CompSci 4Chap 6 Sec 2 – Part 2 Mar 22, 2005Prof. Susan RodgerNote: thanks to Wanda Dann and Steve Cooper for slide ideasAnnouncements• Read Chapter 6 Tips and Techniques for next time• Assignment 7 outLast time – if/else statementsLast time – Boolean functionsUse Function in a method:What we will do today• Lecture on Chap 6, Part 2– Execution control– Nested ifs– Logical Operators• ClassworkRevisit the Zeus world• Recall theZeus world• Testing this world – two problems– Anything clicked gets zapped– Philosophers can get zapped more than onceProblem• We need conditional execution– Check conditions• Is selected object a philosopher?• If so, has philosopher already been zapped?– Conditional execution• Lightning bolt will be shot or notMultiple Conditions• First problem to solve - restrict Zeus to shoot only philosophers• Four possible conditions must be checked– User could click on one of four philosophersOne Condition• Begin with just one philosopher - homerStart with blank if statementDrag in the who tile, then select == and homer== is a relational operator meaning “is equal to”Add in Do in order blockThe Do in order block is nested inside the ifNow, lightning will strike homer, only if user clickson homer. But, what about the other three philosophers?Logical OperatorsUse Boolean logic operators tocheck more than one conditionhomer or plato or socrates or euripidesDemo • Demo Zeus world with if statement and logic operatorsAbstraction• Multiple conditions, as in this example,– become complex– are difficult to read– are difficult to debug• Better solution – write a Boolean function that checks the conditions• This is an example of abstraction – allowing you to think on a higher plane• Used nested if statements• If one of the conditionsis true, function returnstrue, and nothing else in the function is checked!• If none of the conditionsare true, the function returns falseisPhilosopher functionDemo of isPhilospher functionCompleting the Zeus world• Second problem – how to prevent lightning striking the same philosopher more than once• How do we know if a philosopher has already been struck by lightning?– When zapped, color property is set to black– Checking color is convenient way to check for a previous lightning strikePreventing a duplicate strike• Only need to check for a possible duplicate-strike if we already know a philosopher is clicked– Nest a second if statement inside the first!= is a relational operator meaning “is not equal to”Complete shootBoltDemo• Demo of completed Zeus worldClasswork today• New partners• Redo the ball and goals animation–Start
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