python list handling and Lisp list handling

Mark Tarver dr.mtarver at ukonline.co.uk
Fri Apr 24 17:19:22 CEST 2009


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.'

(from http://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.

Mark



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