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8. Comparison operators

10. Lists

# 9. 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().

Programming time!

Run the following program to select a simple world that has Reeborg start in an arbitrary orientation:

```World("/src/worlds/face_north.json", "Face north")
```

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

## 9.1. A few experiments¶

Select world Alone and execute the following program:

```repeat 4:
print("turn_left: ", turn_left() )
print("is_facing_north: ", is_facing_north() )
```

Do you notice anything interesting?

After you are done, execute the following program:

```def interrupted_two_steps():
move()
return
move()

print(interrupted_two_steps())
```

Notice how Reeborg does only one move.

Finally, run the following:

```def three():
return 3

print(three())
```

## 9.2. Getting results from functions¶

Note

If a function has no return statement, or if the return keyword is alone on a line of its own, a Python function will return a value of None, which is another Python keyword.

Tests like is_facing_north() are actually Python functions. They differ from other functions like turn_left() or move() in that they return a potentially useful value.

Try this!

Try the following:

```def north():
return is_facing_north()

while not 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:

```def is_facing_south():
turn_left()
turn_left()
remember = is_facing_north()
turn_left()
turn_left()
return remember

# now, ensure that Reeborg is facing South
while not 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 your program with this world:

```World("/src/worlds/face_west.json", "Face west")
```

## 9.3. How to think about return¶

Suppose we have the following:

```def 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 None.

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.