Problem 6-1

You are given the following superclass. Do not modify this.

class Container(object): """ Holds hashable objects. Objects may occur 0 or more times """ def __init__(self): """ Creates a new container with no objects in it. I.e., any object occurs 0 times in self. """ self.vals = {} def insert(self, e): """ assumes e is hashable Increases the number times e occurs in self by 1. """ try: self.vals[e] += 1 except: self.vals[e] = 1 def __str__(self): s = "" for i in sorted(self.vals.keys()): if self.vals[i] != 0: s += str(i)+":"+str(self.vals[i])+" " return s 

Write a class that implements the specifications below. Do not override any methods of Container.

class Bag(Container): def remove(self, e): """ assumes e is hashable If e occurs one or more times in self, reduces the number of times it occurs in self by 1. Otherwise does nothing. """ # write code here def count(self, e): """ assumes e is hashable Returns the number of times e occurs in self. """ # write code here 

For example,

d1 = Bag() d1.insert(4) d1.insert(4) print(d1) d1.remove(2) print(d1) 

prints

4:2 4:2 

For example,

d1 = Bag() d1.insert(4) d1.insert(4) d1.insert(4) print(d1.count(2)) print(d1.count(4)) 

prints

0 3 

Problem 6-2

Write a method in Bag such that if b1 and b2 were bags then b1+b2 gives a new bag representing the union of the two bags.

For example,

a = Bag() a.insert(4) a.insert(3) b = Bag() b.insert(4) print(a+b) 

prints

3:1 4:2 

Problem 6-3

Write a class that implements the specifications below. Do not override any methods of Container.

class ASet(Container): def remove(self, e): """assumes e is hashable removes e from self""" # write code here def is_in(self, e): """assumes e is hashable returns True if e has been inserted in self and not subsequently removed, and False otherwise.""" # write code here 

For example,

d1 = ASet() d1.insert(4) d1.insert(4) d1.remove(2) print(d1) d1.remove(4) print(d1) 

prints

4:2 # from d1.remove(2) print # (empty) from d1.remove(4) print 

For example,

d1 = ASet() d1.insert(4) print(d1.is_in(4)) d1.insert(5) print(d1.is_in(5)) d1.remove(5) print(d1.is_in(5)) 

prints

True True False