Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Match and Patterns2-15-2012Opening DiscussionMinute essay comments:Can't do “real” problems.Books I read and music.Functions for imaginary numbers.We will learn graphics.Wiggle room on input.Loops would be nice.Using other concepts (like loops) on IcPs.Turning a String to an Int.Distinction between println and return.MoreEntering multiple types of data on the same line.More time coding than it would take to do by hand.Pulling apart tuples.Getting vi coloring in other places.Submitting the assignment?Tracing RecursionThe act of running through code line by line to see what it does is called tracing. It is a very important skill for programmers.Tracing often involves writing out variables and tracing how they change or drawing “pictures”.I want to show you an approach to tracing recursive functions.Deep RecursionThe problem in dealing with 10,000 numbers.Rewrite the code so it doesn't have to remember things.The match ConstructScala has a second conditional called match.expr match {case pattern1 => exprcase pattern2 => expr...}The first case that matches is evaluated.Can put if-guards.PatternsMatches a value to a form.Form can include tuples and many other things.Literals and names starting with capital letters have to match values.Names starting with lower case letters are bound as new values.Need for CollectionsComputers are good at dealing a lot of data. So far we can only store one value in each variable. This is a significant limitation.Collections are types that can store multiple data values.Allow us to remember many things to work on.The collection libraries in a language are very significant.Scala has great collections.SequencesOne variable/name, many values.Integer indexes starting with 0.Our first examples are Lists and Arrays.Basic Arrays and ListsThe two most basic collection types in Scala are arrays and lists.We can make either by following the type name with a parenthesized list of elements.Can create an “empty” array using new.Can build Lists with :: operator. Nil is empty.ComparisonArrays are mutable, but fixed in size.Lists are immutable, but it is easy to add an element and get a new list.Parametric TypesYou should notice that when we make an array or a list, the type is followed by square brackets.These types are parametric. So they take type arguments.In Scala, type parameters are placed in square brackets.Using ArraysWe can get to the elements in an array by putting an index in parentheses. The index is 0-referenced.arr(5)This syntax can be used in expressions to read values.It can also be used in assignments to store values in the array. This is what it means to be mutable.Let's look at some examples of this.Using ListsYou can do direct access on lists, but it is inefficient.The better method is to use the head and tail methods.The elements in a list can't be changed. However, you can efficiently add new elements at the front of the list.Lists work very well with recursion.List and Array PatternsYou can make patterns with Lists and Arrays.For Arrays:Array(1,2,a,b,c)For Lists:List(1,2,a,b,c)h::t - matches any non-empty listNil - matches an empty listMinute EssayQuestions?Quiz #3 is on
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