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

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

.jar.ZIP

karel.jar

ZIP archive

a

b

c

d

e

f

g

META-INF

tiles

a.class

font.png

MainKt.class

Unfortunately, changing file extensions in Windows explorer is not trivial. I recommend downloading

karel.jar

cmd

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

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

opengl

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

Language reference

Primitive commands

| Shortcut | Command | Meaning | | -------- | ----------------- | ------- | | F1 |

moveForward();

turnLeft();

turnAround();

turnRight();

pickBeeper();

dropBeeper();

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:

and use it just like a primitive command:

Repeat

Instead of writing the same sequence of commands multiple times:

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

repeat

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

If/else

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

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

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

Primitive conditions

| Shortcut | Condition | Meaning | | -------- | ---------------- | ------- | | F7 |

onBeeper()

beeperAhead()

leftIsClear()

frontIsClear()

rightIsClear()

If/else if

An

else

if

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

else

if

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

else

frontIsClear()

else

if

true

Not

!

An

if/else

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

!

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

And

&&

An

if

if

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

&&

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

Or

||

An

if/else if

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

||

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

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

while

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

Keyboard shortcuts

| Windows | Effect | Macintosh | | -----------: | :-------------------------: | ---------------- | | F1 |

moveForward();

turnLeft();

turnAround();

turnRight();

pickBeeper();

dropBeeper();

onBeeper()

beeperAhead()

leftIsClear()

frontIsClear()

rightIsClear()