1. CSE 111
Karel the Robot

2. If Then Else Instruction
Format
If condition THEN
BEGIN
instruction(s)
END
ELSE
END;
How does it work?
If condition is met, then perform instructions in THEN clause
If not, perform instructions in ELSE clause

3. If Then Else Instruction
Reserved Words IF
THEN
ELSE
Test Conditions
Same as if/then instruction

4. If Then Else Instruction
As with the if/then instruction, any number of instructions can be placed between the delimiters BEGIN & END

5. If Then Else Instruction
Notice how the semicolon after END indicates the end of the if/then/else statement
If/then
Semicolon after END
If/then/else
Semicolon after second END!

6. If Then Else Instruction
Example
IF next-to-a-beeper THEN
BEGIN
pickbeeper;
END
ELSE
putbeeper;
END;

7. Nested If Statements
If/then or if/then/else statements written within the then or else clause of another instruction
Used for more complex tests
Avoid nesting more than one level deep to keep programs readable

8. Nested If Statements Example Task Code Pick up two beepers
Ensure that if there are not two beepers on the intersection Karel is standing on, the program will NOT end in an error shutoff
Code
IF next-to-a-beeper THEN
BEGIN
pickbeeper;
END
END;

9. Writing Efficient Conditional Statements
Factoring
Programs can be made more readable by removing redundant instructions from inside a conditional statement to outside the statement
It also reduces size of program
Can lead to faster runtimes since memory accesses are reduced
Example #1
The following code fragment can be factored
IF next-to-a-beeper THEN
BEGIN
pickbeeper;
move;
END
ELSE
putbeeper;
END;

10. Writing Efficient Conditional Statements
Factoring
Example #1
After factoring
IF next-to-a-beeper THEN
BEGIN
pickbeeper;
END
ELSE
putbeeper;
END;
move;

11. Writing Efficient Conditional Statements
Factoring
Example #2
The following code fragment can be factored
IF next-to-a-beeper THEN
BEGIN
pickbeeper;
turnleft;
END
ELSE
move;
END;

12. Writing Efficient Conditional Statements
Factoring
Example #2
After factoring
pickbeeper;
IF next-to-a-beeper THEN
BEGIN
turnleft;
END
ELSE
move;
END;

13. References
Richard E. Pattis (revised by Jim Roberts & Mark Stehlik), Karel the Robot, John Wiley & Sons, Inc., 2nd edition, 1995, pp 65-86