Boolean Expressions and Functions 2 1 2012 Opening Discussion Do you have any questions about the quiz Minute essay comments Swing 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 More Reading 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 Motivation I 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 Logic We 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 not Short Circuit Operators The 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 ifs What 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 Math Let 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 Programming The 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 Scala We declare functions in Scala using def Here is the general form def name arg1 Type1 arg2 Type2 Type expression The 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 Decomposition Never 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 down This is the normal approach where you start with the full problem and break it into pieces Bottom up Sometimes you realize that different trivial pieces will be useful and build up from those Minute Essay What are your thoughts so far on the book Have you been reading How much is it helping IcPs will be presented on Friday
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