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Neal Stublen nstublen@jccc.edu

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Human Decisions Think about a decision you've made today? How did you arrive at that decision? Was it a black-and-white decision for you? Our decisions can be complex?

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Computer "Decisions" Computer decisions always reduce down to simple logic comparisons Something is true or something is false It has a boolean value of true or false The decision block in a flow diagram requires a yes/no decision Is the light red? Is the light yellow? Is the light green?

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Boolean Decisions We use boolean expressions to determine a true or false condition We implement a selection structure Dual-alternative selection (if-then-else) Single-alternative selection (if-then)

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If-Then If the condition is true, then perform an action or a sequence of actions if condition then action endif if time is 6pm then begin class endif

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If-Then-Else If the condition is true, then perform an action or a sequence of actions Else perform a different action or sequence of actions if condition then action else another action endif if time is 6pm then begin class else wait endif

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Boolean Expressions An expression is just a statement the computer will evaluate 4 + 3 x / 4 A boolean expression will be evaluated as either true or false Relational operators are used in boolean expressions

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Relational Operators Equivalence, = or == Greater than, > Less than, < Greater than or equal to, >= Less than or equal to, <= Not equal to, <> or != Also called comparison operators

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Relational Operators if customerAge >= 65 then discount = 0.10 else discount= 0.0 endif if customerAge <65 then discount = 0.0 else discount = 0.10 endif

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Negation Operator A negation operator reverses the logic of a true or false expression (NOT or !) if age >= 21 then allow purchase endif if NOT age >= 21 then refuse purchase endif

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Example What logic can we use to implement the following discount table: PurchaseDiscount $0.00 to $25.005% $25.01 to $50.0010% $50.01 to $100.0015% Over $100.0020%

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Implementing Minimal Conditions Selecting from a group of ranges, the last check is never necessary If the table is complete, we rule out one possibility with each check If total is not > 100 and total is not > 50 and total is not > 25, then total must be <= 25.

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Compound Conditions Sometimes we need more complex logic to make a decision if I’m speeding then if I see a police car then slow down immediately endif

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Nested Decisions if condition1 then if condition2 then take action endif if condition1 AND condition2 then take action endif

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AND Operator if x AND y then do something endif “x AND y” is a boolean expression that can be evaluated as true or false xyx AND y True False TrueFalse

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Short-Circuit Evaluation if x AND y then do something endif If we know x is false, we know x AND y is also false. There’s no need to evaluate y.

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Short-Circuit Example if age > 12 AND age < 65 then movieDiscount = 0.0 endif If age is less than or equal to twelve, the computer will not need to determine if age is greater than 65.

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Example How would we implement the following discount table: On Sundays, senior citizens receive an additional 5% discount. PurchaseDiscount $0.00 to $25.005% $25.01 to $50.0010% $50.01 to $100.0015% Over $100.0020%

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OR Operator if x OR y then do something endif “x OR y” is a boolean expression that can be evaluated as true or false xyx OR y True FalseTrue FalseTrue False

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Short-Circuit Evaluation? if x OR y then do something endif Is there a short-circuit evaluation for the OR operator? If we know x is true, we know x OR y is also true. There’s no need to evaluate y.

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OR Efficiency if age < 12 then movieDiscount = 0.10 endif if age > 65 then movieDiscount = 0.10 endif if age 65 then movieDiscount = 0.10 endif

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AND/OR Precedence AND operator is always evaluated before the OR operator c1 OR c2 AND c3 OR c4 c1 OR (c2 AND c3) OR c4

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Precedence Example if age = 65 AND not Friday then discount = 0.10 endif A twelve year old still gets the discount on Friday. if (age = 65) AND not Friday then discount = 0.10 endif Parentheses always clarify intention.

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Summary Boolean expressions Relational operators AND Logic OR Logic Selection within ranges AND/OR precedence

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Date Validation What logic would we need to validate a user’s date input? The user enters separate values for the month, day, and year.

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