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MathGroup Archive 2000

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RE: Sort List of Lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24827] RE: [mg24809] Sort List of Lists
  • From: "Tomas Garza" <tgarza at mail.internet.com.mx>
  • Date: Sun, 13 Aug 2000 23:49:53 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Dana wrote:

> Hello.  Could someone please give me a clue on how to sort a list based on
> the Second Element in a List:
> I think one needs to use a "pure function" but I am too new with
> Mathematica (v4)
> (Trying to learn though) :>)
>
> For Example, I would like to sort this list based on the second element in
> each sub-list in "Descending" order:
>
> {{9, 2}, {1, 6}, {2, 4}}
>
> Results would be:
> { {1,6},{2,4},{9,2} }
>
>
> I think I will learn a lot about Lists, Sorting, and Pure Functions all in
> one shot  if I could get a hint on how to do this.

More on the subject. If you are really keen to use pure functions, take a
look at the definition of Sort: "Sort[list, p] applies the function p to
pairs of elements in list to determine whether they are in order. The
default function p is OrderedQ[{#1, #2}]&".
Now you want to compare pairs of sub-lists in your list a according to the
second element in each sublist. Your ordering function must give True if the
second element in the first sub-list is greater than or equal to the second
element in the second sub-list, right? This translates as
GreaterEqual[#1[[2]], #2[[2]]] &, so that

In[1]:=
a
Out[1]=
{{9, 2}, {1, 6}, {2, 4}}

In[2]:=
Sort[a, GreaterEqual[#1[[2]], #2[[2]]] &]
Out[2]=
{{1, 6}, {2, 4}, {9, 2}}

Tomas Garza
Mexico City



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