While Loop 10 6 2010 Opening Discussion No IcP because the assignment is due today Minute essay comments Length 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 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 Variable Length Argument Lists You 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 Collections It 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 Mutability I 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 Passing When you pass arguments you are really passing references So arguments in functions are aliases to the objects outside the function If the object is mutable the function can change it Pass by Name There 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 Tabulate There 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 Essay Any 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 else
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