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Re: Generalizing Complement to handle multiple occurrences of elements

• To: mathgroup at smc.vnet.net
• Subject: [mg112475] Re: Generalizing Complement to handle multiple occurrences of elements
• From: "Tony Harker" <a.harker at ucl.ac.uk>
• Date: Thu, 16 Sep 2010 06:02:50 -0400 (EDT)

If the elements are ordered, as in your example, then this works.
myComplement[a_, b_] := 
 Flatten[Complement[Split[a], Split[b], 
   SameTest -> #1[[1]] == #2[[1]] &]] 

  Tony

]-> -----Original Message-----
]-> From: Mark Coleman [mailto:markspcoleman at gmail.com] 
]-> Sent: 15 September 2010 09:38
]-> To: mathgroup at smc.vnet.net
]-> Subject: [mg112425] Generalizing Complement to handle 
]-> multiple occurrences of elements
]-> 
]-> Greetings,
]-> 
]-> I'm wondering how one can efficiently generalize the built-in
]-> Complement function to return multiple occurrences of elements. For
]-> instance, the current version generates
]-> 
]-> a={1,2,2,3,4,4,4,4}
]-> b={3,5,7}
]-> 
]-> Complement[a,b]={1,2,4}
]-> 
]-> I'm looking for a way to define
]-> 
]-> myComplement[a,b]={1,2,2,4,4,4,4}
]-> 
]-> My current code is very inefficient!
]-> 
]-> Thanks,
]-> 
]-> Mark
]-> 
]->