Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Boolean Expressions and Functions2-1-2012Opening DiscussionDo you have any questions about the quiz?Minute essay commentsSwing dancing on roller skates.Color and capitalization on “if”.Will people make machines to destroy the other machines?How has programming changed recently and how do I see it changing looking forward?Conditional execution with 3+ options?Password not string enough?MoreReading before or after class?Max on stacking if-else?How do you get Scala to repeat code without cut and paste?Let's finish what we were doing last class.MotivationI want to have a function that tells me if two squares intersect.The function will be given the x and y location of the center of each square as well as the length of the side of each square.It should return a Boolean telling if they intersect.Conditional LogicWe talked about comparisons of values in the last class.We can also combine Boolean expressions together using Boolean logic.There are four Boolean operators:&& for and|| for inclusive or^ for exclusive or! for notShort Circuit OperatorsThe && and || operators are short circuit operators.This means that if the value is known after evaluating the first operand, the second operand won't be evaluated.This can prevent errors.Let's look at an example of this with division by zero.Nesting ifsWhat you put in an if can be any expression or statement.As a result, you can put an if inside of another if.As we will see, Scala doesn't care what you nest inside of things. You write the logic that makes sense to you and says what you want to say.Functions in MathLet's review the concept of functions from math.In algebra a function would take one or more values and give you back a value. The values were generally numbers.In higher level math this is generalized with things like sets.In math functions the same input always leads to the same result.Functions in ProgrammingThe concept of a function is critically important to programming.Functions can take one or more arguments and give us back values. (Most languages allow only one return value.)Let's think of some examples of functions that we could write.Functions in ScalaWe declare functions in Scala using def. Here is the general form.def name(ar g1:Type1, arg2:Type2, ...):Type = expressionThe argument list can have zero or more elements. If there are zero even the parentheses can be left off.Function arguments must have types.The return type is optional, but it is recommended.Why Functions?Functions are used in programs for a number of reasons.Reduce code duplication. You can call the same function multiple times and only write it once.Improve readability and maintainability. Good function names make it easier to read. Small functions are easier to test and debug.Break problems down/problem decomposition.Problem DecompositionNever solve a hard problem. If a problem is hard, break it into smaller problems that are easier. Repeat until you are only solving trivial problems.Top-downThis is the “normal” approach where you start with the full problem and break it into pieces.Bottom-upSometimes you realize that different trivial pieces will be useful and build up from those.Minute EssayWhat are your thoughts so far on the book? Have you been reading? How much is it helping?IcPs will be presented on
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