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Re: list mutability (very basic question)

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  • Subject: [mg131712] Re: list mutability (very basic question)
  • From: Itai Seggev <itais at>
  • Date: Mon, 23 Sep 2013 21:57:09 -0400 (EDT)
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> I just realized that I am unclear on a very basic point: in what ways are
> lists mutable?
> Example:
> {0}[[1]]=1; (* a. this is an error *)
> x={0}
> x[[1]]=1 (* b. this is not an error *)
> ReplacePart[x, 1 -> 2]  (* c. this is differently not an error *)
>ndices.html It is important to understand that ReplacePart always creates a
> new list. It does not modify a list that has already been assigned to a
> symbol the way does.

There two levels here; what I'll call user-mutability and internal-mutability.  
On a user level, Set modifies the (definition of) the variables it acts upon, 
so after it has evaluated the variable will have a new value, and it returns 
the new value of the variable.  ReplacePart does not; it will first make a 
copy, modify it, and then return that new expression.  I think this is 
relatively clear

The there's internal mutability, whether and when expressions can be shared and 
mofied in place.  This depends very sensitivily on your state.  For example, if 

a = {1,0}
b = a
b[[1]]= 2

Then a and b can point the same list after the 2nd input is evaluated, but not 
after the third.  So in this case it would likely generate a copy and b would 
be updated to point to the new location.  For a user perspective how, it is 
still "b that was modified". 

> I think this means that after b. (i.e., Set[Part[x,1],1]) that x still
> references the same location in memory. If so, why is a. an error? (I
> realize that someone will think that saying that {0} is not a L-value will
> be a helpful answer, and perhaps it should be, but I'm looking for a
> different angle on the answer.)

I'm confused by this comment.  Do you think that 0=1 should not be an error?  If 
you think it should be an error (as it is in every language I'm familiar with), 
then why should {0}[[1]]=1, which would essentially be the same thing, not be 
an error?


Itai Seggev
Software Technology

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