Match and Patterns 2 15 2012 Opening Discussion Minute 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 More Entering 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 Recursion The 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 Recursion The problem in dealing with 10 000 numbers Rewrite the code so it doesn t have to remember things The match Construct Scala has a second conditional called match expr match case pattern1 expr case pattern2 expr The first case that matches is evaluated Can put if guards Patterns Matches 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 Collections Computers 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 Sequences One variable name many values Integer indexes starting with 0 Our first examples are Lists and Arrays Basic Arrays and Lists The 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 Comparison Arrays are mutable but fixed in size Lists are immutable but it is easy to add an element and get a new list Parametric Types You 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 Arrays We can get to the elements in an array by putting an index in parentheses The index is 0referenced 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 Lists You 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 Patterns You 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 list Nil matches an empty list Minute Essay Questions Quiz 3 is on Friday
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