Patterns Set and Maps 11 17 2010 Opening Discussion Do you have any questions about the quiz IcP solutions Simple description of a pattern Something you probably did in math in elementary school was looking at repeating patterns A pattern specifies a certain form for something A match occurs if a value fits the given pattern Pattern Matching We have used three types of patterns previously Value literals Tuples Type matches The last two start to show the power of pattern matching In particular they show that part of a pattern can be a variable name that binds to part of the pattern Variable Binding When a pattern is matched any words that start lower case are assumed to be variable names you want bound Use an underscore for anything you want to match stuff but ignore the value Use to bind a name to a match you are also further specifying To match the value of an outside variable put the variable name in backticks XML Patterns You can use patterns to pull out parts of XML or match on different types of nodes Simply put the variable names you want inside of curly braces val a s a node Case Class Patterns The real power of case classes in Scala comes from the fact they can be used in matches stu match case Student n q t a You can do this type of matching on events to pull out the fields you care about if you don t want the full event case MouseMoved source point mod List and Collection Patterns You can also make patterns with collections case Array a b c use a b and c Even more cool is what you can do with Lists case h t h is head and t is tail case a b Nil two element List This can be ideal for recursive methods on lists def len lst List Int lst match case Nil 0 case h t 1 len t Patterns Everywhere Patterns are used in a lot of places in Scala not just cases and matches The initial declaration of variables is a pattern match That is why we could assign from tuples The variable name in a for loop is actually a pattern If the pattern isn t matched by an element that element is skipped Sets Maps and Buffers The Scala collections library is a lot richer than just Lists and Arrays I want to introduce three other types of collections to you as they can make your life a lot easier for certain tasks They are all parametric so they can work on a variety of types Sets This is a collection that isn t ordered and doesn t allow duplicates There are both mutable and immutable sets By default you get the mutable version Buffers A buffer is a sequence like an array or a list but it is mutable like an array and grows like a list You find these in the scala collection mutable package Maps This collection type has two type parameters for a key and a value type You store values and look them up by key The keys are unique There are both mutable and immutable maps By default you get the mutable version Minute Essay What questions do you have about stuff Interclass Problem Use a Scanner to read words from a text file Add them to a Set to get all the unique words from the page For more of a challenge use a Map to count how many times each word occurs
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