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list mutability (very basic question)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131695] list mutability (very basic question)
  • From: Alan <alan.isaac at gmail.com>
  • Date: Sat, 21 Sep 2013 04:38:30 -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 *)

http://reference.wolfram.com/mathematica/tutorial/ManipulatingListsByTheirIndices.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.

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.)

Thanks,
Alan Isaac



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