Project: who can enter the club?

What you will be able to do

After this lesson you can

• turn a set of rules in plain words into a program,
• build a table of all the cases before you write any code,
• write the decision flat, with the combined conditions, and
• test every case and extend the program.

The rules

A club has these entry rules.

1. A person under 18 is never allowed, not even with a ticket or a membership.
2. A person of 18 or more needs a ticket.
3. A member does not need a ticket. But the member must be 18 or more as well.

Step 1: the table of cases

Three things can change the answer: the age (under 18 or not), the ticket (yes or no), the membership (yes or no). That gives 2 times 2 times 2, which is eight cases. Write the expected answer for each, from the rules (not from a program).

Look at the table. The first four rows are the same. The age decides everything for the young. For adults, we need has_ticket or is_member.

Step 2: the program

Notice how the order helps. Because the first check removed the young, the second condition does not have to ask about the age again.

Step 3: test all eight cases

Change the three values at the top, one row of the table at a time, and compare the result with the table. You can also use a loop (Chapter 9) to run all eight at once, as you did in the previous chapter.

Three of the eight cases are welcome. The whole bottom row is too young, whatever the ticket and the membership.

Step 4: extend it

Pick one, and do it without looking at a solution. After each change, add rows to your table. With every new yes/no question, the number of cases doubles.

1. A closing time. If the hour is 23 or later, nobody enters, not even a member.
2. A guest rule. A member can bring one guest of 18 or more, who needs no ticket.
3. A student price. An adult who is a student and has a ticket pays less. Add a variable price.
4. Different messages. A young person who is a member gets a special message about the age limit.

Common mistakes

• Putting the "too young" rule last. Then a young member may be let in by the member rule.
• Mixing and and or without brackets in the middle of a long condition.
• Testing only a few cases. Eight cases take two minutes.
• Repeating the age test in every branch.

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:

1. Warm-up. Open the lab Project: who can enter the club? The checker runs your decision on all eight cases.
2. Build. Do Step 4 for one extension, and add rows to the table.
3. Stretch. Write the club rule as a single if with and and or that gives True or False, with no elif. Check it with the table.

Remember these

• Write the rules in words, then the table of every case, then the program.
• Put the check that must always win first.
• With three yes/no questions there are eight cases. Test them all.
• Each new yes/no question doubles the number of cases.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many cases does the table have if you add the closing-time question?

2. Why does the "too young" check come first?

3. A 17-year-old member with a ticket arrives. What is the result?