Re: Python and Lisp : car and cdr
On 06/18/11 00:45, Franck Ditter wrote: Hi, I'm just wondering about the complexity of some Python operations to mimic Lisp car and cdr in Python... def length(L) : if not L : return 0 return 1 + length(L[1:]) Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice a copy of a segment of L, or do I actually get a pointer to something inside L ? Is the above function length O(n) or probably O(n^2) ? Where are such implementation things (well) said ? Thanks, franck Your function does not mimic Lisp's car/cdr. This one does: def car(L): return L[0] def cdr(L): return L[1] def length(L): if not L: return 0 return 1 + length(cdr(L)) L = (a, (b, (c, (d, None length(L) is O(n) -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
Lie Ryan wrote: On 06/18/11 00:45, Franck Ditter wrote: Hi, I'm just wondering about the complexity of some Python operations to mimic Lisp car and cdr in Python... def length(L) : if not L : return 0 return 1 + length(L[1:]) Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice a copy of a segment of L, or do I actually get a pointer to something inside L ? Is the above function length O(n) or probably O(n^2) ? Where are such implementation things (well) said ? Thanks, franck Your function does not mimic Lisp's car/cdr. This one does: def car(L): return L[0] def cdr(L): return L[1] IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ? def length(L): if not L: return 0 return 1 + length(cdr(L)) How is this different from regular ol' 'len' ? ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
On Sun, Jun 19, 2011 at 10:56 PM, Ethan Furman et...@stoneleaf.us wrote: Lie Ryan wrote: def cdr(L): return L[1] IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ? In LISP, a list is a series of two-item units (conses). L = (a, (b, (c, (d, None This represents the LISP equivalent of [a, b, c, d] in Python. A list is a linked list, not an array (as in Python). IANAL either though, someone else may wish to clarify the advantages of this system. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
On Sun, 19 Jun 2011 05:56:27 -0700, Ethan Furman wrote: Lie Ryan wrote: On 06/18/11 00:45, Franck Ditter wrote: Hi, I'm just wondering about the complexity of some Python operations to mimic Lisp car and cdr in Python... def length(L) : if not L : return 0 return 1 + length(L[1:]) Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice a copy of a segment of L, or do I actually get a pointer to something inside L ? Is the above function length O(n) or probably O(n^2) ? Where are such implementation things (well) said ? Thanks, franck Your function does not mimic Lisp's car/cdr. This one does: def car(L): return L[0] def cdr(L): return L[1] IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ? No. Each cell in a Lisp-style linked list has exactly two elements, and in Python are usually implemented as nested tuples: (head, tail) # Annoyingly, this is also known as (car, cdr). where head is the data value and tail is either another Lisp-style list or a marker for empty (such as the empty tuple () or None). So a one-element linked list might be given as: (42, None) A two element list: (42, (43, None)) Three element list: (42, (43, (44, None))) and so forth. So while you could harmlessly use a slice L[1:], there is no point, since L[1:] will have at most a single element. def length(L): if not L: return 0 return 1 + length(cdr(L)) How is this different from regular ol' 'len' ? The point is to duplicate Lisp's implementation, not to be useful :) Regular len will return 2, no matter how many elements you have in the linked list, because it doesn't look at the tail recursively. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
On Sun, 19 Jun 2011 15:56:27 +0300, Ethan Furman et...@stoneleaf.us wrote: Lie Ryan wrote: def length(L): if not L: return 0 return 1 + length(cdr(L)) How is this different from regular ol' 'len' ? It's better, because len() can't overflow the stack. ;) -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
Ethan Furman et...@stoneleaf.us writes: def car(L): return L[0] def cdr(L): return L[1] IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ? Not for the linked list implementation he presented. def length(L): if not L: return 0 return 1 + length(cdr(L)) How is this different from regular ol' 'len' ? len would just return 2 for every linked list, and would raise an exception for empty list (represented by None in Lie's implementation). A more Pythonic implementation would represent the linked list as a first-class objects with car and cdr being attributes, allowing for fairly natural expression of __len__, __iter__, etc. For example: class List(object): __slots__ = 'car', 'cdr' def __init__(self, it=()): it = iter(it) try: self.car = it.next() except StopIteration: pass else: self.cdr = List(it) def __len__(self): if not hasattr(self, 'cdr'): return 0 return 1 + len(self.cdr) def __iter__(self): head = self while hasattr(head, 'cdr'): yield head.car head = head.cdr def __repr__(self): return %s(%r) % (type(self).__name__, list(self)) l = List([1, 2, 3]) l List([1, 2, 3]) l.car 1 l.cdr List([2, 3]) l.cdr.cdr.car 3 l.cdr.cdr.cdr List([]) tuple(l) (1, 2, 3) -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
Ethan Furman wrote: IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ? Ah, thanks all for the clarification. ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
On 6/19/2011 9:24 AM, Steven D'Aprano wrote:
No. Each cell in a Lisp-style linked list has exactly two elements, and
in Python are usually implemented as nested tuples:
(head, tail) # Annoyingly, this is also known as (car, cdr).
