Karol the robot

Eric Roberts
Department of Computer Science
Stanford University
September 2005

Chapter 1
Introducing Karel the Robot
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. Drawing inspiration from the success of Seymour Papert’s LOGO project at MIT, Rich designed an introductory programming environment in which students teach a robot to solve simple problems. That robot was named Karel,
after the Czech playwright Karel Capek, whose 1923 play R.U.R. (Rossum’s Universal Robots) gave the word robot to the English language. Karel the Robot was quite a success. Karel was used in introductory computer science courses all across the country, to the point that Rich’s textbook sold well over 100,000
copies. Many generations of CS106A students learned how programming works by putting Karel through its paces. But nothing lasts forever. In the middle of the 1990s, the simulator we had been using for Karel the Robot stopped working. We were, however, soon able to get a version of Karel up and running in the Thetis interpreter we were using at the time. But then, a year ago, CS106A switched to Java, and Karel again vanished from the scene. For the last three quarters, the hole in the curriculum left by Karel’s
departure has been competently filled by Nick Parlante’s Binky world, but it seems about time to bring Karel back. The new implementation of Karel is designed to be compatible with both Java and the Eclipse programming environment, which means that you’ll get to practice using the Eclipse editor and debugger from the very beginning of the course. What is Karel?
Karel is a very simple robot living in a very simple world. By giving Karel a set of commands, you can direct it to perform certain tasks within its world. The process of specifying those commands is called programming. Initially, Karel understands only a very small number of predefined commands, but an important part of the programming process is teaching Karel new commands that extend its capabilities.
When you program Karel to perform a task, you must write out the necessary commands in a very precise way so that the robot can correctly interpret what you have told it to do. In particular, the programs you write must obey a set of syntactic rules that define what commands and language forms are legal. Taken together, the predefined commands and syntactic rules define the Karel programming language. The Karel
programming language is designed to be as similar as possible to Java so as to ease the transition to the language you will be using all quarter. Karel programs have much the same structure and involve the same fundamental elements as Java programs do. The critical difference is that Karel’s programming language is extremely small, in the sense that it has very few commands and rules. It is easy, for example, to teach the entire Karel language in just a couple of hours, which is precisely what we do in CS106A. At the end
of that time, you will know everything that Karel can do and how to specify those actions in a program. The details are easy to master. Even so, you will discover that solving a problem can be extremely challenging. Problem solving is the essence of programming; the rules are just a minor concern along the way.