Bengt Richter wrote:
> On 19 Jan 2006 01:19:06 -0800, "Kay Schluehr" <[EMAIL PROTECTED]> wrote:
>
> >I want to manipulate a deeply nested list in a generic way at a
> >determined place. Given a sequence of constant objects a1, a2, ..., aN
> >and a variable x. Now construct a list from them recursively:
> >
> >L = [a1, [a2, [....[aN, [x]]...]]
> >
> >The value of x is the only one to be changed. With each value of x a
> >new instance of L is needed so that we actually have a function:
> >
> >L = lambda x: [a1, [a2, [....[aN, [x]]...]]
> >
> >I'm seeking for an efficient implementation that does not recompute the
> >whole list each time. Any ideas?
> >
> Make a custom class factory that makes a class with a1..aN and can be
> instantiated
> with x, producing an object that behaves like L
Hi Bengt,
I made such an attempt already yesterday but I disliked the result
because the __getitem__ method of the class that encapsulated a1...aN
worked as an object factory that was as expensive as list creation. My
second try is much cheaper and shifts only references:
def BinList(flat):
proto = None
class _BinList(object):
def __init__(self, x=None):
self.x = x
if proto:
self.first = proto.first
self.next = proto.next
def _nest(self, lst):
self.first = lst.pop(0)
self.next = None
if lst:
self.next = _BinList()
self.next._nest(lst)
def __getitem__(self, i):
if i==0:
return self.first
elif i==1:
if self.next:
self.next.x = self.x
return self.next
else:
return self.x
else:
raise IndexError,i
def __repr__(self):
if self.next is None:
return "[%s, %s]"%(self.first,self.x)
else:
return "[%s, %s]"%(self[0], self[1])
# instantiate prototype
bl = _BinList()
bl._nest(flat)
proto = bl
return _BinList
>>> A = BinList([1,2,3])
>>> a0 = A(7)
>>> a1 = A(9)
>>> a0
[1, [2, [3, 7]]]
>>> a1
[1, [2, [3, 9]]]
> So the question in my mind is, how are you actually going to use these "L"
> things?
The "L" things are fragments of listified Python parse trees ;)
>
> Regards,
> Bengt Richter
Regards too,
Kay
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