Re: evaluate to True?

*To*: mathgroup at smc.vnet.net*Subject*: [mg125987] Re: evaluate to True?*From*: clhotka at fundp.ac.be*Date*: Wed, 11 Apr 2012 18:19:47 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <201204061001.GAA23045@smc.vnet.net>

Hello, thank you for pointing out that MatchQ already exists (see also [mg125948]). I am suprised that after more than 8 years of "heavy" usage of Mathematica I did never use this function (directly). In fact in most of my codes I prefer to use the construct Not[FreeQ[#]]& instead of MemberQ. It is interesting to note that it still has the same drawback as MemberQ: LiteralMemberQ[expr_, form_] := MemberQ[expr, Verbatim[form]] myMemberQ[expr_, form_] := Not[FreeQ[expr, form]] LiteralMemberQ[{0, 1, 2}, 2] True myMemberQ[{0, 1, 2}, 2] True LiteralMemberQ[{0, 1, 2}, expr_] False myMemberQ[{0, 1, 2}, expr_] True LiteralMemberQ[{0, 1, expr_}, expr_] True myMemberQ[{0, 1, expr_}, expr_] True As you can see the difference in the above examples is that LiteralMemberQ[{0,1,2},expr_] returns False while myMemberQ[{0,1,2},expr_] returns True. I am still thinking that both functions should return neither True nor False (see argumentation below) in this case. It took me some time to design the following function: myMemberQ[expr_, form_] := Switch[Head[form], Pattern, If[Intersection[ Cases[expr, Verbatim[form], \[Infinity]], {form}] != {}, True, Undefined], _, Not[FreeQ[expr, form]]] The behaviour is now: myMemberQ[{0, 1, 2}, 3] False myMemberQ[{0, 1, 2}, 2] True myMemberQ[{0, 1, 2}, expr_] Undefined myMemberQ[{0, 1, expr_}, expr_] True with the additional nice property that myMemberQ[{0, 1, 2^expr_}, expr_] True LiteralMemberQ[{0, 1, 2^expr_}, expr_] False while MemberQ[{0, 1, 2^expr_}, expr_] returns True not because of expr_ in 2^expr_ but because every element of {0,1,2^expr_} matches expr_. Please let me know your opinion, Christoph Quoting Andrzej Kozlowski <akozlowski at gmail.com>: > Of course there is already a function called "MatchQ". Second, > MemberQ can actually be used for "literal" selection of the kind you > seem to be thinking about. > > For example: > > MemberQ[{0, 1, 2}, expr_] > > True > > but > > MemberQ[{0, 1, 2}, Verbatim[expr_]] > > False > > MemberQ[{0, 1, expr_}, Verbatim[expr_]] > > True > > Thus we can define a "literal" membership checking function as follows: > > LiteralMemberQ[expr_, form_] := MemberQ[expr, Verbatim[form]] > > Now it behaves the way you expected: > > LiteralMemberQ[{0, 1, 2}, 2] > > True > > LiteralMemberQ[{0, 1, 2}, expr_] > > False > > and even: > > LiteralMemberQ[{0, 1, expr_}, expr_] > > True > > Andrzej Kozlowski > > > On 7 Apr 2012, at 11:59, Christoph Lhotka wrote: > >> Hello, >> >> yes you are right, I mean MemberQ rather than ModuleQ (please see my >> correction of the post [mg125913]). >> >> In fact the behaviour is consistent with the information you get for >> MemberQ: >> >> In[1]:= ?MemberQ >> >> "MemberQ[list, form] returns True if an element of list matches form, >> and False otherwise." >> >> In fact the function name is misleading (at least to me): form is never >> a member of list if MemberQ >> returns True. If this would be the case my argumentation (below, >> original post) would bring the >> behaviour of the function in troubles if form is the "expression for >> everything". >> >> The misinterpretation of the function due to the name can be the cause >> of severe bugs as seen >> in message [mg125911]. Maybe a name like MatchQ would be more >> appropriate for future versions >> of Mathematica. >> >> Best, >> >> Christoph >> >> >> >> >> On 04/06/2012 02:38 PM, Bob Hanlon wrote: >>> You mean MemberQ rather than ModuleQ. In MemberQ[list, expr_] a blank >>> (with or without a name for the blank) matches anything. >>> >>> {MemberQ[{a}, _], >>> MemberQ[{"a"}, _], >>> MemberQ[{Indeterminate}, _], >>> MemberQ[{ComplexInfinity}, _], >>> MemberQ[{Plot[x, {x, 0, 1}]}, _]} >>> >>> {True, True, True, True, True} >>> >>> >>> Bob Hanlon >>> >>> On Fri, Apr 6, 2012 at 6:01 AM, Christoph Lhotka >>> <christoph.lhotka at fundp.ac.be> wrote: >>>> Hello, >>>> >>>> I found and interesting subject of discussion in the post >>>> >>>> "Bug in pattern test, or I did something wrong?" >>>> >>>> >>>> I could trace back the problem to an issue with ModuleQ. >>>> >>>> Question: Why does >>>> >>>> In[12]:= ModuleQ[{0,1,2},expr_] >>>> >>>> Out[12]:= True >>>> >>>> evaluate to True? >>>> >>>> >>>> My argumentation is as follows: >>>> >>>> On the one hand there could be a chance that expr_ is 0,1 or 2 but on >>>> the other >>>> hand the probability that expr_ is not 0,1 or 2 is even higher. As a >>>> conclusion it should neither >>>> evaluate to True nor to False. >>>> >>>> In other words: Is there any reason why the expression of everything >>>> (named expr) >>>> is contained in the set {0,1,2} ? >>>> >>>> Best, >>>> >>>> Christoph >>>> >>> >>> >> >> > >

**References**:**Why does ModuleQ[{0,1,2}, expr_] evaluate to True?***From:*Christoph Lhotka <christoph.lhotka@fundp.ac.be>