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MathGroup Archive 2007

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Re: Picking Arguments

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74012] Re: Picking Arguments
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Mon, 5 Mar 2007 04:58:18 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <esdka9$ou3$1@smc.vnet.net>

Mr Ajit Sen wrote:
> Dear MathGroup,
> 
>   Given a list of functions 
> 
>   A={f[x],g[p,q,r,s],h[u,v,w]},
> 
>  I'd like to pick out their arguments as a list.
> 
>    Cases[A,_[args__]:>{args}]
> 
>  works fine returning  {{x}, {p,q,r,s}, {u,v,w}}.
> 
>   How do I achieve the same thing with Select ?

Might be difficult and kind of convoluted since Select expects that its 
second argument returns a boolean value True or False. (Indeed, any 
value returned by the test function that is not explicitly the symbol 
True will be deemed as a failure.)

One way to implement your request would be to build a test function that 
always returns False to Select (in doing so it prevents Select to build 
a list) and with side effect (that is a test function that also 
construct a list).

You would be better off by using something else. Here is some examples:

In[1]:=
A = {f[x], g[p, q, r, s], h[u, v, w]};
Cases[A, _[args__] :> {args}]
Cases[A, _[args__] -> {args}]
List@@@A
Apply[List, A, {1}]
Replace[A, _[args__] :> {args}, {1}]
Replace[A, _[args__] -> {args}, {1}]

Out[2]=
{{x}, {p, q, r, s}, {u, v, w}}

Out[3]=
{{x}, {p, q, r, s}, {u, v, w}}

Out[4]=
{{x}, {p, q, r, s}, {u, v, w}}

Out[5]=
{{x}, {p, q, r, s}, {u, v, w}}

Out[6]=
{{x}, {p, q, r, s}, {u, v, w}}

Out[7]=
{{x}, {p, q, r, s}, {u, v, w}}

Regards,
Jean-Marc


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