Mark Tarver <dr.mtar...@ukonline.co.uk> writes:
> But are Python lists also indistinguishable from conventional
> Lisplists for list processing.
>
> For example, can I modify a Python list non-destructively?
No.
> Are they equivalent to Lisp lists. Can CAR and CDR in Lisp be thought
> of as
>
> def car (x):
> return x[0]
>
> def cdr (x):
> return x[1:]
Not in the presence of side-effects, no.
The slice-extration operation on lists constructs a copy of the
original, and mutating the original doesn't affect the copy. Here's a
simple example.
[Load your definitions...]
In [1]: def car (x): return x[0]
...:
In [2]: def cdr (x): return x[1:]
...:
[Python] [Common Lisp]
In [3]: a = [1, 2, 3, 4] CL-USER> (setf a (list 1 2 3 4))
(1 2 3 4)
In [4]: b = cdr(a) CL-USER> (setf b (cdr a))
(2 3 4)
In [5]: a[2] = 'banana' CL-USER> (setf (third a) "banana")
"banana"
In [6]: a CL-USER> a
Out[6]: [1, 2, 'banana', 4] (1 2 "banana" 4)
In [7]: b CL-USER> b
Out[7]: [2, 3, 4] ! (2 "banana" 4)
Also, note:
In [8]: b is cdr(a) CL-USER> (eq b (cdr a))
Out[8]: False ! T
Your Python `cdr' operation conses. Until you create it explicitly,
there is no Python value corresponding to `all but the first element of
this list'.
But, apart from the performance and sharing characteristics, they're the
same. I think those are pretty big differences, though.
-- [mdw]
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