karel the robot nested if statements while loops copyright © 2008 by helene g. kershner

Karel The RobotKarel The Robot

Nested If StatementsNested If Statements

While LoopsWhile Loops

Copyright © 2008 by Helene G. Kershner

Karel the Robot: Nested Ifs and WhileKarel the Robot: Nested Ifs and While

Five Block Square MazeFive Block Square Maze Problem statement; Problem statement; Karel Karel

needs to move through the needs to move through the five block square maze five block square maze and stop when he gets to and stop when he gets to the opening.the opening.

Define Output:Define Output: Karel Karel stops when he locates the stops when he locates the opening in the maze.opening in the maze.

Define Input.Define Input. Karel is at Karel is at the beginning of a 5-block the beginning of a 5-block square maze, facing east.square maze, facing east.


Karel the Robot: Nested Ifs and While Karel the Robot: Nested Ifs and While

Version 1Version 1:: If we didn’t know more If we didn’t know more

than the primitive than the primitive instructions we would do instructions we would do something like:something like:

Move;Move; Move;Move; Move;Move; Move;Move; Move;Move; Turnleft;Turnleft; Move;Move; Move;Move; Move;Move; Move;Move; Move;Move; Turnleft;Turnleft; ……


Karel the Robot: Nested Ifs and WhileKarel the Robot: Nested Ifs and WhileVersion 2Version 2::

How do we get Karel to move around the maze with less of How do we get Karel to move around the maze with less of the programmer’s help? the programmer’s help?

What question would Karel need to ask to determine IF it What question would Karel need to ask to determine IF it were time to turn left?were time to turn left?

If front-is-clear then If front-is-clear then

beginbegin

move;move;

endend

elseelse

beginbegin

turnleft;turnleft;

end;end;Copyright © 2008 by Helene G. Kershner


What happens when we enter that code?What happens when we enter that code? Karel moves exactly one block. Karel moves exactly one block.

Why?Why? Because, we only told Karel to move one Because, we only told Karel to move one

time or turnleft one time. time or turnleft one time.


Karel the Robot: Nested Ifs and WhileKarel the Robot: Nested Ifs and WhileVersion 3Version 3:: How do we get Karel to move a bunch of times? How do we get Karel to move a bunch of times?

Iterate 40 timesIterate 40 times

begin begin

If front-is-clear then If front-is-clear then

beginbegin

move;move;

endend

elseelse

beginbegin

turnleft;turnleft;

end;end;

end;end;

Karel certainly moves around the maze when we combine the Karel certainly moves around the maze when we combine the iterate command with the IF statement. iterate command with the IF statement.


Karel the Robot: Nested Ifs and WhileKarel the Robot: Nested Ifs and WhileAre we solving the problem we were asked to solve? Are we solving the problem we were asked to solve?

Problem statement; Problem statement; Karel needs to move through the five Karel needs to move through the five block square maze and stop when he gets to the block square maze and stop when he gets to the opening.opening.

Reading the problem statement carefully you’ll notice we Reading the problem statement carefully you’ll notice we are NOT solving the right problem.are NOT solving the right problem.

Why?Why?



Well, Well,

1. Karel stops when the iterate command finishes, 1. Karel stops when the iterate command finishes, wherever he happens to be. wherever he happens to be.

2. Karel does not stop when he reaches the 2. Karel does not stop when he reaches the opening as the problem statement required. opening as the problem statement required.

3. In fact according to this program, Karel has no 3. In fact according to this program, Karel has no idea whether he has reached the opening or not. idea whether he has reached the opening or not.



How does Karel know when the opening is How does Karel know when the opening is reached? reached?

How does Karel know when he has completed How does Karel know when he has completed his walk through the maze?his walk through the maze?

We, the programmer, could count the number of We, the programmer, could count the number of intersections, but that sort of misses the point. intersections, but that sort of misses the point.

What statement do we use to allow Karel to What statement do we use to allow Karel to make a decision? make a decision?



We need to create a decision (IF) that asks We need to create a decision (IF) that asks essentially “Am I done?”essentially “Am I done?” How can Karel know when to stop? How can Karel know when to stop? How does Karel know Karel is at the opening?How does Karel know Karel is at the opening?

Remember Karel can test any of the following conditionsRemember Karel can test any of the following conditionsfront-is-clearfront-is-clear front-is-blockedfront-is-blocked

left-is-clearleft-is-clear left-is-blockedleft-is-blocked

right-is-clearright-is-clear right-is-blockedright-is-blocked

next-to-a-beepernext-to-a-beeper not-next-to-a-beepernot-next-to-a-beeper

any-beepers-in-beeper-bagany-beepers-in-beeper-bag no-beepers-in-beeper-no-beepers-in-beeper-bagbag



We are looking for a test thatWe are looking for a test thatrecognizes that when the recognizes that when the

openingopening is to Karel’s is to Karel’s RightRight, , then he is done.then he is done.

