Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Loops2-24-2012Opening DiscussionMinute essay comments:What books would I read if I had time?Impact of the power outage on our schedule. (IcPs)Cheat sheet can have front and back.Special characters in encryption/decryption.Looking at code: Google Code and Github.All scripts ARE online.Significant functions: map and filter.How to practice and study.List of List?MoreReading BigInts.Sorting – that is later in the semester.Coding augmented reality and CV.Uses of HUD.“Books website”Breaking out in dance.5 IcPsIs stumbling along “normal”?Games in majors lab.The unzip method.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.while LoopRecursion 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) st at ementThe condition is evaluated first. If it is true the statement (possibly a block) executes.This repeats until the condition is false.do-while LoopThe partner to the while loop is the do-while loop.do {statement} while(condi t ion)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 LoopThe most commonly used loop in most languages is the for loop. The Scala version is a bit different from most.Often used for counting:for(i <- 1 to 10) { ... }In general it is a “for each” loop that goes through a collection.for(e <- co ll) { ... }Variable takes on value of each element in the collection.Range TypeRange types provide an easy way to make collections for counting.“to” and “until” operate on numeric types to produce ranges.1 to 100 until 10Use “by” to change the stepping in a range.1 to 100 by 210 to 1 by -1'a' to 'z' by 3yieldThe 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 <- co ll) yield exprWhat you get back will be a collection that is generally of the same type as what you iterated over.if GuardsYou can put conditions in the for that will cause some values to be skipped.for(n <- nums; if(n%2==0)) ...Multiple GeneratorsYou 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 ArraysYou 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)Minute EssayAny questions?Midterm is on Monday. What times over the weekend work well for you to have a review session? I will also do a Google+ hangout review session during the
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