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Re: Clever way to manipulate lists

• To: mathgroup at smc.vnet.net
• Subject: [mg94418] Re: [mg94366] Clever way to manipulate lists
• From: DrMajorBob <btreat1 at austin.rr.com>
• Date: Fri, 12 Dec 2008 06:56:24 -0500 (EST)
• References: <200812110845.DAA01163@smc.vnet.net>
• Reply-to: drmajorbob at longhorns.com

Here's an example where both lists have deletions:

one = Drop[Array[{#, #} &, 10], {3}]
two = Drop[Array[{#, #^2} &, 10], {2, 5}]

{{1, 1}, {2, 2}, {4, 4}, {5, 5}, {6, 6}, {7, 7}, {8, 8}, {9, 9}, {10,
   10}}

{{1, 1}, {6, 36}, {7, 49}, {8, 64}, {9, 81}, {10, 100}}

and here's a simple way of pruning them to common x values:

oneMod = With[{xVals = two[[All, 1]]},
   Select[one, MemberQ[xVals, #[[1]]] &]]
twoMod = With[{xVals = one[[All, 1]]},
   Select[two, MemberQ[xVals, #[[1]]] &]]

{{1, 1}, {6, 6}, {7, 7}, {8, 8}, {9, 9}, {10, 10}}

{{1, 1}, {6, 36}, {7, 49}, {8, 64}, {9, 81}, {10, 100}}

or

oneMod = Intersection[one, two, SameTest -> (Equal @@ First /@ {##} &)]
twoMod = Intersection[two, one, SameTest -> (Equal @@ First /@ {##} &)]

{{1, 1}, {6, 6}, {7, 7}, {8, 8}, {9, 9}, {10, 10}}

{{1, 1}, {6, 36}, {7, 49}, {8, 64}, {9, 81}, {10, 100}}

Note: I've seen no documentation to suggest those Intersections should be  
different. But they are, fortunately!

Also, both methods depend (to some extent) on having no duplicates among  
the x values.

Bobby

On Thu, 11 Dec 2008 02:45:28 -0600, guerom00 <guerom00 at gmail.com> wrote:

> 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 :)
>



-- 
DrMajorBob at longhorns.com

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