fredoverflow / Karel

Background

Karel The Robot is a simple teaching environment for imperative programming basics. The original idea was developed in the 1970s by Richard Pattis at Stanford University:

In the 1970s, a Stanford graduate student named Rich Pattis decided that it would be easier to teach the fundamentals of programming if students could somehow learn the basic ideas in a simple environment free from the complexities that characterize most programming languages.

In sophisticated languages like Java, there are so many details that learning these details often becomes the focus of the course. When that happens, the much more critical issues of problem solving tend to get lost in the shuffle. By starting with Karel, you can concentrate on solving problems from the very beginning. And because Karel encourages imagination and creativity, you can have quite a lot of fun along the way.

This project started in 2012 due to dissatisfaction with the available Karel environments. Since then, thousands of German university students have been introduced to the basics of imperative programming via this project.

Problem solving

Problem solving means translating human-understandable problem descriptions into machine-executable programs. Ideally, machine-executable programs should also be human-understandable; we attain that ideal with abstractions.

Abstractions aid tremendously in developing solutions to problems in a top-down (decomposing a complex problem into simpler subproblems) or bottom-up (composing simple subsolutions into a complex solution) fashion.

Abstractions

Humans like to organize processes (for example, doing the laundry) in hierarchical levels of abstraction:

do laundry:

• wash laundry 🧼
• wait 1 hour ⏳
• hang laundry 🧺

wash laundry: 🧼

• put clothes into washing drum
• apply laundry detergent
• close washing drum
• put plug into socket
• choose temperature
• press start button

hang laundry: 🧺

• open washing drum
• put clothes into laundry basket
• remove plug from socket
• put clothes onto clothes line

If we keep delving deeper into lower levels of abstraction until we reach individual muscle movements, even the most simple-minded being can do the laundry by following the given instructions carefully. And how do we call such beings? Robots!

Getting started

Install Java

Karel The Robot requires Java version 8 or later: https://adoptopenjdk.net

The pre-selected version and JVM are usually fine, no need to change them.

Download Karel

Download karel.jar (~250 kb)

Start Karel (Microsoft Windows)

Open Windows explorer, navigate to the Downloads folder and double-click on karel.jar.

If double-clicking does not start Karel, the most probable causes are:

1. Java is not installed correctly:
   • You skipped the Java requirement above
   • You downloaded Java, but did not install it
   • You picked an inferior alternative to https://adoptopenjdk.net
2. karel.jar was silently renamed to karel.jar.ZIP

Apparently, some Windows browsers silently rename .jar files to .jar.ZIP during download. Since Windows explorer hides file extensions by default, you probably won't even notice the wrong extension; the file name will show up as karel.jar, and the file type as ZIP archive. Double-clicking on that file will not start Karel, but open the ZIP archive instead. You will see a, b, c, d, e, f, g, META-INF, tiles, a.class, font.png and MainKt.class.

Unfortunately, changing file extensions in Windows explorer is not trivial. I recommend downloading karel.jar from the Windows command line instead. Press the Windows key (the key on the bottom left with the Windows logo on it), write cmd and confirm with Enter. Then enter the following lines:

cd Downloads
curl -o karel.jar https://raw.githubusercontent.com/fredoverflow/karel/release/karel.jar
java -jar karel.jar

The last line verifies the download by starting Karel. If that fails, enter java -version. If that also fails, Java is not installed correctly.

Start Karel (other operating systems)

Open a terminal in the download folder and write:

java -jar karel.jar

Java animations tend to stutter on Linux. Replacing xrender with opengl may help:

java -jar -Dsun.java2d.opengl=True karel.jar

Autosave

Your code is automatically saved to a new file each time you click the start button. The save folder is named karel, and it is located in your home directory. The full path is displayed in the title bar.

Language reference

Primitive commands

Custom commands

Sometimes the same sequence of commands appears multiple times:

void roundTrip()
{
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();

    turnAround();

    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
}

You can extract such a sequence of commands into a new, custom command:

void moveAcrossWorld()
{
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
    moveForward();
}

and use it just like a primitive command:

void roundTrip()
{
    moveAcrossWorld();
    turnAround();
    moveAcrossWorld();
}

Deciding when a sequence of commands is worth extracting and choosing a good name for the custom command are essential development skills you will acquire over time.

Repeat

Instead of writing the same sequence of commands multiple times:

void dance()
{
    moveForward();
    turnLeft();
    moveForward();
    turnLeft();
    moveForward();
    turnLeft();
    moveForward();
    turnLeft();
}

you can use repeat and only write it once:

void dance()
{
    repeat (4)
    {
        moveForward();
        turnLeft();
    }
}

If/else

Sometimes you only want to do something if some condition holds:

if (onBeeper())
{
    pickBeeper();
}

Optionally, you can also specify what to do in case the condition does not hold:

if (onBeeper())
{
    pickBeeper();
}
else
{
    dropBeeper();
}

Note that conditions are only checked when control flow actually reaches them (when the corresponding line is highlighted in the code editor). Conditions are not periodically checked in the background! In large programs with lots of potentially contradicting conditionals, such periodic background checks would quickly lead to incomprehensible program behavior.

Primitive conditions

If/else if

An else with nothing but another if inside:

if (leftIsClear())
{
    turnLeft();
}
else
{
    if (rightIsClear())
    {
        turnRight();
    }
}

can be simplified by leaving out the block between the else and if:

if (leftIsClear())
{
    turnLeft();
}
else if (rightIsClear())
{
    turnRight();
}

Note that without the else, Karel might turn left and then immedately turn right again, given frontIsClear() originally held. The else prevents the second if from executing in case the first condition was already true.

Not !

An if/else with an empty first block:

if (onBeeper())
{
}
else
{
    dropBeeper();
}

can be simplified by negating the condition with a leading !:

if (!onBeeper())
{
    dropBeeper();
}

And &&

An if with nothing but another if inside:

if (frontIsClear())
{
    if (beeperAhead())
    {
        moveForward();
        pickBeeper();
    }
}

can be simplified by combining both conditions with &&:

if (frontIsClear() && beeperAhead())
{
    moveForward();
    pickBeeper();
}

Or ||

An if/else if with identical blocks:

if (!frontIsClear())
{
    turnRight();
}
else if (beeperAhead())
{
    turnRight();
}

can be simplified by combining both conditions with ||:

if (!frontIsClear() || beeperAhead())
{
    turnRight();
}

Summary compound conditions

While

if checks the condition and then executes the block at most once:

void moveForwardSafely()
{
    if (frontIsClear())
    {
        moveForward(); // This line is executed 0 or 1 times
    }
}

while re-checks the condition after the block is executed:

void moveToWall()
{
    while (frontIsClear())
    {
        moveForward(); // This line is executed 0 to 9 times
    }
}

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