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161 lines
5.2 KiB
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161 lines
5.2 KiB
Markdown
---
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id: karel
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slug: /recursion/karel
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title: Recursion and backtracking with Robot Karel
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description: |
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A problem with too many restrictions.
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tags:
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- python
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- karel
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- recursion
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- backtracking
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last_update:
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date: 2022-11-29
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---
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- [Sources](pathname:///files/algorithms/recursion/karel-1.tar.gz)
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## Introduction
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In this exercise we will be working with a Robot Karel and with a »very« limited
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resources. The point of this exercise is to show how powerful recursion and
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backtracking can be even without anything else at your hand.
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## Your environment and problem description
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### Environment
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You are given a robot that is present in a maze and is looking for an exit. Maze
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consists of different walls and exit is marked with a single so-called “beeper”.
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Walking into a wall results in a permanent damage of the robot.
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### Interface
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You can control the robot using the following interface:
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- actions — you can use them to change the current state of the robot and its
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surroundings
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- `robot.step()` — moves robot one step further
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- `robot.turn_left()` — turns robot 90-degrees counter-clockwise
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- notice that you are not given `turn_right` or `turn_around`, but feel free
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to implement them yourself
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- `robot.pick_beeper()` and opposite operation `robot.put_beeper()` — that
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allows you to either pick or put “beeper” from or onto the current position
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- queries — you can use them to check the current state of the robot and its
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surroundings
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- `robot.beepers_present()` — to check if there are any beepers at the robot's
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current location
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- `robot.left_is_clear()` — to check if you can step to the left
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- analogically for `front` and `right`
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:::warning[caution]
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Helper functions / procedures are allowed. Return values are allowed.
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**Variables are prohibited!**
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:::
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### Problem
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Your task is to decide whether there is an exit from the maze or not. You can see
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an example of a maze here:
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## Simple problem to get familiar with the robot
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If you feel completely lost after the previous description, let me start you off
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with a simpler problem.
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You are standing in front of the stairs, your task is to walk up the stairs.
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You can see an example of such map here:
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## Brainstorm the idea
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As a first step write down any ideas and things that you have noticed or came to
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your mind. Ideally:
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1. Write down a nested list of the problems
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2. Write down list of problems that can happen
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3. Write down **anything** you consider important to solving the problem
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:::tip Example
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**Problem**: I want to find out whether the display on smartphone should rotate.
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- nested list of problems
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- Check if display has been rotated
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- Read data from some sensor
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- From what sensor
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- In what format are the data I have read?
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- How do I communicate with the sensor?
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- What is the meaning of the data that I got?
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- How can I process it?
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- any problems that can happen
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- What if the sensor doesn't work?
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- What if the data doesn't conform to the specification?
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- What if my formulas are wrong?
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- anything important
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- I could probably use gyroscope.
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- I should probably look up the datasheet for that module.
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- I could write some tests to verify that my computations are correct.
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:::
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## »Rough« pseudocode
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As a next step write a **mock up** of a pseudocode solving the problem, you are
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allowed to use comments as placeholders for bigger chunks of code.
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Those comments are also a very good hints for decomposition and short, but
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descriptive, commnets (if they are short enough and you decide not to factor them
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out to separate functions).
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:::tip
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The smaller the function is, the easier it is to test it and argue about its
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correctness.
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:::
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## »Proper« pseudocode
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If you are satisfied with the _»rough« pseudocode_, it's time to convert it into
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a proper one. Get rid of the uncertain pieces of functionality and replace them
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with proper pseudocode, i.e. list of the things that should happen in its place.
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## Library
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If you got here, and you **actually** wrote down the pseudocode, you can try your
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solution after downloading the sources linked at the beginning. If you download
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the ZIP-file, you can there:
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- `generate_mazes.py` - that was used to generate the same maze with beepers in
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different locations
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- `karel_tk.py` - library which can run Karel given the his world
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- documentation can be found [here](https://www.fi.muni.cz/~xfocko/ib111/10/docs/)
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- also requires Tk Python library to be installed (it should be included in
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majority of Python installations)
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- `*.kw` - which represent multiple worlds for Karel I have prepared
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- `skeleton.py` - skeleton for your solution, needs to be put in the same directory
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as `karel_tk.py` and takes path to the world as a first argument, example usage:
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```
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$ python3 skeleton.py stairs.kw
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```
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- of course, this file can be renamed ;)
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## Solution
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Solution to this problem will be released as a second part, so that you can try
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it out by yourself without any influence of “example solution”.
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If you want to get any feedback, feel free to mail me your solution (including
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all the steps that lead to your final solution, if you wish to get feedback on
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those too).
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