Re: Pure functions and Select[] sequential elements of list
- To: mathgroup at smc.vnet.net
- Subject: [mg119325] Re: Pure functions and Select[] sequential elements of list
- From: "Scot T. Martin" <smartin at seas.harvard.edu>
- Date: Mon, 30 May 2011 06:36:55 -0400 (EDT)
I agree that this kind of functionality is not easily accessed in Mathematica. It seems to be a common need that requires some obscure formulation to get the needed effect, at least so far as I have figured out. Here are two examples of what I use:
1. I sometimes use Partition[...] first to get this effect:
mList Partition[{1, 2, 3, 4, 5, 7, 6, 8}, 2, 1];
First /@ Select[mList, (#[[1]] > #[[2]]) &]
2. Pattern matching can also do the trick, though can be slow depending on the size of your task.
mList = {1, 2, 3, 4, 5, 7, 6, 8};
ReplaceList[mList, Rule[{___, firstValue: _, secondValue: _, ___} /; (firstValue> secondValue), firstValue]]
________________________________________
From: McHale, Paul [Paul.McHale at excelitas.com]
Sent: Sunday, May 29, 2011 07:38
To: mathgroup at smc.vnet.net
Subject: [mg119325] [mg119298] Pure functions and Select[] sequential elements of list
Hi,
Does anyone know of a way to use a pure function to access current and next element of list with uses like Select[]? I know I might be dazed and confused, but I thought there was a way. I.e.
mList={1,2,3,4,5,7,6,8};
Select[mList, (#? > #??)&]
Where #? Is element N and #?? Is element N+1. I would expect the previous code (assuming N?, N?? were valid) to return 7.
Maybe there is no way to do this with a pure function. I know there are many other ways. This would be one more way to not require a looping construct to resolve this. If someone knows of straightforward ways other than Table[] or For[], Do[]... looping contructs, I would appreciate it the general knowledge. I constantly struggle against my C language beginning ;)
Best Regards,
Paul
Paul McHale | Electrical Engineer, Energetics Systems | Excelitas Technologies Corp.
Phone: +1 937.865.3004 | Fax: +1 937.865.5170 | Mobile: +1 937.371.2828
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