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MathGroup Archive 2007

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Re: Mathematica 6.0 easier for me ... (small review)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg76581] Re: Mathematica 6.0 easier for me ... (small review)
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 24 May 2007 06:06:51 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f30vpv$mkn$1@smc.vnet.net>

Paul at desinc.com wrote:
[snip]
> 4.  At work, I haven't had to resort to other programs because
> Mathematica wasn't the best choice.  This is extremely nice.  BTW,
> There's a "temporal" advantage with procedural programming that hasn't
> been apparant to me in functional or rule.  Still working on it. Maybe
> someone can help.  If I have
> lis={{1,10},{2,10},...{9,10},{11,20},{12,20}...{19,20}
> 
> How do I use functional and/or rule to determine where the second
> number (lis[[i,2]]) jumped from 10 to 20 to 30 and save the pair.
> Assuming there was noise, I only want to store the first 10->20, then
> look for 20->30 and so on.  So in time, I want my search to change as
> I progress through the list.  Any input appreciated!
[snip]

The following expression should be a good start:

In[1]:=
lst = {{1, 10}, {2, 10}, {3, 10}, {4, 10}, {8, 10}, {9, 10}, {11, 20},
        {12, 20}, {13, 20}, {16, 20}, {17, 20}, {18, 20}, {25, 30},
        {26, 30}, {27, 30}, {28, 30}, {29, 30}};

(First[Take[##1, -1]] & ) /@
  Drop[Split[lst, Last[#1] == Last[#2] & ], -1]

Out[2]= {{9, 10}, {18, 20}}

Regards,
Jean-Marc



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