Re: list referencing with [[]][[]]
- To: mathgroup at smc.vnet.net
- Subject: [mg14024] Re: list referencing with [[]][[]]
- From: mefisher at my-dejanews.com
- Date: Wed, 16 Sep 1998 14:12:04 -0400
- Organization: Deja News - The Leader in Internet Discussion
- References: <6ten4b$3ea@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
In article <6ten4b$3ea at smc.vnet.net>,
Allan Hayes <hay at haystack.demon.co.uk> wrote:
> 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
Do you just know this from experimentation or is there somewhere to look
this up (and evaluation issues like this)?
--Mark.
mark.fisher at atl.frb.org
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