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Alan
Posts:
95
Registered:
3/29/11


list mutability (very basic question)
Posted:
Sep 21, 2013 4:21 AM


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 Lvalue will be a helpful answer, and perhaps it should be, but I'm looking for a different angle on the answer.)
Thanks, Alan Isaac



