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5. World

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4. Newspaper delivery revisited again

Let’s go back to our newspaper delivery examples, but this time we will only look at the deliveries where money is paid for the newspaper.

Below is an attempt at a solution that would work both Ms. Lovelace’s world Newspaper 1, and Mr. Babbage’s Newspaper 2:

from library import turn_right, turn_around

def climb_up_one_floor():

def climb_down_one_floor():

def get_money():
    while object_here() :

# === End of definitions ===

while not object_here():

put()   # leave Star newspaper
while not at_goal() :

Important test!

Reproduce the above program, run it and note the result.

As you will see, when Reeborg attempts to execute the instruction put() at the line highlighted in yellow above, it stops and shouts I carry too many different objects. I don't know which one to put down!.

So, the problem is that Reeborg is carrying both money (tokens) and a newspaper (star).

A solution?

Change the program so that Reeborg puts down the newspaper before it takes the money. Does this work?

4.1. Function arguments

When there are two or more types of objects at a given location, for example a star and some tokens in the worlds mentioned above, and we instructs Reeborg to take(), Reeborg cannot know which type of object it must take. Similarly, if Reeborg carries many types of objects and we simply ask him to put(), it cannot know which one it should put down.

The solution is simple: we need to be more specific.

We have seen function arguments. And, by a happy coincidence (!), it just happens that the functions take() and put() can accept an argument. In the case that interests us, the Star newspaper is represented by a star, and the argument to use is "star" as in


For money, we use "token".

Your turn!

Change the program above to specify which type of object Reeborg must take or put down. Make sure that your program works correctly for both worlds, Newspaper 1 and Newspaper 2. You might want to use think() to occasionally speed up or slow down Reeborg’s motion.

Another option

Change again your program to replace object_here() by object_here("token") and make sure it still works.