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29. Surprises

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31. Summary

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30. Revisiting some challenges

It is time to solve some of the challenges introduced early on.

Using while, if, else, etc., you will write programs that will be able to accomplish various tasks. In writing these programs, you should keep the following in mind:

  1. Remember rule #2 and choose names that are descriptive for the problems you are attempting to solve
  2. Follow the principles given in rule #4: start by solving simpler problems first and generalize your solution so that it can be applied to more than one problem
  3. Remember rule #3: avoid repeating code as much as possible.

Now, remember rule #1 ... and make sure that, this time, you solve all the challenges mentioned below.


Make sure you save your various solutions as you might want to come back to them later.

30.1. Before harvesting

Select world Harvest 3 which occurred shortly after the garden was seeded. Some carrot seeds sprouted well, others yielded two or three plants while some did not sprout at all. Help Reeborg weed out the excess seedlings, and plant some new seeds at locations where some were missing.

Do it!

Write a program as described above.

30.2. Ready to harvest

The weeding went well (I hope!), the growing season is over and it is time to harvest. Select world Harvest 1 and/or Harvest 2 and have Reeborg collect all the carrots. You should write a single program that would work for both worlds.

30.3. Stormy weather

It was a beautifully sunny day. Reeborg was playing outside with his friend. Suddenly, it started to rain and Reeborg remembered that the windows in his house were all open. So Reeborg went back to his house and stopped in front of the door, unsure of how to proceed.

Do it!

Using the build_wall() instruction and either wall_on_right() or wall_in_front(), or both, but not right_is_clear() nor front_is_clear(), help Reeborg close the windows of his house. When Reeborg finishes his task, he will stand in the doorway, watching the rain fall, waiting for it to stop before he can go back and play outside. The world is Rain 1.

30.4. Reeborg’s friend’s turn.

Erdna, Reeborg’s friend, lives in a bigger house as shown on Rain 2. Erdna was playing outside with Reeborg when it started raining. Help Erdna close the windows in her house.

Try it!

Your challenge is to write a single program that will work for both Reeborg and Erdna, again without using right_is_clear() nor front_is_clear()... however, it might be a bit tricky.


To determine if there’s a window at a given location, a robot may have to take an extra step and, depending on the result, may have to turn around, go back and close that window.

Do it again!

This time, write a program that does make use of right_is_clear() or front_is_clear(), in addition to wall_on_right() or wall_in_front() to allow Reeborg and Erdna to close the windows in their houses. The number of steps (move() and turn_left()) taken by the robots should be reduced since they can get more information about their world directly.

30.5. After the storm

The wind blew really hard last night. There are fallen leaves everywhere outside Reeborg’s house. His parents asked him to go and clean up the path leading to the curb, Storm 1, as well as the driveway: Storm 2.

Your turn!

Reeborg must collect all the leaves and put them in the compost bin, making sure to close the lid using build_wall(). Reeborg can determine if it carries objects (dead leaves here) by using the condition carries_object() either in an if statement or a while statement.

30.6. More yard work!

Reeborg’s parents are so proud of his work, that they ask him to pick up all the leaves that got blown away in their backyard during the windstorm, as illustrated on Storm 3. Have Reeborg pick up all the leaves and put it in the compost bin.

Your turn!

Write a program that will work in all three Storm worlds.

30.7. Finding the center

Have a look at the worlds Center 1 to Center 3.

Do this!

Write a single program that has Reeborg put a token at the geometric center of the rectangular room he finds himself in. Begin with the simplest case, making progressive improvements to tackle the more complicated cases. Note that this is a more challenging task.


You may find it useful to have Reeborg, who carries two tokens, drop one token at each end of a line. Then, by picking one token at one end and moving it one step, going back and forth until both tokens are at the same position, the location of the center can be found. When you know how to do this in one dimension (along a horizontal line), you can use that location as the starting point to find the location along the other location (a vertical line).