Re: Pure functions and Select[] sequential elements of list
- To: mathgroup at smc.vnet.net
- Subject: [mg119354] Re: Pure functions and Select[] sequential elements of list
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Tue, 31 May 2011 07:49:32 -0400 (EDT)
Pure functions are not "function dependent"... they are argument dependent.
Select tests individual elements (hence passes ONE argument to the test
function), while Split compares adjacent PAIRS of elements (hence passes
two arguments to the test function).
It's a fundamental difference in their mission and behavior.
Bobby
On Mon, 30 May 2011 05:34:24 -0500, McHale, Paul
<Paul.McHale at excelitas.com> wrote:
> 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.
>
> Phone: +1 937.865.3004 | Fax: +1 937.865.5170 | Mobile: +1
> 937.371.2828
> 1100 Vanguard Blvd, Miamisburg, Ohio 45342-0312 USA
> Paul.McHale at Excelitas.com<mailto:Paul.McHale at perkinelmer.com>
> www.excelitas.com<http://www.excelitas.com>
>
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