Answer: Debug the Robot #53

Here’s the bug

Step 2 is an extra Forward that shouldn’t be there. Beep walks one square too far.

Did you find a different fix that also gets Beep to the battery? That counts too! There is often more than one way to fix a bug. The important thing is that you tested it step by step.

The fixed program

1. Forward
2. Forward
3. Turn left
4. Forward

What the buggy program did

Here is what happens if Beep follows the program with the bug, step by step. Beep doesn’t bump into anything, but ends up in the wrong square.

Step	Command	Beep is now at	Facing	Note
1	Forward	column 2, row 3	right
2	Forward	column 3, row 3	right
3	Forward	column 4, row 3	right
4	Turn left	column 4, row 3	up
5	Forward	column 4, row 2	up

Check it with Python

This Python program pretends to be Beep. It follows the fixed program and checks for walls on the way. In Python, squares are counted from 0, so column 1, row 1 is (0, 0).

program = "FFLF"
walls = [(1, 3), (2, 3), (3, 0)]
width, height = 4, 4
x, y, facing = 0, 2, "right"

moves = {"up": (0, -1), "right": (1, 0), "down": (0, 1), "left": (-1, 0)}
turn_left = {"up": "left", "left": "down", "down": "right", "right": "up"}
turn_right = {"up": "right", "right": "down", "down": "left", "left": "up"}

for step in program:
    if step == "F":
        dx, dy = moves[facing]
        if (x + dx, y + dy) in walls or not (0 <= x + dx < width and 0 <= y + dy < height):
            print("Bump! Beep hit a wall at", (x, y))
            break
        x, y = x + dx, y + dy
    elif step == "L":
        facing = turn_left[facing]
    elif step == "R":
        facing = turn_right[facing]

print("Beep finished at", (x, y), "facing", facing)

When you run it, it prints:

Beep finished at (2, 1) facing up

The battery is at (2, 1) in Python’s counting, so Beep made it! Try putting the buggy program, "FFFLF", back in and see what Python says.