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CS162 Week 2

CS162 Week 2. Kyle Dewey. Overview. More on Scala Assignment I review Secure information flow. More on Scala. Running the REPL. Just type scala at the command line Pretty nifty to quickly check to see what an expression does. Unit.

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CS162 Week 2

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  1. CS162 Week 2 • Kyle Dewey

  2. Overview • More on Scala • Assignment I review • Secure information flow

  3. More on Scala

  4. Running the REPL • Just type scala at the command line • Pretty nifty to quickly check to see what an expression does

  5. Unit • Essentially void, but there is an object associated with it • If you define a method without an equals, (i.e. def foo {}), then the return type is Unit • Can say () to refer to the Unitobject

  6. Seq • Some Sequence • In many ways very similar to Java’s List • List, Vector, and related classes are subclasses • Note: Scala’s List refers specifically to a linked list, more like Java’s LinkedList

  7. null • In general, null is an excellent wonderful/terrible feature • Often poorly documented whether or not null is possible • Checking for impossible cases • Not checking for possible cases

  8. Option • A solution: encode null as part of a type • For some type, say Object, if null is possible say we have a NullPossible<Object> • Scala has this, known as Option • In general, if null is possible, use Option

  9. Tuples • For when you want to return more than one thing • Can be created by putting datums in parenthesis • Can pattern match on them

  10. Sequence Processing Functions • AKA: Why while is rare and for isn’t for

  11. foreach • Applies a given function to each element of a Seq

  12. map • Like foreach, in that it applies a given function to each element of a sequence • However, it also returns a new sequence that holds the return values of each of the function calls

  13. filter • Takes a predicate, i.e. a function that returns true or false • Applies the predicate to each item in a list • A new list is returned that contains all the items for which the predicate was true

  14. foldLeft • Extremely flexible, but sometimes unwieldy • Takes a base element • Takes a function that takes a current result and a current list element • The function will manipulate result with respect to the current element

  15. flatMap • Like map, but made especially for functions that return Seqs • Will internally “flatten” all of the inner Seqs into a single Seq • More on this later in the course

  16. for Comprehensions • Much like Python’s list comprehensions • Internally translated into a series of foreach, flatMap, map, and filter operations

  17. Assignment I Review

  18. Secure Information Flow Assignment

  19. Missing File • As written, there is a missing file: util.scala • Option 1: Download zip file from the course website (under “Interpreter Code”), copy util.scala, and add it to the makefile • Option 2: Remove all mentions of the pretty printer (from util.scala)

  20. Adding a Field for the Label

  21. pc Stack • Define an object named pc • It internally has a mutable stack • There are many ways to do this, but scala.collection.mutable.Stack is probably the easiest

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