a tricky List manipulation problem
- To: mathgroup at smc.vnet.net
- Subject: [mg16211] a tricky List manipulation problem
- From: hanssen at zeiss.de
- Date: Fri, 5 Mar 1999 00:40:45 -0500
- Sender: owner-wri-mathgroup at wolfram.com
Hi, MathGroup
suppose I have two lists of lists which are structured like this:
l1={{s1,x1},....,{si,xi}},
l2={{t1,y1},....,{tk,yk}}.
where the lengths of the two lists may be different. The first
entries in the lists serve als "Tags", the second are "values".
Suppose, no tag appears twice in l1 or twice in l2.
I am looking for the list of elements {sh,xh,yh} of elements
with the following conditions:
{sh,xh} is element of l1 AND
{sh,yh] is element of l2 (i.e. combine elements with same "tag")
Without loss of generality suppose, both lists are sorted by their tags.
A procedural approach to this problem would be to climb
down the ladder starting with {i,j}={1,1}, comparing pairs l1[[i,1]] and l2[[j,1]]
and taking found tag matches to a result list. This requires some caution,
since tags from l1 may be missing in l2 and vice versa.
My question is, if there is a better pattern matching or set
manipulation approach to this problem, since the lists l1 and l2
might become quite large. Timing for this is the main concern.
I am looking for a solution which avoids any loop construct.
Best regards
Dipl.-Math. Adalbert Hanszen
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