assign_8.txt
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Exercise for week 8, Tues Nov 29, bring exercise to turn in (hardcopy)

Reading: see course web page

Exercise: Write a module bag.py that defines a class named bag.  A bag (also
called a multiset) is a collection without order (like a set) but with
repetition (unlike a set) --- an element can appear one or more times
in a bag.  Implement bag as a subclass of dictionary where each bag
element is a key and its value is an integer that represents its
multiplicity (the number of repetitions).  For example in b1 =
bag({'a':2, 'b':3}), b1 is a bag where 'a' occurs twice (has
multiplicity 2) and 'b' occurs three times.  Provide a bag union
operator + (plus sign) that operates on two bags and returns a third
bag, their union.  The bag union contains all of the elements of both
bags, with their multiplicities added.  For example, after b2 =
bag({'b':1, 'c':2}), then b1 + b2 == bag({'a':2, 'b':4', 'c':2})