CSE 111
Karel the Robot
Loops
Repeating a sequence of instructions
Unconditional
Repeat a set number of times
ITERATE
Conditional
Repeat while a condition is true
WHILE…DO
Unconditional Loops
Format
ITERATE positive-number TIMES
BEGIN
Body of loop (sequence of instructions)
END;
How does it work?
Repeat the body of the loop a given number of
times
New Reserved Words
ITERATE
TIMES
Unconditional Loops
Example
Let’s rewrite the turnright instruction
Original Definition
DEFINE-NEW-INSTRUCTION turnright AS
BEGIN
turnleft;
turnleft;
turnleft;
END;
New Definition
DEFINE-NEW-INSTRUCTION turnright AS
BEGIN
ITERATE 3 TIMES
BEGIN
turnleft;
END;
END;
Conditional Loops
Format
WHILE condition DO
BEGIN
Body of loop (sequence of instructions)
END;
How does it work?
Repeat the body of the loop while the given
condition is met
Conditions are the same as the conditions for
IF/THEN/ELSE
New Reserved Words
WHILE
DO
Conditional Loops
Condition is checked BEFORE loop body is
executed
It is possible that the loop body may never be
executed
Constructing WHILE DO loops
Identify condition for which loop should terminate
Negate termination condition for loop condition
Determine what must be done before and after the
loop
Perform minimal number of instructions in the
loop
Conditional Loop Example
Task
Put down a line of beepers, between Karel & the
closest wall in front of Karel
Initial World
Streets
Karel starts with 10 beepers in bag
10
9
8
7
6
5
4
3
2
1
N
W
E
S
Karel’s Initial
Position
(facing east)
1 2 3 4 5 6 7 8 9 10
Avenues
Conditional Loop Example
Streets
Final World
10
9
8
7
6
5
4
3
2
1
N
W
E
S
1 1 1 1 1 1 1 1
Karel’s Final
Position
1 2 3 4 5 6 7 8 9 10
Avenues
Conditional Loop Example
BEGINNING-OF-PROGRAM
BEGINNING-OF-EXECUTION
putbeeper;
WHILE front-is-clear DO
BEGIN
move;
putbeeper;
END;
turnoff;
END-OF-EXECUTION
END-OF-PROGRAM
Conditional Loop Example
Streets
What happens if the initial world is as shown
below?
10
9
8
7
6
5
4
3
2
1
N
W
E
S
Karel’s Initial
Position
(facing east)
1 2 3 4 5 6 7 8 9 10
Avenues
References
Richard E. Pattis (revised by Jim Roberts &
mark Stehlik), Karel the Robot, John Wiley &
Sons, Inc., 2nd edition, 1995, pp 93-115