On Apr 25, 12:07 am, Mark Tarver <dr.mtar...@ukonline.co.uk> wrote: > On 25 Apr, 05:01, Carl Banks <pavlovevide...@gmail.com> wrote: > > > > > On Apr 24, 8:19 am, Mark Tarver <dr.mtar...@ukonline.co.uk> wrote: > > > > This page says that Python lists are often flexible arrays > > > >http://www.brpreiss.com/books/opus7/html/page82.html > > > > but also says that their representation is implementation dependent. > > > As far as I see this should mean that element access in Python should > > > run in constant time. Now if so this is a boon, because generally > > > > 'A list is a sequence of elements, but it is not a single primitive > > > object; it is made of cons cells, one cell per element. Finding the > > > nth element requires looking through n cons cells, so elements farther > > > from the beginning of the list take longer to access. But it is > > > possible to add elements to the list, or remove elements.' > > > > (fromhttp://www.chemie.fu-berlin.de/chemnet/use/info/elisp/elisp_7.html) > > > > But are Python lists also indistinguishable from conventional > > > Lisplists for list processing. For example, can I modify a Python > > > list non-destructively? 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:] > > > > The idea of a list in which elements can be accessed in constant time > > > is novel to me. > > > That's because Python lists aren't lists. > > > It might be an interesting exercise to compare Python lists and Lisp > > lists, but you should do it under the understanding that they are not > > analogous types. (A Python list is analogous to a Lisp vector.) The > > two objects have almost no similarity in typical their manner of use; > > even the way you iterate through them is different. > > > You could, as you've tried to do here, operate on Python lists the > > same way you operate on Lisp lists, but you'd just be doing things the > > hard way. Whatever you're trying to do with cons, car, and cdr, > > chances are Python has a high-level way to do it built in that > > performs a lot better. > > > Then again, Lispers seem to like to reimplement high-level operations > > from low-level primitives every time they need it. So if you liked > > doing that you might not mind doing a lot of extra work using your > > homebrew cons, car, and cdr. > > > Carl Banks- Hide quoted text - > > > - Show quoted text - > > OK; I guess the answer to the question > > "Assuming the following Python encodings, and ignoring questions > of performance, would Python and Lisp lists then be observationally > indistinguishable? i.e. would these then be fair encodings?" > > is a 'yes'. Any disagreement?
In Python cdr([]) == [] And I'd think that'd be an exception in Lisp depending on variants and such. That's the only difference I can think of. Carl Banks -- http://mail.python.org/mailman/listinfo/python-list
