Step 3: Turning Left and Stepping Through Code


Reeborg’s Bugs

In computer jargon, a bug is an error that causes a program to behave in an unexpected way. If you are writing computer programs, you are going to have bugs in them sooner or later - everybody does. Good programmers seek to “remove” bugs or “fix” them as soon as they find that their program behaves unexpectedly.

Not so good programmers state that “bugs” are not really bugs but that they are “features” of their programs. You are going to be a good programmer, unlike the maker of Reeborg, whose program is littered with bugs. Some of the problems with Reeborg are:

  1. Reeborg has an oil leak. Oil leaks are damaging for the environment and inconvenient for Reeborg who must replenish its supplies when it’s not busy accomplishing tasks. The maker of Reeborg claims that it is a feature, as it enables you to follow Reeborg’s path, just like any programmer can learn to “trace” a program.

  2. Reeborg’s steering mechanism is not handled properly by Reeborg’s program: it can only turn left. The maker of Reeborg, once again, claims that this is a feature as it present you with an opportunity to learn about functions. You will soon learn how to program a workaround solution, enabling Reeborg to turn right, although in a wasteful fashion.

  3. Reeborg has a compass, enabling it to determine which direction it is facing. Unfortunately, yet again, the program that enables Reeborg to get the information from the compass has a bug: it only tells Reeborg if it is facing North … or not. Once again, you will learn how to implement a workaround solution soon.

  4. Reeborg can see if a wall is in front of it, and can also turn its head to the right to see if there is a wall there. However, a software “glitch” (which is another weasel term that software manufacturers use to avoid having to say that their product has a bug) prevents Reeborg’s program from properly registering a wall when it turns its head left.

Stepping Through Code

Sometimes to find the cause of bugs, it can help to break the normal flow of the program. To this end you may do one or more of the following:

  1. You can pause a program as it is running by pressing the pause button. This is similar to what people refer to as setting a breakpoint in a computer program


  2. Instead of actually pressing the pause button, you can type in the instruction pause() at any point inside a program and Reeborg will pause, awaiting your permission to continue.

  3. You can step through a program, one instruction at a time, by pressing the execute one instruction and pause, or step button. By default, the line about to be executed is highlighted; you can turn off the highlighting by clicking on a button above the code editor.


  4. You can stop a program at any point by pressing the stop button; this unfortunately may not work if you create what is known as an infinite loop, outside of Reeborg’s control. If worse comes to worst, you can always just reload the web page.


Your Turn

Open Step 3 on the Reeborg environment.

As mentioned above, Reeborg can only turn in one direction: left. To tell Reeborg to turn, you can use the turn_left() function.


Use the move() function and the turn_left() function to make Reeborg walk around the lake.

Try this!

To see what happens, let’s do some things intentionally wrong. For example, make Reeborg walk into the lake. You might also want to try not walking all the way around the lake to see what the error message will be.

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