(for other links, see the Directory of Documents)
Why study Karel the Robot?
* as an elaboration of Insight
III:
sequence, choice,
repetition, & naming
1. Karel: a "robot" (actually, an arrow, which I'll
represent here as: < )
2. Karel's "world": a grid of streets
and avenues,
with walls and beepers
(which I won't try
to draw here; see the
Karel
website for better pictures)
3. You should run Karel;
for information on downloading
your own copy of Karel,
see the website:
How
to Download Your Own Copy of
Karel the Robot onto a Windows PC
or use the CIT Windows machines.
4. Karel can:
* move forward (in the direction
he faces)
- i.e., change location, but not direction
* turn; i.e., change direction, but not location
* perceive:
- look ahead 1/2 block, without turning
- look left 1/2 block, without turning
- look right 1/2 block, without turning
- hear beeper on same corner
- the direction he's facing
* pick up & put down beepers
* determine if he is carrying any
beepers in his
beeper bag
5. Sample tasks:
* go to
corner of 6th St. & 25th Ave.
* pick
up beeper on 3rd St. & 6th Ave.
* escape
from a room with only one door
6. To get Karel to do things (to "perform tasks"),
we give him a program
* i.e., an algorithm expressed in his language
7. To describe a task for him to perform
(a problem for him to solve),
we use "situtations":
* exact descriptions of Karel's world:
-
Karel's current position
(= location + direction)
- location & size of all walls
- location & amount of all beepers
* initial
situation: when Karel turns on
* final
situation: when Karel turns off
* task:
gets Karel from initial situation
to final situation
- i.e., solves problem
8. Karel has 5 "primitive" or basic instructions:
| SYNTAX
(what the instruction looks like in Karel's lang |
SEMANTICS
(what the instruction means (i.e., what Karel does) |
|---|---|
| move | IF Karel sees no walls,
THEN Karel moves forward 1 block in direction he's facing ELSE IF Karel sees a wall, THEN "error shutoff" |
| turnleft | Karel pivots 90° left |
| pickbeeper | IF there is at least one beeper
on Karel's corner, THEN Karel picks up one beeper & puts it in his bag ELSE error shutoff |
| putbeeper | IF there is at least one beeper
in Karel's bag, THEN Karel puts it on his corner ELSE error shutoff |
| turnoff | Karel turns himself off |
Note: All programs must have turnoff as last
instruction.
9. e.g., init. sit. = K at origin, facing N;
1 beeper at 2nd St. & 2nd Ave.
final sit = K at 2nd St. & 2nd Ave.,
facing N,
no beepers there
possible solution:
move; turnleft; turnleft; turnleft; move; pickbeeper; turnleft; turnoffBut, to tell this to Karel, we must be polite,
beginning-of-program beginning-of-execution move; turnleft; turnleft; turnleft; move; pickbeeper; turnleft; turnoff end-of-execution end-of-programThere are several other programs that will do the
10. Things that can go wrong ("bugs"):
* lexical
(vocabulary) errors:
- using a word not in Karel's language
e.g., "turnright"
* syntactic
(grammatical) errors:
- bad grammar or punctuation
e.g., no semicolon between instructions,
no "beginning-of-execution"
* error
shutoff:
- result of an incorrect program
- possible causes:
> move beyond wall or boundary
> pickbeeper where there is no
beeper
> putbeeper when Karel has no
beepers
> no turnoff
* semantic
(meaning) errors:
- Karel's program is correct,
& does exactly what we tell him,
successfully,
BUT:
we didn't tell him what we meant
e.g.: for above problem:
move;
turnleft;
turnleft;
turnleft;
move;
turnoff
OOPS! We forgot to tell him
"pickbeeper"
Debugging = finding & fixing bugs