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Re: Pure functions and Select[] sequential elements of list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg119311] Re: Pure functions and Select[] sequential elements of list
  • From: "McHale, Paul" <Paul.McHale at excelitas.com>
  • Date: Mon, 30 May 2011 06:34:24 -0400 (EDT)

Thanks for the posts.  I think this is the part I am missing. 

When I do this:
mList = Range[10];
mList[[5]] = 9;
mList
mList = Split[mList, (#1 > #2) &]

I get:
{1, 2, 3, 4, 9, 6, 7, 8, 9, 10}
{{1}, {2}, {3}, {4}, {9, 6}, {7}, {8}, {9}, {10}}

So it appears Split[] considers #1 = mList[[N]]  and    #2 = mList[[N+1]].

However:
mList = Range[10];
mList[[5]] = 9;
mList
mList = Select[mList, (#1 > #2) &]

I get errors.
{1, 2, 3, 4, 9, 6, 7, 8, 9, 10}
Function::slotn: "Slot number 2 in #1>#2& cannot be filled from (#1>#2&)[1]".>>
..
General::stop: Further output of Function::slotn will be suppressed during this calculation. >>

Looks like pure functions are treated differently depending on which built in function is using them.  Not saying it is wrong.  Just explains confusion.  I am sure I used #N before.  I just didn't realize it was function dependent.



_______________________________________
From: Scot T. Martin [smartin at seas.harvard.edu]
Sent: Sunday, May 29, 2011 11:45 AM
To: mathgroup at smc.vnet.net
Cc: McHale, Paul
Subject: [mg119311] RE: [mg119298] Pure functions and Select[] sequential elements of list

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: [mg119311] [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.

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