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MathGroup Archive 1998

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Re: list referencing with [[]][[]]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13993] Re: list referencing with [[]][[]]
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Sat, 12 Sep 1998 16:59:17 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Tim Dellinger wrote:
> 
> According to "the book" section 1.8.4,
> 
> t[[1]][[2]] "is equivalent to" t[[1,2]], "but is clumsier to write"
> 
> and that is all that I can find concerning the difference between the
> two ways of refering to a list element.
> 
> But...
> 
>  In[37]:=
>  alist = {{1,2},{3,4}}; alist[[2,1]] = 7 ; alist
> 
>  Out[37]=
>  {{1,2},{7,4}}
> 
> So far so good, and yet...
> 
>  In[39]:=
>  anotherlist = {{1,2},{3,4}}; anotherlist[[2]][[1]] = 7
>  Set::"setps":
>     "\!\(anotherlist \\[LeftDoubleBracket] 2 \\[RightDoubleBracket]\) in
> \
>  assignment of part is not a symbol."
>  Out[39]=
>  7
> 
> What's the problem here?  Is there a difference between the two ways of
> referencing list elements that "the book" neglects to mention?

Tim:

t[[1]][[2]]  is equivalent to t[[1,2]] for extraction of parts, as
indicated by  your example.
But for part resetting, as in your second example, it in not equivalent.
The catch is that part resetting like
                        a[[3]] = 7
works only when 
1) a is a symbol that has been assigned an expression expr with
                        a = expr
                        (a:= expr will not do) 2) expr has a part 3

During evaluation,
                anotherlist[[2]][[1]] = 7  
                (that is (anotherlist[[2]])[[1]] = 7 ) becomes          

                {3,4} [[1]] = 7
which does not meet requirment 1).

Allan
-- 
Allan Hayes
Training and Consulting
Leicester, UK
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44 (0)116 271 8642



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