where head is the data value and tail is either another Lisp-style list
or a marker for empty (such as the empty tuple () or None).
So a one-element linked list might be given as:
(42, None)
A two element list: (42, (43, None))
Three element list: (42, (43, (44, None)))
and so forth. So while you could harmlessly use a slice L[1:], there is
no point, since L[1:] will have at most a single element.
It should be noted that the head element of any 'list' can also be a
'list' (as with Python lists),
t = { { (1,None), (2,(3,None)) ), ( (4,(5,None)), (6,None) ) )
so that the structure is actually a tree, which is a much more general
data structure than a true sequence of atoms. But TREP (for
tree-processing) is not as catchy as LISP (for list processing).
--
Terry Jan Reedy
--
http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
* 2011-06-19T12:20:32-04:00 * Terry Reedy wrote: On 6/19/2011 9:24 AM, Steven D'Aprano wrote: No. Each cell in a Lisp-style linked list has exactly two elements, and in Python are usually implemented as nested tuples: (head, tail) # Annoyingly, this is also known as (car, cdr). where head is the data value and tail is either another Lisp-style list or a marker for empty (such as the empty tuple () or None). It should be noted that the head element of any 'list' can also be a list' (as with Python lists), Both the head and tail elements of a cons cell can refer to any Lisp objects. Cons cell is a general-purpose primitive data type but it is _often_ used to build lists and trees so the tail element often refers to another cons cell (or nil that terminates the list). Let's not forget that Lisp's program code itself is built on such trees of cons cells. Lisp code itself is represented in this primitive Lisp data type. That's why Lisp is so powerful in meta programming. It's trivial to use Lisp functions to create Lisp code. -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
On Fri, 17 Jun 2011 16:45:38 +0200, Franck Ditter wrote: Hi, I'm just wondering about the complexity of some Python operations to mimic Lisp car and cdr in Python... def length(L) : if not L : return 0 return 1 + length(L[1:]) Python's lists are arrays/vectors, not linked lists. Should I think of the slice L[1:] as (cdr L) ? No. I mean, is the slice a copy of a segment of L, Yes. or do I actually get a pointer to something inside L ? No. Is the above function length O(n) or probably O(n^2) ? O(n^2). And Python doesn't do tail-call optimisation, so you're likely to run out of stack for a large list. -- http://mail.python.org/mailman/listinfo/python-list
Python and Lisp : car and cdr
Hi, I'm just wondering about the complexity of some Python operations to mimic Lisp car and cdr in Python... def length(L) : if not L : return 0 return 1 + length(L[1:]) Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice a copy of a segment of L, or do I actually get a pointer to something inside L ? Is the above function length O(n) or probably O(n^2) ? Where are such implementation things (well) said ? Thanks, franck -- http://mail.python.org/mailman/listinfo/python-list
Re: Python and Lisp : car and cdr
On Fri, Jun 17, 2011 at 8:45 AM, Franck Ditter fra...@ditter.org wrote: Hi, I'm just wondering about the complexity of some Python operations to mimic Lisp car and cdr in Python... def length(L) : if not L : return 0 return 1 + length(L[1:]) Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice a copy of a segment of L, or do I actually get a pointer to something inside L ? The slice is a copy of a segment of L. Is the above function length O(n) or probably O(n^2) ? O(n^2). If you want to implement Lisp-style list processing in Python, Python lists are not the most efficient data type to do it with. I would suggest using 2-element tuples to represent cons cells and building up from there. Also note that Python does not do tail recursion optimization, so recursion in general is inefficient and prone to stack overflow if the data structure is large enough. Where are such implementation things (well) said ? http://docs.python.org/tutorial/introduction.html#lists -- http://mail.python.org/mailman/listinfo/python-list
