Loops 2 24 2012 Opening Discussion Minute 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 More Reading BigInts Sorting that is later in the semester Coding augmented reality and CV Uses of HUD Books website Breaking out in dance 5 IcPs Is stumbling along normal Games in majors lab The unzip method 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 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 The for Loop The most commonly used loop in most languages is the for loop The Scala version is a bit different from most Often used for counting In general it is a for each loop that goes through a collection for i 1 to 10 for e coll Variable takes on value of each element in the collection Range Type Range types provide an easy way to make collections for counting to and until operate on numeric types to produce ranges 1 to 10 0 until 10 Use by to change the stepping in a range 1 to 100 by 2 10 to 1 by 1 a to z by 3 yield The 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 coll yield expr What you get back will be a collection that is generally of the same type as what you iterated over if Guards You can put conditions in the for that will cause some values to be skipped for n nums if n 2 0 Multiple Generators You 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 Arrays You 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 Essay Any 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 weekend
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