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9. Rule number 3

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8. Newspaper delivery

Reeborg has a new job: he delivers the Northern Star newspaper. Many of Reeborg’s customers do not live on the ground floor of their building. For each of these customers, Reeborg has to do the following

  1. Pick up a single copy of the newspaper (it would be too heavy to carry them all up the stairs).
  2. Climb up the required number of floors to reach the customer’s door.
  3. For some customiers, get the money (tokens) left by the customer.
  4. Leave a copy of the newspaper (simply nicknamed the star) behind.
  5. Climb down to the ground level


In computer programming, we generally start counting at zero instead of 1. Since these worlds refer to two famous people in computer science, I thought it would be appropriate to number these worlds starting at zero.

While Reeborg has many clients, I only included two with their corresponding worlds:

  • Dr. Richard Pattis, creator of Karel (an ancestor of Reeborg); Reeborg always leaves a free copy of the newspaper at Pattis’s home, so that Dr. Pattis has learned not to leave any money. This world is Newspaper 0.
  • The nice Ada Lovelace, who often leaves some extra tokens as a gift to Reeborg; hers is world Newspaper 1.

Writing programs for worlds containing two types of objects (like a star and some tokens) that can be found at the same place is more complicated than what we have seen so far; we will see how to do this later. For now, let’s focus on world Newspaper 0 which has only one object. Later we will learn how to teach Reeborg to do deliveries in both worlds Newspaper 0 and Newspaper 1.

Try it!

Make Reeborg deliver a newspaper to Dr. Pattis.


Save your program; you will come back and use it again soon.