Project: a ticket office

What you will be able to do

After this lesson you can

• write the rules of a real decision before you write any code,
• build a program in two steps, where the second decision uses the result of the first,
• test it with a table, and
• extend the program with a new rule and test again.

The problem

A small museum sells tickets. The rules are:

1. Children under 5 enter free.
2. Children from 5 to 12 pay 50 rupees.
3. People from 13 to 59 pay 100 rupees.
4. People of 60 and more pay 70 rupees.
5. On a weekend, every ticket that is not free costs 20 rupees more.

Step 1: write the test table first

Before the program, work out the expected price for some inputs.

The edges are 5, 13 and 60. The rule of the weekend has its own special case: the free ticket. This table is the answer key of the project.

Step 2: the program in two decisions

The price by age is the ladder of the last lesson. Then a second decision adds the weekend.

Step 3: run the test table

Change age and weekend for each row of the table, and compare the price with your expected one. For the row 4, True: the first step gives 0, and the second step does not add 20, because price > 0 is False. This is the case that a program without the protection gets wrong, and it is why we put it in the table.

The jumps in the graph are the edges: 5, 13 and 60. Those are the ages to put in the test table.

Step 4: extend it

A project is yours when you change it. Choose one, and do it without looking at a solution. After each change, add rows to the test table.

1. A group rule. A group of 10 people or more gets 10 percent off the total.
2. A student rule. A student of 13 to 22 pays 80 rupees on a weekday. Where does it go in the ladder?
3. A closed day. On Monday, the museum is closed. The program shows a message and no price.
4. A coupon. A learner who types the code FREE20 gets 20 rupees off.

Common mistakes

• Writing the program before the table. You test what you wrote, not what is needed.
• Forgetting the free ticket in the weekend rule. The table has a row for it, for a reason.
• A new if where you need elif. In step 1, the choices exclude each other.
• Not testing the edges. 5, 13 and 60 are the likely mistakes.
• A missing else. The variable price may not exist for an age that is not caught.

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: the ticket office. The checker runs your program for ten pairs of age and weekend, and shows the pairs that fail.
2. Build. Do Step 4 for one extension.
3. Stretch. Open the problem The roller coaster from the next course, And, Or, Not, and use and to write the rule in one line.

Remember these

• Write the rules first, then the test table, then the program.
• Use a ladder for the price, and a second decision for the extra.
• The special cases (a free ticket) need a protection and a row in the table.
• Test the edges, and test again after every change.

Check yourself

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

1. Why is the age 4 on a weekend an important test?

2. Which age values are the edges?

3. Why do we use elif in the price ladder, and a separate if for the weekend?