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Chapter 2: Programming Karel

The simplest style of Karel program uses text to specify a sequence of built-in commands that should be executed when the program is run. Consider the simple Karel program below. The text on the left is the program. The state of Karel's world is shown on the right:

Code Editor

# File: FirstKarel.py # ----------------------------- # The FirstKarel program defines a "main" # function with three commands. These commands cause # Karel to move forward one block, pick up a beeper # and then move ahead to the next corner. from karel.stanfordkarel import * def main(): move() pick_beeper() move()

Ready to run

Karel's World

Position: (1, 1) | Direction: East | Beepers: 0

Press the "Run" button to execute the program. Programs are typically written in a special application called an Integrated Development Environment (IDE) and most Karel programs are written in an IDE called PyCharm. Like an IDE, this reader has the ability to execute programs in order to help you see how things work as you learn.

Understanding the Program Structure

The program is composed of several parts. The first part consists of the following lines:

# File: FirstKarel.py # ----------------------------- # The FirstKarel program defines a "main" # function with three commands. These commands cause # Karel to move forward one block, pick up a beeper # and then move ahead to the next corner.

These lines are an example of a comment, which is simply text designed to explain the operation of the program to human readers. Comments in both Karel and Python begin with the characters # and include the rest of the line. In a simple program, extensive comments may seem silly because the effect of the program is obvious, but they are extremely important as a means of documenting the design of larger, more complex programs.

The second part of the program is the line:

from karel.stanfordkarel import *

This line requests the inclusion of all definitions from the karel.stanfordkarel library. This library contains the basic definitions necessary for writing Karel programs, such as the definitions of the standard operations move() and pick_beeper(). Because you always need access to these operations, every Karel program you write will include this import command before you write the actual program.

Function Definitions

The final part of the Karel program consists of the following function definition:

def main(): move() pick_beeper() move()

These lines represent the definition of a new function, which specifies the sequence of steps necessary to respond to a command. As in the case of the FirstKarel program itself, the function definition consists of two parts that can be considered separately: The first line constitutes the function header and the indented code following is the function body.

The first word in the function header, def, is part of Python's syntactic structure. It says that you are creating a new function. The next word on the header line specifies the name of the new function, which in this case is main. Defining a function means that Karel can now respond to a new command with that name.

The main() command plays a special role in a Karel program. When you start a Karel program it creates a new Karel instance, adds that Karel to a world that you specify, and then issues the main() command. The effect of running the program is defined by the body of the main() function, which is a sequence of commands that the robot will execute in order.

Note: Try modifying the code in the editor above and clicking "Run" to see how Karel responds to different commands!

Available Karel Commands

# Movement commands move() # Move forward one square turn_left() # Turn 90 degrees to the left # Beeper commands pick_beeper() # Pick up a beeper from current position put_beeper() # Place a beeper at current position # World queries front_is_clear() # Check if front is clear left_is_clear() # Check if left is clear right_is_clear() # Check if right is clear beepers_present() # Check if beepers are at current location