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CSE 111

CSE 111. Karel the Robot. If Then Else Instruction. Format If condition THEN BEGIN instruction(s) END ELSE BEGIN instruction(s) END; How does it work? If condition is met, then perform instructions in THEN clause

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CSE 111

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  1. CSE 111 Karel the Robot

  2. If Then Else Instruction • Format If condition THEN BEGIN instruction(s) END ELSE BEGIN instruction(s) 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 IFnext-to-a-beeperTHEN BEGIN pickbeeper; END ELSE BEGIN 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 • 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 IFnext-to-a-beeperTHEN BEGIN pickbeeper; IFnext-to-a-beeperTHEN 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 IFnext-to-a-beeperTHEN BEGIN pickbeeper; move; END ELSE BEGIN putbeeper; move; END;

  10. Writing Efficient Conditional Statements • Factoring • Example #1 • After factoring IFnext-to-a-beeperTHEN BEGIN pickbeeper; END ELSE BEGIN putbeeper; END; move;

  11. Writing Efficient Conditional Statements • Factoring • Example #2 • The following code fragment can be factored IFnext-to-a-beeperTHEN BEGIN pickbeeper; turnleft; END ELSE BEGIN pickbeeper; move; END;

  12. Writing Efficient Conditional Statements • Factoring • Example #2 • After factoring pickbeeper; IFnext-to-a-beeperTHEN BEGIN turnleft; END ELSE BEGIN 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

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