Remember our choices:Remember our choices:front-is-clearfront-is-clear

front-is-blockedfront-is-blockedleft-is-clearleft-is-clear

left-is-blockedleft-is-blockedright-is-clearright-is-clear

right-is-blockedright-is-blockednext-to-a-beepernext-to-a-beeper

not-next-to-a-beepernot-next-to-a-beeperany-beepers-in-beeper-bagany-beepers-in-beeper-bag

no-beepers-no-beepers-in-beeper-bagin-beeper-bag


Opening


When the right-is-clear Karel is done.When the right-is-clear Karel is done.

We could write the following:We could write the following: If right-is-blocked thenIf right-is-blocked then

BeginBegin

-- Move through the maze-- Move through the maze

End;End;

But,But, How do we move through the maze? How do we move through the maze?



Earlier we solved moving through the maze using the Earlier we solved moving through the maze using the following IF statement:following IF statement:

This is how we move throught the maze, one intersection at This is how we move throught the maze, one intersection at a time. a time.

If front-is-clear then If front-is-clear then beginbegin move;move;endend

elseelsebeginbegin turnleft;turnleft;end;end;



For Karel to travel through the maze and do all the For Karel to travel through the maze and do all the checking himself, the following questions need to be checking himself, the following questions need to be asked ? asked ? Am I done yet?Am I done yet? Do I move forward?Do I move forward? Do I turn left?Do I turn left?

Karel’s code would need to look like this:Karel’s code would need to look like this:



If right-is-blocked thenIf right-is-blocked thenbeginbegin If front-is-clear then If front-is-clear then

beginbeginmove;move;

endend elseelse beginbegin

turnleft;turnleft; end;end;

end;end;



VersionVersion 4 4: Using nested Ifs, : Using nested Ifs, how might we solve our how might we solve our original problem? original problem?

Using Iterate, we still need Using Iterate, we still need to “guess” in advance to “guess” in advance how many intersections how many intersections Karel needs to go Karel needs to go through.through.

Iterate 40 timesIterate 40 timesbeginbegin If right-is-blocked thenIf right-is-blocked then beginbegin If front-is-clear then If front-is-clear then

beginbegin move;move;

endend elseelse

beginbegin turnleft;turnleft;

end;end; end;end;end;end;



What happens when Karel reaches the opening in the What happens when Karel reaches the opening in the Maze?Maze?

Karel knows not to turnKarel knows not to turn Karel knows not to move aheadKarel knows not to move ahead

BUT, Karel keeps repeating his tests because BUT, Karel keeps repeating his tests because the ITERATE statement tells him to keep the ITERATE statement tells him to keep testing. testing.

So, while Karel has correctly solved the required problem So, while Karel has correctly solved the required problem he has not done so efficiently. he has not done so efficiently.



The While Instruction or While LoopThe While Instruction or While Loop With ITERATE, the programmer needs to determine in With ITERATE, the programmer needs to determine in

advance of running the program how many time Karel advance of running the program how many time Karel will perform an activitywill perform an activity

Karel uses the WHILE instruction whenever a task is to Karel uses the WHILE instruction whenever a task is to be repeated an undetermined number of times.be repeated an undetermined number of times.

The WHILE instruction essentially combines an IF with The WHILE instruction essentially combines an IF with an Iterate. This allows Karel to determine, without an Iterate. This allows Karel to determine, without human intervention, when the required task has been human intervention, when the required task has been accomplished. accomplished.



The While Instruction or While LoopThe While Instruction or While Loopwhile <while <testtest> DO> DO

beginbegin

<<instruction(s)instruction(s)>;>;

endend

Ex: Ex: while front-is-clear DO while front-is-clear DO

beginbegin

putbeeper;putbeeper;

move;move;

end;end;



Our maze problem is perfectly suited to using While. Our maze problem is perfectly suited to using While. Remember, a while loop essentially combines a decision Remember, a while loop essentially combines a decision

(IF) with a repeat (Iterate) statement. (IF) with a repeat (Iterate) statement. Karel is already testing for the ending condition (right-is-Karel is already testing for the ending condition (right-is-

blocked) and doing so iteratively or repeatively. blocked) and doing so iteratively or repeatively. We can replace both the Iterate statements and the IF We can replace both the Iterate statements and the IF

test with a while statement. test with a while statement. A while statement is often called a While Loop because A while statement is often called a While Loop because

like the Iterate statement it repeats a set of instructions.like the Iterate statement it repeats a set of instructions.



Version 4Version 4::Iterate 40 timesIterate 40 timesbeginbegin If right-is-blocked thenIf right-is-blocked then beginbegin If front-is-clear then If front-is-clear then


endend elseelse


end;end; end;end;end;end;

Version 5Version 5::While right-is-blocked DOWhile right-is-blocked DO beginbegin If front-is-clear then If front-is-clear then


endend elseelse


end;end; end;end;

Be sure to match the Begin/End Be sure to match the Begin/End statementsstatements



NoteNote: Looking at the : Looking at the code, we can see that code, we can see that it will solve more than it will solve more than just our starting maze.just our starting maze.

This code will work on This code will work on any “left-handed” any “left-handed” maze. maze.

HINT:HINT: The basic The basic solution for this maze solution for this maze works for your final works for your final project! project!


karel the robot nested if statements while loops copyright © 2008 by helene g. kershner

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