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MathGroup Archive 1996

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Re: Re: Re: How to get rid of the outer {}?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg5616] Re: [mg5574] Re: [mg5562] Re: [mg5507] How to get rid of the outer {}?
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Fri, 27 Dec 1996 23:43:43 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

Andrei Constantinescu <constant at athena.polytechnique.fr>
Subject: [mg5616] [mg5574] Re:  [mg5562] Re: [mg5507] How to get rid of the  
outer {}?
asks the attached questions about Outer and Apply.

Andrei

The clue is what happens before Outer is evaluated:
Using A in place of outer we get

mylist = {{0,1}, {0,1}, {0,1}, {0,1}};
	
A[List,Sequence[mylist]]	(*Sequence disappears*)
	
	A[List, {{0, 1}, {0, 1}, {0, 1}, {0, 1}}]
	
A[List,Apply[Sequence,mylist]] (*List mylist becomes a sequence*)

	A[List, {0, 1}, {0, 1}, {0, 1}, {0, 1}]

A[Sequence[mylist]]	  (*Sequence disappears*)

	A[{{0, 1}, {0, 1}, {0, 1}, {0, 1}}]

Allan Hayes
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk

*******************
Forwarded message

... one little remark about Outer , Sequence & the question !

There are two possibilities about applying Sequence:

In[5]:= mylist = {{0,1}, {0,1}, {0,1}, {0,1}}

Out[5]= {{0, 1}, {0, 1}, {0, 1}, {0, 1}}

In[6]:= Outer[ List , Sequence[mylist] ]

Out[6]= {{{0}, {1}}, {{0}, {1}}, {{0}, {1}}, {{0}, {1}}}

... which is not the good answer, :-) , and :

In[9]:= Outer[ List, Apply[Sequence,mylist] ]

Out[9]= {{{{{0, 0, 0, 0}, {0, 0, 0, 1}}, {{0, 0, 1, 0}, {0, 0, 1, 1}}}, 

>     {{{0, 1, 0, 0}, {0, 1, 0, 1}}, {{0, 1, 1, 0}, {0, 1, 1, 1}}}}, 

>    {{{{1, 0, 0, 0}, {1, 0, 0, 1}}, {{1, 0, 1, 0}, {1, 0, 1, 1}}}, 

>     {{{1, 1, 0, 0}, {1, 1, 0, 1}}, {{1, 1, 1, 0}, {1, 1, 1, 1}}}}}


.... which is what we'd like to have !

Unfortunately, I'm not capable of explaining why this happens, by may be
someone else could, :-) !

Another try did:

In[2]:= mylist = {List, {0,1}, {0,1}, {0,1}, {0,1}}

Out[2]= {List, {0, 1}, {0, 1}, {0, 1}, {0, 1}}

In[3]:= Outer[Sequence[mylist]]

Outer::argmu: Outer called with 1 argument; 2 or more arguments are  
expected.

... which seems also to be strange. I couldn't stop this happen  
using Release
or Evaluate, which might be a question of the inner functiong of  
Outer ???


  Cheers,

     Andrei




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