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Pure functions and Select[] sequential elements of list

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  Pure functions and Select[] sequential elements of list "McHale, Paul" <Paul.McHale@excelitas.com> - 2011-05-29 11:38 +0000
    Re: Pure functions and Select[] sequential elements of list Ray Koopman <koopman@sfu.ca> - 2011-05-30 10:33 +0000
    Re: Pure functions and Select[] sequential elements of list "Chris Degnen" <degnen@cwgsy.net> - 2011-05-30 10:35 +0000

#2780 — 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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#2803

On May 29, 4:38 am, "McHale, Paul" <Paul.McH...@excelitas.com> wrote:
> 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 ;)

There are many ways to do this. Probably the simplest (but
not the fastest) is to Partition the list using an offset:

First /@ Select[ Partition[mList,2,1], #[[1]] > #[[2]]& ]

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#2810

Paul McHale wrote:
>
> 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.e.
> 
> mList={1,2,3,4,5,7,6,8};
> Select[mList, (#? > #??)&]
> 

I would use this:

Part[mList, 
 Flatten[Position[#1 > #2 & @@@ Partition[mList, 2, 1], True]]]

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