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RE: Clever way to manipulate lists

• To: mathgroup at smc.vnet.net
• Subject: [mg94425] RE: [mg94366] Clever way to manipulate lists
• From: "Scot Martin" <smartin at seas.harvard.edu>
• Date: Fri, 12 Dec 2008 06:57:40 -0500 (EST)
• Organization: Harvard University
• References: <200812110845.DAA01163@smc.vnet.net>
• Reply-to: <scot_martin at harvard.edu>

Not sure if this is what you're looking for, but it works:

In[1]:=
list1 = {{x1, y1}, {x2, y2}, {x3, y3}, {xN, yN}};
list2 = {{x1, z1}, {x3, z3}, {x4, z4}, {xN, zN}};
masterlist = Intersection[First /@ list1, First /@ list2];
smalllist1 = DeleteCases[list1, {_?(! MemberQ[masterlist, #] &), _}];
smalllist2 = DeleteCases[list2, {_?(! MemberQ[masterlist, #] &), _}];

In[6]:= smalllist1
Out[6]= {{x1, y1}, {x3, y3}, {xN, yN}}

In[7]:= smalllist2
Out[7]= {{x1, z1}, {x3, z3}, {xN, zN}}



-----Original Message-----
From: guerom00 [mailto:guerom00 at gmail.com] 
Sent: Thursday, December 11, 2008 3:45 AM
To: mathgroup at smc.vnet.net
Subject: [mg94425] [mg94366] Clever way to manipulate lists

Hi everyone,

I'm still struggling through lists manipulation. I'll take a concrete
example to illustrate my point.
Let's say I have a first list, say coordinates on a regular grid :

list1={{x1,y1},{x2,y2},{x3,y3}...{xN,yN}}

This obviously has a Length of N. Now, let's say I have a second list.
In this one, there are fewer than N elements, some points are
missing... Let's say it misses a point at x2 :

list2 ={{x1,z1},{x3,z3},{x4,z4}...{xN,zN}}

Now, since those two lists are not of the same length, I cannot add
them, substract them or something. But list2 is included in list1 (in
the sense of set theory). Now, what I want to do is, in this example,
remove the point {x2,y2} from list1 and then the two list will have
the same length and I'll be able to manipulate them as I want.
Right now, I do that with For loops (detect elements which are in
list1 and not in list2 and delete them, etc...) and that works but it
is not elegant.
I'm looking for a concise, elegant way to do that if somebody sees
what I mean...

Thanks in advance :)

• References:
  • Clever way to manipulate lists
    • From: guerom00 <guerom00@gmail.com>