Argument Passing 2 18 2011 Opening Discussion Minute essay comments Study guide for the midterm Average of numbers under 100 Methods return new collection New variable is best Could use recursion Review some of the standard methods on sequences Higher Order Methods The most powerful methods are ones you can pass functions into exist forall Boolean checks like for math filter partition separate collection based on Boolean map apply function to all the elements reduceLeft apply function moving through collection foldLeft apply function moving through but allows initial value so it can return a different type This is curried Let s Put These Into Action I want to spend some time playing with these methods and seeing what we can do with them A String is a collection so you can do these things with a String as well 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 What questions do you have about collections Remember to do IcP 4
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