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The Three Main Execution Options
- Execution Mode
- Execution Speed
- Display Mode
- Starting Execution
Now that you know how Karel's world is displayed on your screen, it is time to focus
your attention on the simulator's message line (the one at the top of the screen). Before
the simulator can execute your program, there are three Execution Options that it must
request: Execution Mode, Execution Speed, and Display Mode. This chapter discusses the
possible choices for each of these options.
The simulator first asks you to supply the Execution Mode you want. This request
appears as:
Enter Execution Mode([A]utomatic, [M]onitor, or [E]xplain)[M]:_
The following discussion contains a brief explanation of each of the possible Execution
Mode answers.
 | In Automatic Execution Mode, the simulator automatically executes your program from
start to finish, without any possible intervention from you. Think of this mode as the
normal way that Karel executes our programs. In Automatic Execution Mode, the simulator
continues executing your program until Karel executes a turnoff instruction or is forced
to perform an error shutoff. |
| In Monitor Execution Mode, the simulator continually waits for you to OK the execution
of the next instruction in Karel's program. By pressing the return key, you can direct
Karel to execute one more instruction; the simulator then asks you again if it is OK for
Karel to execute the new next instruction. When executing in Monitor Execution Mode, you
can also change Karel's speed, make him go backwards in the program (unexecute some
instructions), or make him execute large bunches of instructions either forward or
backward. We shall continue our discussion of the details of Monitor Execution Mode in
Chapters 6 and 8. |
| In Explain Execution Mode, the simulator uses the Explain line (fourth from the top on
your screen) to explain, in great detail, how Karel is executing your program. In this
mode the simulator tells you when Karel starts to execute defined instructions; when he
starts BEGIN/END blocks;when he starts primitive, IF, ITERATE, and WHILE instructions;
when he performs tests, what the outcome of each test is, etc. If you are unsure about
exactly how Karel is executing your program, run the simulator in Explain Execution Mode
for a very lengthy and detailed description. |
Notice that in the above request for Execution Mode, Monitor Execution Mode is the
default mode. This default mode was chosen because there are special commands in Monitor
Mode that easily allow you to enter either Automatic or Explain Mode (and return to
Monitor Mode from Explain Mode). Initially, run the simulator in Automatic Mode to reduce
the amount of new material that you must learn. Before you try using Monitor Mode, read
the chapter on Simple Stepping in Monitor Mode (Chapter 6).
The second execution option that the simulator requests is Execution Speed. The
following request appears on the message line at the top of your screen:
Enter Execution Speed([F]ast, [M]edium, or [S]low)[F]:_
At the fast speed, Karel moves around the screen, but the simulator does not update its
status line until Karel stops. At medium speed, each time Karel executes an instruction,
the status line is automatically updated. This updating slows down the simulation by about
a factor of 20 from its fast speed. Finally, at slow speed the simulator updates Karel's
status and also prints a message after each instruction that Karel executes (the message
tells which instruction he has just executed). Slow speed is about 5 times slower than
medium speed, making it about 100 times slower than fast speed. The default speed is F,
and this is a good speed to start with. If you find that the simulator is executing your
program too quickly, you can re-run your program at a slower speed. Also, as mentioned
before, if you are in Monitor Mode, you can easily change the simulator's speed. Finally,
if you decide to choose Explain Mode, you will not have a choice of speeds: Explain Mode
runs at only the slow speed.
The final execution option is Display Mode. The simulator's request for this option
appears as:
Enter Display Mode([R]egular or [S]ticky)[R]:_
In Regular Display Mode, whenever Karel moves off a corner, that corner's contents are
redisplayed on the screen (that is, the number of beepers on that corner is redisplayed).
In Sticky Display Mode, whenever Karel moves off a corner, his image remains stuck there.
Running the simulator in Sticky Mode is useful for seeing the path that Karel takes, but
it can be confusing, as you don't see the beepers on any corners where Karel has been.
Also, if Karel repeatedly moves through a corner, only his last image through that corner
would be displayed. Start with Regular Display Mode, the default answer. Again, if you are
in Monitor Mode, you can easily change the Display Mode between Regular and Sticky.
Finally, after you have entered all three of the necessary execution options, the
simulator prints the following statement on your screen's message line:
The simulator is ready to execute Karel's program.*
After you press the return key, the simulator will start executing your program in the
initial situation, according to the options that you requested. f you are in Automatic
Execution Mode, the simulator will continue executing your program until Karel executes a
turnoff instruction or performs an error shutoff. If you are in Monitor or Explain Mode,
you must keep stepping the simulator forward until Karel executes a turnoff instruction or
performs an error shutoff. The next chapter discusses what you can do after the simulator
has halted Karel's execution.
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