Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2013

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: list mutability (very basic question)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131717] Re: list mutability (very basic question)
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Mon, 23 Sep 2013 21:58:50 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • Delivered-to: l-mathgroup@wolfram.com
  • Delivered-to: mathgroup-outx@smc.vnet.net
  • Delivered-to: mathgroup-newsendx@smc.vnet.net
  • References: <20130921083830.751226A65@smc.vnet.net> <CAEtRDScmG_YsoM-NJHfkvJV74gKmLeQCB8xknARWW9L3v31zaQ@mail.gmail.com> <20130922064701.8199B6A68@smc.vnet.net>

I missed the beginning of this thread, but I just wanted to note that the confusion is deepened when you contrast evaluating

  {0}[[1]]

which causes an error, with evaluating each of the following two sequences of inputs:

  x = {0};
  x[[1]] = "Hello"'
  x
{Hello}

  {0}[[1]]
0

 
On Sep 22, 2013, at 2:47 AM, Alan G Isaac <alan.isaac at gmail.com> wrote:

> On 9/21/2013 10:15 AM, Bob Hanlon wrote:
>> Attributes[Set]
>> {HoldFirst, Protected, SequenceHold}
>
>
> Thanks, but I'm still unclear:
> why does ``{0}[[1]]=1`` raise an error?
>
> As a point of reference for my confusion,
> Python behaves in a way I find more intuitive:
> ``[0,1][1]=0`` does not raise an error.
> (This becomes an interesting difference when
> we get to argument passing in Mathematica, where HoldFirst
> becomes useful as a way to allow "in place" modification.)
>
> Back to Mathematica, am I understanding correctly that after
> ``Set[Part[x,1],1])`` that x still references the
> same location in memory?  How can I easily see that?
> (E.g., in Python, I would us the ``id`` function.)
>
> Thanks,
> Alan Isaac

---
Murray Eisenberg                                    =
murray at math.umass.edu
Mathematics & Statistics Dept.      
Lederle Graduate Research Tower           
University of Massachusetts 
710 North Pleasant Street  
Amherst, MA 01003-9305








  • Prev by Date: Re: list mutability (very basic question)
  • Next by Date: ExtendUnits package still works with old symbolic units
  • Previous by thread: Re: list mutability (very basic question)
  • Next by thread: Re: list mutability (very basic question)