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MathGroup Archive 1998

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Pattern matching more than once


  • To: mathgroup@smc.vnet.net
  • Subject: [mg11991] Pattern matching more than once
  • From: asari@math.uiuc.edu (ASARI Hirotsugu)
  • Date: Fri, 17 Apr 1998 03:40:27 -0400
  • Organization: University of Illinois at Urbana-Champaign

I have been stuck with the following problem for about 10 days.  I can't
think of an elegant solution.

Input: Two lists, from some universal set Output: Many "spliced" lists
obtained from the input.

e.g.	{{a,b,c},{d,e,f}} --> {{a,b,c},{d,e,f}} (no common element)

	{{a,b,c},{d,b,f}} --> {{a,b,c},{d,b,f},{a,b,f},{d,b,c}}

	{{a,b,c,d,e},{f,b,g,d,h}} -->
			{{a,b,c,d,e},{a,b,c,d,h},{a,b,g,d,e},{a,b,g,d,h},
			 {f,b,c,d,e},{f,b,c,d,h},{f,b,g,d,e},{f,b,g,d,h}}

I will regard the input as list of chains (totally ordered set) coming
from a partially ordered set, and I would like to extend these chains
as much as possible.  If there is at most one common element in the
inputs, the following will do:

extendList[{list1:{pre1___,x_,post1___},list2:{pre2___,x_,post2___}}]:=
	{list1,list2,{pre1,x,post2},{pre2,x,post1}};

The problem is that when the lists have more than one common element,
the second common element will not be considered at all.  I suppose I
could just write, for instance,

extendList2[{list1:{pre1___,x_,mid1___,y_,post1___},
	list2:{pre2___,x_,mid2___,y_,post2___}}]:=
	Union[
		{list1,{pre1,x,mid1,y,post2},{pre1,x,mid2,y,post1},
		 {pre1,x,mid2,y,post2},list2,{pre2,x,mid1,y,post1},
		 {pre2,x,mid1,y,post2},{pre2,x,mid2,y,post1}}]; But this is very ugly.

I suppose I could write some For[] loop through
Intersection[list1,list2], but I would rather avoid it if I could.

Any help would be appreciated.

-- 
ASARI Hirotsugu                 //   http://www.math.uiuc.edu/~asari/
finger://math.uiuc.edu/asari   //    ph://ns.uiuc.edu/asari
	"We are what we pretend to be, so we must be careful
		about what we pretend to be."	--Kurt Vonnegut



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