lastname@example.org>
On Mar 11, 10:23 am, Albert Retey <a... at gmx-topmail.de> wrote: > Hi, > > > can you elaborate a bit. According to the definition of Verbatim , > > > Verbatim[Condition][_, _]] should match literally "Condition][_, _]" an= d > > > not Condition[a, b]. > > you need to be very carefully to where the Verbatim ends. I think > everything is in order (this time even the documentation, although it is > rather on the short side): > > Match the Condition-Head verbatim, but treat its arguments _,_ as a patte= rn: > > In:= MatchQ[Condition[a,b],Verbatim[Condition][_,_]] > Out= True > > Match the whole expression Condition[_,_] verbatim: > > In:= MatchQ[Condition[a,b],Verbatim[Condition[_,_]]] > Out= False > In:= MatchQ[Condition[_,_],Verbatim[Condition[_,_]]] > Out= True > > > Therefore it looks more like another bug to me and > > > adds to the mystery of why MatchQ[Condition[a, b], Condition[_, _]] > > > evaluates to False. > > I can see no bug and no mystery here. The problem is that patterns in > Mathematica are built as Mathematica expressions just like everything > else. If you want to match one of building blocks for > pattern-expressions (or expressions containing them) verbatim, you need > to wrap them with Verbatim, which seems rather straightforward once one > has seen it. It's a little like the escape characters in string patterns > that are unavoidable if you use strings to describe the patterns, as has > been discussed in another thread. > > hth, > > albert Finally, I think this is the point. It happens with all pattern constructions (I recently realize that Condition is a pattern ): MatchQ[Pattern[a, b], HoldPattern[Pattern][_, _]] MatchQ[Except[a], HoldPattern[Except][_]] They give False if we apply HoldPattern to all the second argument. I also realized that when doing pattern matching, patterns are "evaluated" inside the kernel even with HoldComplete wraped around. MatchQ[HoldComplete[Except[a]], HoldComplete[Except[_]]] of course, if we just hold the head we prevent evaluation: MatchQ[Hold[Except][a], Hold[Except][_]] Cheers and thanks all you, Cesar