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MathGroup Archive 1995

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Overriding system functions (was: Programming Options)

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg877] Overriding system functions (was: Programming Options)
  • From: wagner at bullwinkle.cs.Colorado.EDU (Dave Wagner)
  • Date: Thu, 27 Apr 1995 01:49:40 -0400
  • Organization: University of Colorado, Boulder

In article <3nf21g$kgm at news0.cybernetics.net>,
Allan Hayes  <hay at haystack.demon.co.uk> wrote:
>
>In mg[802] JackGoldberg asks
>
>> Should (and can!) one add options to a built in command?  I do  
>

Some comments on Alan Hayes' example using Plot:

>		prtd = MemberQ[Attributes[Plot],Protected];
>		Unprotect[Plot];
>		...
>		If[prtd, Protect[Plot]];		

The purpose of this code fragment is to leave the Plot symbol in
the same state (w.r.t. being Protected) as it was before the code
executed.  There's a slicker way to do this that is also more general.
Unprotect returns a list of the symbol(s) that it unprotected.
So the above could be accomplished by:

		prtd = Unprotect[Plot];
		...
		Protect @@ prtd;

The big advantage to using this idiom comes when you are modifying a
few system symbols at a time.  It is very compact:

		prtd = Unprotect[Plot, ParametricPlot, ContourPlot, ...];
		...
		Protect @@ prtd;

This will protect exactly those symbols that were protected before
the call to Unprotect, and leave the rest unprotected.

Second, if you find yourself slinging options around a lot,
you might benefit from using the FilterOptions functions that
is defined in the standard package Utilities`FilterOptions`:

In:
    FilterOptions[Plot, PlotPoints->25, Bogus->Never, AspectRatio->1]
Out:
    Sequence[PlotPoints -> 25, AspectRatio -> 1]

You'll note that FilterOptions takes a sequence (not a list!) of
options and returns another sequence.  The head "Sequence" goes away
whenever you insert a sequence into another expression.  For example,

In:
    {a, %, b}
Out:
    {a, PlotPoints -> 25, AspectRatio -> 1, b}

Sequence is also the head of objects that match patterns like "__" and
"___".  This makes FilterOptions perfect for collecting options from
one function call and pasting them into another. For example:

    myPlot[f_, range_List, opts___Rule] :=  (* opts is a Sequence *)
    Module[{mungedF, mungedR},
	{mungedF, mungedR} = doMyOwnThing[f, range, opts];
	(* now call the real plot *)
	Plot[mungedF, mungedR, FilterOptions[Plot, opts]];
    ]

Both the Protect/Unprotect technqiue and FilterOptions were invented by
Roman Maeder, which should come as no surprise to anybody!

	
		Dave Wagner
		Principia Consulting
		(303) 786-8371
		princon at csn.net
		http://www.csn.net/princon


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