Sets are lists with no duplicate entries. Let's say you want to collect a list of words used in a paragraph:

print set("my name is Eric and Eric is my name".split())

This will print out a list containing "my", "name", "is", "Eric", and finally "and". Since the rest of the sentence uses words which are already in the set, they are not inserted twice.

Sets are a powerful tool in Python since they have the ability to calculate differences and intersections between other sets. For example, say you have a list of participants in events A and B:

a = set(["Jake", "John", "Eric"])
b = set(["John", "Jill"])

To find out which members attended both events, you may use the "intersection" method:

>>> a.intersection(b)
set(['John'])
>>> b.intersection(a)
set(['John'])

To find out which members attended only one of the events, use the "symmetric_difference" method:

>>> a.symmetric_difference(b)
set(['Jill', 'Jake', 'Eric'])
>>> b.symmetric_difference(a)
set(['Jill', 'Jake', 'Eric'])

To find out which members attended only one event and not the other, use the "difference" method:

>>> a.difference(b)
set(['Jake', 'Eric'])
>>> b.difference(a)
set(['Jill'])

To receive a list of all participants, use the "union" method:

>>> a.union(b)
set(['Jill', 'Jake', 'John', 'Eric'])

In the exercise below, use the given lists to print out a set containing all the participants from event A which did not attend event B.

a = ["Jake", "John", "Eric"] b = ["John", "Jill"]

set(['Jake', 'Eric'])

a = ["Jake", "John", "Eric"] b = ["John", "Jill"]

A = set(a) B = set(b)

print A.difference(B)