New user questions ... be gentle

[Alex Martelli]

"bushi" <bushi at spiritone.com> wrote in message
news:399FC343.FECFA832 at spiritone.com...
    [snip]
> I use pointers and linked lists a lot in the other languages.  I have
> seen some references that hint to a way to do this but it isn't very
> clear.  Any pointers on this would be very helpful. *grins*

In a sense, Python has "nothing-but" pointers, aka references.  Any
variable, any 'slot' in a Python list, any 'slot' in a Python dict,
any attribute of a Python object, ..., is-a/holds-a reference to
something, which acts much as a pointer does in Pascal.

Python's high-level structures, lists and dictionaries, reduce your
need for any explicit 'thinking about' these pointers/references as
such, to much rarer cases than it would be in Pascal or other lower
level languages.  However, it still happens, and, when it does, you
just have to _remember_ what these references ARE.  After which, you
can proceed much like in Pascal (at worst; there are often useful
shortcuts, but, you don't _have_ to use them).

For example, in Pascal you would construct a linked-list out of
records, each representing a 'node' of the linked-list, and also
holding a pointer to the 'next' node (and maybe to the 'previous'
one too, if you want a _doubly_ linked list) as well as some
'payload' which is the actual data for this node.

OK, you can do that in Python, too, if you wish:

class node:
    def __init__(self, payload, next=None):
        self.payload = payload
        self.next = next

Given a value head, that can be None or else a node (the first one
in a list), you prepend a new node carrying payload 'foo' to the
(possibly-now-empty) list by:

    head = node('foo',head)

To remove the head-node from a non-empty linked list built this
way,
    if head:
        removed,head = head,head.next
        print 'Removed:',removed.payload
    else:
        print 'List is empty'


Etc, etc.  You don't normally need to do this, but, if you do need
(or want, e.g. for didactical purposes), it's as easy as this.


Alex