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linkedbag.py
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linkedbag.py
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"""
Author: Leigh Stauffer
Project 4
File: linkedbag.py
"""
from node import Node
from abstractbag import AbstractBag
class LinkedBag(AbstractBag):
"""A link-based bag implementation."""
# Constructor
def __init__(self, sourceCollection = None):
"""Sets the initial state of self, which includes the
contents of sourceCollection, if it's present."""
self._items = None
AbstractBag.__init__(self, sourceCollection)
# Accessor methods
def __iter__(self):
"""Supports iteration over a view of self."""
cursor = self._items
while not cursor is None:
yield cursor.data
cursor = cursor.next
# Mutator methods
def clear(self):
"""Makes self become empty."""
self._items = None
def add(self, item):
"""Adds item to self."""
self._items = Node(item, self._items)
self._size += 1
def count(self, item):
"""Expects an item as an argument and returns the number of instances
of the item in the bag."""
probe = self._items
itemCount = 0
while probe != None:
if item == probe.data:
itemCount += 1
probe = probe.next
return itemCount
def remove(self, item):
"""Precondition: item is in self.
Raises: KeyError if item in not in self.
Postcondition: item is removed from self."""
# Check precondition and raise if necessary
if not item in self:
raise KeyError(str(item) + " not in bag")
# Search for the node containing the target item
# probe will point to the target node, and trailer
# will point to the one before it, if it exists
probe = self._items
trailer = None
for targetItem in self:
if targetItem == item:
break
trailer = probe
probe = probe.next
# Unhook the node to be deleted, either the first one or one
# thereafter
if probe == self._items:
self._items = self._items.next
else:
trailer.next = probe.next
# Decrement logical size
self._size -= 1