13. Newspaper delivery revisited

Let’s go back to our newspaper delivery example; we’ll consider the delivery to Ms. Ada Lovelace in Newspaper 1. Below is a solution to that problem, with a few added comments.

 take("star");

 // climb up first floor
 turn_left();
 move();
 turn_left();
 turn_left();
 turn_left();
 move();
 move();

 // climb up second floor
 turn_left();
 move();
 turn_left();
 turn_left();
 turn_left();
 move();
 move();

 // climb up third floor
 turn_left();
 move();
 turn_left();
 turn_left();
 turn_left();
 move();
 move();

 // get money
 take("token");
 take("token");
 take("token");
 take("token");
 take("token");

 // leave paper
 put("star");

 // turn around
 turn_left();
 turn_left();

 // climb down floor
 move();
 move();
 turn_left();
 move();
 turn_left();
 turn_left();
 turn_left();

 // climb down floor
 move();
 move();
 turn_left();
 move();
 turn_left();
 turn_left();
 turn_left();

 // climb down floor
 move();
 move();
 turn_left();
 move();
 turn_left();
 turn_left();
 turn_left();

This solution is quite long ... and it is easy to make mistakes when typing it. We note however that there are quite a few repeated code segments for which we could create functions. We have already defined turn_right() and turn_around(); let’s use them and define a few others.

 import_lib();

 function climb_up_one_floor() {
     turn_left();
     move();
     turn_right();
     move();
     move();
 }

 function climb_up_three_floors() {
     climb_up_one_floor();
     climb_up_one_floor();
     climb_up_one_floor();
 }

 function climb_down_one_floor() {
     move();
     move();
     turn_left();
     move();
     turn_right();
 }

 function climb_down_three_floors() {
     climb_down_one_floor();
     climb_down_one_floor();
     climb_down_one_floor();
 }

 function get_money() {
     take("token");
     take("token");
     take("token");
     take("token");
     take("token");
 }

 // === End of definitions ===

 take("star");
 climb_up_three_floors();
 get_money();
 put("star"); // leave paper
 turn_around();
 climb_down_three_floors();

Each function contains no more than 5 instructions; it’s much easier to verify that each function does what it is supposed to do than verifying an entire list of commands like we have previously. Once we know that the functions do what they are supposed to do, making use of them allow us to write a complete program in 6 more lines of code - again, much easier to verify that it is right. All together, using functions to avoid repetitions, we end up with a shorter program that is also much easier to read.

It should be fairly simple to modify the above function definitions so that we could deliver a newspaper to Mr. Babbage, in world Newspaper 2.

Do it!

Modify the above program so as to deliver a newspaper to Mr. Babbage.

Later, we will see how we can write a single program, shorter than the above, that will enable us to deliver newspapers to either Ms. Lovelace or Mr. Babbage.

Since functions like get_money(), climb_up_three_floors, etc., are specific to this problem, it is probably not a good idea to save them in the library; you don’t want to have too many functions in your library so that you are always able to remember all that are there. If you do find that you use a function many times in different programs, then it is a good idea to put it in your library.