Handout: 3_Intro_1_karel

Reeborg, our Karel simulator, and Python, its language

The Reeborg GUI

Let’s start it up and solve a mission: click here.

First, identify the different parts of the interface:

Karel’s world

• this is the grid where Karel lives
• the cells may be empty or contain an object:
  • one or more tokens that Karel can pick up
  • an ’empty’ token where we should put tokens to end the mission correctly
  • Karel’s destination, or ‘home’ square
• there can be walls between cells
• On the top-left area, we can see how many tokens Karel has in its bag

code editor

Control buttons

Solving a mission

We are now going to solve a mission based on the world that we saw before. Karel needs to pick up the yellow token, put it in the basket and then go home. So let’s get started.

Comments

show it to me

A comment is text that is not part of the program; it has information for the programmer, not for the computer, i.e., it is like a margin note. In Python, any text that follows a hashmark # is a comment.

Let’s start the program with the following comment:

# The goal of this mission is to put the token
# in a basket and then go home

Now, when we run the program (i.e., when we press the ‘play’ button), the program will run and fail because, although the program is valid, it does not achieve any of the goals of the mission, i.e., there is no token in the basket and there is no robot on the green square.

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System commands

show it to me

The following are a few functions that will appear often in our missions.

• sound( True/False ): turns the sound effects on and off
• think(n): sets a delay between commands, in milliseconds. Delays shorter or equal to 250 ms turn off the sound effects.
• say(message): prints a message in a pop-up box.
• pause(): pauses the program until we press the play or step button

Let’s add them in our program:

sound(True)
think(1000)

# main program
msg = "I am ready to start. Press play to continue"
say(msg)
pause()

With these commands we are:

1. turning the sounds on
2. setting the delay between commands to 1 second
3. assigning a string to the variable msg
4. printing the message to the screen with the function say()
5. pausing the program

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Built-in commands

show it to me

An actuation command (or function) allows a robot to act. Karel has 4 built-in actuation functions:

move()       # move to the cell in front of the droid
turn_left()  # do a 90° turn to the left (in place)
take()       # pick one token from floor and put in bag
put()        # get token from bag and put it on floor

Let’s use these commands to accomplish the first task, i.e., to put the token in the basket, adding the following code:

move()       # step on the cell that has the token
take()       # take token and put it in bag
turn_left()  # turn to left of the robot, not our left
move()       # step on the cell that has the basked
put()        # put on the floor the token in the bag

When we run the program now, it still fails but indicates that the tokens are now in their correct final location.

Important:

A mission fails if…
…we crash Karel against a wall
…we ask him to take a token while standing on an empty cell
…we ask him to put a token but his bag of tokens is empty

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Repeat

show it to me

Repeat() repeats a single command multiple times:

repeat( command, n )  # repeat command n times

For example, Karel has placed the token in the basket; now he needs to turn around to go home. To turn around we can tell him to turn_left() twice or, better, use the repeat() command to tell him to do turn_left() 2 times.

In summary, instead of writing this:

turn_left()
turn_left()

we are going to write this:

repeat(turn_left, 2)  # turn_around

Both pieces of code do the same thing but the second version is better because does the same thing in a more clear way. Again:

the 2nd version is better because it is clearer than the first one, not because it is shorter.

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The library

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Although Karel only knows 4 commands when we turn him on, we can teach him new commands that are based on the 4 commands that he already knows. We have provided you with 3 additional commands that you can find in the library:

• left()
• right()
• turn_around()

In particular, turn_around() does exactly what we just did ourselves: turns the robot around by using repeat(). Thus, instead of repeating the code, we just use the command already written in the library. We tell Python that we want to use the commands in the library placing the following line at the top of our program:

from my_lib import  *

which means ‘allow my program to use everything in my_lib‘.

Hence, we can modify our program to use the library like this:

from my_lib import  *
....
turn_around()

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3rd grade – Almost there

Work it out

Answer: Show it to me

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4th grade – A dozen chicks

work it out

Answer: show it to me

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5th / 6th grade

There are no 5th or 6th grade mission this week because we covered a lot of material. If you were able to do the 4th level mission you are perfectly on pace. Congratulations.