Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11While Loop10-6-2010Opening DiscussionNo IcP because the assignment is due today.Minute essay commentsLength of assignments.What does reduce left do?Why are they called “collections”?Is it a problem if not many CS majors are learning Scala?How could you use map to multiply some components and add others?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.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.Minute EssayAny questions?Midterm is on Monday. I'll post a review sheet and try to schedule a review session.Interclass problem:Read in lines of input until the user enters a line that says just “quit”. Keep track of at least two of the following for all input.Occurrences of the word “the”.Occurrences of the letter 'e'.Number of words.Something
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