On 02/14/2014 12:08 PM, dave em wrote:
Hello,
Background: My twelve y/o son and I are still working our way through Invent
Your Own Computer Games with Python, 2nd Edition.
(We finished the Khan Academy Javascript Tutorials is the extent of our
experience)
He is asking a question I am having trouble answering which is how a variable
containing a value differs from a variable containing a list or more
specifically a list reference.
I tried the to explain as best I can remember is that a variable is assigned to
a specific memory location with a value inside of it. Therefore, the variable
is kind of self contained and if you change the variable, you change the value
in that specific memory location.
However, when a variable contains a list reference, the memory location of the
variable points to a separate memory location that stores the list. It is also
possible to have multiple variable that point to the memory location of the
list reference. And all of those variable can act upon the list reference.
Question: Is my explanation correct? If not please set me straight :)
And does anyone have an easier to digest explanation?
Thanks in advance,
Dave
You've got it backwards. In Python, /everything/ is a reference. The
variable is just a "pointer" to the actual value. When you change a
variable, you're just changing the memory location it points to.
Strings, ints, tuples, and floats behave differently because they're
/immutable/. That means that they CANNOT modify themselves. That's why
all of the string methods return a new string. It also means that, when
you pass one two a function, a /copy/ of it is made and passed instead.
So, back to the original subject. Everything is a reference. When you do
this:
|x = [1,2,3]
x = [4,5,6]
|
x now points to a different memory location. And, when you do this:
|x[0] =99000
x[0] =100
|
you're just changing the memory location that |x[0]| points to.
--
--Ryan
If anybody ever asks me why I prefer C++ to C, my answer will be simple: "It's
becauseslejfp23(@#Q*(E*EIdc-SEGFAULT. Wait, I don't think that was nul-terminated."
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