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Re: Zero or one

• To: mathgroup at smc.vnet.net
• Subject: [mg62990] Re: Zero or one
• From: "dkr" <dkrjeg at adelphia.net>
• Date: Sat, 10 Dec 2005 06:02:59 -0500 (EST)
• References: <200512060503.AAA02671@smc.vnet.net><dn5oan$nj4$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

The following definitions are not equivalent:

In[1]:=
optional1[expr_] := expr | ___ ? (Length[{#}] == 0&);
optional2[expr_]:=expr|___?(#==Null&);

In[3]:=
MatchQ[{1,,3},{1,optional1[2],3}]
Out[3]=
False
In[4]:=
MatchQ[{1,,3},{1,optional2[2],3}]
Out[4]=
True

Some alternatives to optional1 are:

In[5]:=
optionalAlt1[expr_]:=expr|___?(!MatchQ[#,_]&);
optionalAlt2[expr_]:=expr|(a___/;!MatchQ[{a},{__}]);

In[7]:=
{MatchQ[{1,3},{1,#[2],3}],MatchQ[{1,2,3},{1,#[2],3}],
      MatchQ[{1,2,2,3},{1,#[2],3}],
      MatchQ[{1,"foo",3},{1,#[_String|_Integer],3}],
      MatchQ[{1,"foo","bar",3},{1,#[_String|_Integer],3}],

MatchQ[{1,,3},{1,#[2],3}]}&/@{optionalAlt1,optionalAlt2,optional1}

Out[7]=
{{True,True,False,True,False,False},{True,True,False,True,False,False},{True,
    True,False,True,False,False}}

In optionalAlt1, the pattern test rules out all non null sequences.  If
the sequence is the null sequence, then the pattern test is ignored, as
evidenced by the following:

In[8]:=
MatchQ[{},{___?(False&)}]
Out[8]=
True

dkr

• Follow-Ups:
  • Re: Re: Zero or one
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>

• References:
  • Zero or one
    • From: "Trevor Baca" <trevorbaca@Gmail.com>