34. Return

Note

North is toward the top of the screen; East is toward the right, West toward the left and South toward the bottom.

As you know, Reeborg is not exactly in good shape. He can only turn left, has an oil leak, can only see walls when they are right in front of him or immediately to his right, and can see tokens only when he is literally standing on top of them. Reeborg has also a (somewhat broken) compass which he can used to find out if is is facing north ... or not. To find out if he is facing north, you can ask Reeborg to do the test is_facing_north().

Try this!

Choosing an appropriate world, write a short program that will ensure that Reeborg will turn left until he faces north, no matter what his starting orientation is.

34.1. Getting results from functions

Tests like is_facing_north() are actually Javascript functions. They differ from other functions like turn_left() or move() in that they return a value. Let’s start by considering a simple example:

Try this!

function interrupted_two_steps() {
    move();
    return;
    move();
}

interrupted_two_steps();

If you use the strict version of Javascript, you will get a warning Unreachable ‘move’ after ‘return’ and see that Reeborg only takes one step. You can get rid of this warning by using the “regular” version of Javascript.

The return keyword can actually be accompanied by something else.

Try this!

For example, try the following:

function north(){
   return is_facing_north();
}

while (!north()) {
    turn_left();
}

As you have tried it, you noticed that north() was giving the same result as is_facing_north(); that is the effect of the return statement. We can use this to have Reeborg be able to identify orientations other than North. First, note that if Reeborg turns left 4 times, he will be back to its initial orientation; we do want Reeborg to end the test in the same orientation as that which he had at the start. Now, suppose we would like to know if Reeborg was facing South. We could ask Reeborg to turn left twice, note if his orientation is North (which it should be if he was facing South) or not, turn left twice more, to go back to its original orientation, and tell us what he remembered using the return statement. One thing we need to do: have Reeborg use a variable to remember its orientation after two left turns:

function is_facing_south(){
    var remember;
    turn_left();
    turn_left();
    remember = is_facing_north();
    turn_left();
    turn_left();
    return remember;
}

// now, ensure that Reeborg is facing South
while (!is_facing_south()) {
    turn_left();
}

Try it!

It will not take you long, and you will be ready for the next exercise!

The above way works ... but, depending on its initial orientation, you might get dizzy if you keep track of all left turns that Reeborg has to make: when its orientation is not South, for each left turn that he makes to change its orientation, he must make 4 more to determine its new orientation!

In a future tutorial, when we talk about Object-Oriented Programming, we will find a way, by digging in Reeborg’s built-in program, to fix its compass and have it determine its orientation without getting dizzy.

Mini-quiz!

Write a program that has Reeborg face West, no matter what his original orientation is. Test it with Reeborg in various starting orientations, by giving him a few turn_left() instructions first.

34.2. How to think about return

Suppose we have the following:

function some_function () {
    ...
    return something;
}

... = some_function();

In this case, the call to some_function() on the last line gets replaced by the value of something which is what follows the return keyword. If nothing follows return the result is undefined.

More returns

Reeborg can determine if there is a wall in front of him, using front_is_clear(), or if there is a wall to his right, using right_is_clear(). Write a test that has Reeborg turn left 4 times, so that he ends up back in the same orientation that he started with, but that returns true if there is no wall to his left.

Challenges!

Use the test you have written to have Reeborg get out of worlds Maze 1 and Maze 2 by following the left wall. Do the same for solving challenges for worlds Storm 1 and Storm 2, that is, go around the one-room houses in the opposite direction compared with your previous solutions.