Course 1.2Lesson 5 of 5

Project: the school clubs

In this project we manage the clubs of a school. The members of each club are a set, and the clubs are kept in a dictionary. With the set operations the program finds all the students, the students who are in every club, the students who are only in one club, how many clubs each student joins, and the overlap of each pair of clubs. We plan the questions and work out the expected answers by hand, build the program in steps, test the unusual cases such as an empty club, and extend it with a club that members can join and leave.

about 25 min1 simulation2 practice questions

Lesson 5 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

keep related sets in a dictionary,
answer questions about groups with the set operations,
combine a loop with sets to cover all the clubs, and
test the unusual cases and extend the program.

The goal

A school has three clubs: cricket, chess and music. The principal wants to know

who are all the students that are in at least one club,
who is in every club,
who is only in the cricket club,
how many clubs each student has joined, and
how many students each pair of clubs shares.

Step 1: the data and the expected answers

The members of each club are a set (nobody is a member twice, and the order does not matter). The clubs are in a dictionary, with the club name as the key.

Club	Members
cricket	Asha, Ravi, Meera, Dev
chess	Ravi, Kabir, Meera
music	Asha, Kabir, Ana, Meera

Work out the answers by hand.

All the students: Asha, Ana, Dev, Kabir, Meera, Ravi. That is 6.
In every club: only Meera.
Only in cricket: Dev (Asha is also in music, Ravi and Meera are also in chess).
Clubs per student: Meera 3, Asha 2, Kabir 2, Ravi 2, Ana 1, Dev 1.
Cricket and chess share Meera and Ravi. Cricket and music share Asha and Meera. Chess and music share Kabir and Meera.

Step 2: the program

Run it, and compare each line with your answers. Then look at how it is built.

- All the students. Start with an empty set, and take the union with the members of each club. clubs.values() gives the sets, one by one.

- In every club. Start with everyone, and take the intersection with each club in turn. A student who is missing from any club drops out. We copy it with set(everyone), so that the first set is not changed.

- Only in cricket. clubs["cricket"] - clubs["chess"] - clubs["music"] takes away the two other clubs from cricket.

- Clubs per student. This is the counting pattern with a dictionary, used on the members of every club. A set tells us who is in a club, and the dictionary counts the clubs. Each tool is used for what it does best.

- Pairs of clubs. Two loops, with j starting at i + 1, visit each pair once. Without that, we would see both cricket and chess and chess and cricket, and also each club with itself.

Step 3: test the unusual cases

Change the data, and work out the answers by hand first.

Test	Expected
one club only	everyone is in every club, and the "only in" answer is all
an empty club	no one is in every club, and the union is not changed
a student in a single club three times in the data	still only once, because a set
two clubs with the same members	the difference is empty

The second row is the edge. Does the program still work if a club is set()? The intersection with an empty set is empty, which is the right answer: no one belongs to every club.

For each pair, the project finds the members of both clubs with &.

Step 4: extend it

Pick one or two, and do them without looking at a solution. Write the expected answers first.

Join and leave. Write functions join(clubs, club, student) and leave(clubs, club, student). Use add and discard.
Exactly two clubs. Use the counting dictionary to list the students who joined exactly two clubs.
Suggest a club. For a student, find the club that shares the most members with the clubs that the student already joined.
The biggest club. Find the club with the most members, with the "best so far" pattern.
A club that nobody left. Use ^ to find the students who are in exactly one of two clubs.

Common mistakes

Changing the first set when you meant to make a new one. in_all = everyone is a second name for the same set. Use set(everyone) or everyone.copy().
Printing a set and expecting the same order each time. Use sorted.
Visiting each pair twice. Start the second loop at i + 1.
Using a list of members and getting a student twice.
Not testing an empty club.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the lab Project: the school clubs. The checker runs your functions on clubs that are different from the example.
Build. Do Step 4 for one extension.
Stretch. Open the problem Who is in exactly two clubs? and solve it with the counting dictionary.

Remember these

A set for the members, a dictionary for the clubs: each container for what it does best.
Union with a loop covers all the groups. Intersection with a loop finds who is in all.
Copy a set before you change it.
Test the empty group, and the group that has everyone.