Sets are lists with no duplicate entries. Let's say you want to collect a list of words used in a paragraph. 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:
print(set("my name is Eric and Eric is my name".split()))
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(["Jeff", "Mark", "Alex"])
print(a)
b = set(["John", "Alex"])
print(b)
To find out which members attended both events, you may use the "intersection" method:
a = set(["Jeff", "Mark", "Alex"])
b = set(["John", "Alex"])
print(a.intersection(b))
print(b.intersection(a))
To find out which members attended only one of the events, use the symmetric_difference method:
a = set(["Jeff", "Mark", "Alex"])
b = set(["John", "Alex"])
print(a.symmetric_difference(b))
print(b.symmetric_difference(a))
To find out which members attended only one event and not the other, use the difference method:
a = set(["Jeff", "Mark", "Alex"])
b = set(["John", "Alex"])
print(a.difference(b))
print(b.difference(a))
To receive a list of all participants, use the union method:
a = set(["Jeff", "Mark", "Alex"])
b = set(["John", "Alex"])
print(a.union(b))
Exercise
Use the given lists to print out a set containing all the participants from event A which did not attend event B.
a = ["Michael", "John", "Marie", "Cindy"]
b = ["John", "Jack", "Rowel"]
a = ["Michael", "John", "Marie", "Cindy"]
b = ["John", "Jack", "Rowel"]
A = set(a)
B = set(b)
print(A.difference(B))
test_output_contains("{'Cindy', 'Marie', 'Michael'}")
success_msg("Excellent!")