Loops and Reading from Files 3 5 2012 Opening Discussion Midterm answers Minute essay comments Arrays if matrices Using a HUD while driving Advice on low cost gaming machine How fast can I skate 20 laps and do I play roller hockey Consider watching instead of following while Loop Recursion is sufficient for making repetition but in imperative languages it isn t the normal approach Instead people use loops The simplest loop is the while loop while condition statement The condition is evaluated first If it is true the statement possibly a block executes This repeats until the condition is false do while Loop The partner to the while loop is the do while loop do statement while condition This loop is post check instead of the pre check of the normal while loop Always happens once The while loop might never happen The for Loop The most commonly used loop in most languages is the for loop The Scala version is a bit different from most Often used for counting In general it is a for each loop that goes through a collection for i 1 to 10 for e coll Variable takes on value of each element in the collection Range Type Range types provide an easy way to make collections for counting to and until operate on numeric types to produce ranges 1 to 10 0 until 10 Use by to change the stepping in a range 1 to 100 by 2 10 to 1 by 1 a to z by 3 yield The for loop can be used as an expression if you put yield between the end of the for and the expression after it for e coll yield expr What you get back will be a collection that is generally of the same type as what you iterated over if Guards You can put conditions in the for that will cause some values to be skipped for n nums if n 2 0 Multiple Generators You can also put multiple generators in a for loop for i 1 to 10 j i to 10 You can combine as many generators and guards as you want You can also declare variables in the middle of the for The thing you assign into is like a val so it can be a pattern We have only seen this with tuples so far Multidimensional Arrays You can have collections of collections A common example would be something like Array Array Double to represent a matrix Both fill and tabulate can be used to make these val ident Array tabulate 3 3 i j if i j 1 0 else 0 0 Motivation Programs are more useful when they can interact with files Everything that isn t in a file is lost when the program stops running I O Redirection Using I O redirection gives you some very basic ability to read from and write to files It has big limitations though because there is only one file each way More over that one file blocks the ability to use either standard input or output Packages and Imports To read from a file we will be using the scala io Source type To understand what that means we need to talk about packages Packages provide a way to organize code and group things of like functionality Import statements let you use things without typing in their fully specified names The API To get a sense of the different package in Scala it is helpful to look at the API There are still lots of things in the API you won t fully understand That isn t a problem as you aren t expected to get too much from it right now scala io Source Call Source fromFile fileName String to get a Source object that reads from a file There are other methods in the main Source object that we will learn about later The fromFile method technically gives you BufferedSource This is for efficiency Iterators Both Source and BufferedSource are of the type Iterator Char An Iterator has most of the methods you are used to from List and Array However you can only go through it once Fundamentally uses hasNext and next methods getLines This will give you an Iterator String that will go through the file one line at a time instead of a character at a time You will often find this more useful Minute Essay What questions do you have IcP 5 on Friday note this is moving back a class
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