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

Download Karel

Download karel.jar (~250 kb)

Some Windows browsers seem to have trouble downloading jar files. In that case, 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 the jar file. If that fails, enter java -version. If that also fails, Java is not installed correctly.

Start Karel

Most Windows users can start karel.jar by double-clicking on it.

On other operating systems (or if double-clicking fails), 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

Shortcut	Command	Meaning
F1	moveForward();	Karel moves one square forward in the direction he currently faces. Fails if a wall blocks the way.
F2	turnLeft();	Karel turns 90° to the left.
F3	turnAround();	Karel turns 180° around.
F4	turnRight();	Karel turns 90° to the right.
F5	pickBeeper();	Karel picks a beeper from the square he currently stands on. Fails if there is no beeper.
F6	dropBeeper();	Karel drops a beeper onto the square he currently stands on. Fails if there already is a beeper.

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

Shortcut	Condition	Meaning
F7	onBeeper()	Karel checks whether a beeper is on the square he currently stands on.
F8	beeperAhead()	Karel checks whether a beeper is on the square immediately in front of him.
F9	leftIsClear()	Karel checks whether no wall is between him and the square to his left.
F10	frontIsClear()	Karel checks whether no wall is between him and the square in front of him.
F11	rightIsClear()	Karel checks whether no wall is between him and the square to his right.

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

Condition	Meaning
!a	holds if a does not hold (and vice versa)
a && b	holds if both a and b hold
a || b	holds if a or b (or both) hold
a || !b && c	a || ((!b) && c)

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
    }
}

Keyboard shortcuts

Windows	Effect	Macintosh
F1	moveForward();	F1
F2	turnLeft();	F2
F3	turnAround();	F3
F4	turnRight();	F4
F5	pickBeeper();	F5
F6	dropBeeper();	F6
F7	onBeeper()	F7
F8	beeperAhead()	F8
F9	leftIsClear()	F9
F10	frontIsClear()	F10
F11	rightIsClear()	F11
F12	start step into reset	F12
Tab Enter	auto-indent	Tab Enter
Ctrl Space	auto-complete	Command Space
Ctrl Alt R	rename command	Command Option R
Ctrl D	delete line	Command D
Ctrl C	copy	Command C
Ctrl X	cut	Command X
Ctrl V	paste	Command V
Ctrl Z	undo	Command Z
Ctrl Y	redo	Command Y