Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Argument Passing2-18-2011Opening DiscussionMinute essay comments:Study guide for the midterm?Average of numbers under 100. Methods return new collection. New variable is best. Could use recursion.Review some of the standard methods on sequences.Higher Order MethodsThe most powerful methods are ones you can pass functions into.exist, forall – Boolean checks like for math.filter, partition – separate collection based on Boolean.map – apply function to all the elements.reduceLeft – apply function moving through collectionfoldLeft – apply function moving through, but allows initial value so it can return a different type. This is curried.Let's Put These Into ActionI want to spend some time playing with these methods and seeing what we can do with them.A String is a collection so you can do these things with a String as well.Variable Length Argument ListsYou can make functions that don't specify exactly how many arguments they take.These are often called var-args.To do this, but a * after the type. It can only be the last argument in a list.Calling Var-Args with CollectionsIt is often helpful to call a var-args method passing a collection for the variable length arguments.You can do this, but you have to tell Scala what you are doing.Follow the collection with :_* to do this.The : is like specifying a type.The _ says you don't care about the exact type.The * is like the * in var-args declarations.Aliasing and MutabilityI argue that immutable collections like Lists can be safer than mutable ones like Arrays.One of the big reasons for this is aliasing.An alias in programming is just like in normal life. It is a second name for something.Variables are really references to objects.If a second variable is assigned the same value as the first, they are aliases to that object.Let's play with this and draw on the board.Aliasing for Argument PassingWhen you pass arguments, you are really passing references.So arguments in functions are aliases to the objects outside the functionIf the object is mutable, the function can change it.Pass-by-NameThere is another way to pass things in Scala called pass-by-name.When you pass something by name, it isn't evaluated at the time it is passed. Instead it is turned into a function and that function is evaluated every time the variable is used.The syntax is to put an => before a type, but not have an argument list before the arrow.Fill and TabulateThere are two other ways of creating collections: fill and tabulate. Both are curried. Second argument to fill is by name, second argument to tabulate is a function.The fill method on Array or List takes a first argument of how many elements. After that is a by-name parameter that gives back the type you want in the array or list.Tabulate also takes a size first. After that is a function that takes the index.Minute EssayWhat questions do you have about collections?Remember to do IcP
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