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Re: list mutability (very basic question)

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  • Subject: [mg131717] Re: list mutability (very basic question)
  • From: Murray Eisenberg <murray at>
  • Date: Mon, 23 Sep 2013 21:58:50 -0400 (EDT)
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I missed the beginning of this thread, but I just wanted to note that the confusion is deepened when you contrast evaluating


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

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


On Sep 22, 2013, at 2:47 AM, Alan G Isaac <alan.isaac at> 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
Mathematics & Statistics Dept.      
Lederle Graduate Research Tower           
University of Massachusetts 
710 North Pleasant Street  
Amherst, MA 01003-9305

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