Презентация на тему Scala. Lists

occupy sequential locations in memory
This makes random access (for

example, getting the 37th element) very fast--O(1)
Lists are composed of values linked together
All access starts from the head (first element) and follows links
Random access takes linear time

access is via the head, creating a “new” list

is a fast operation

• myLongerList looks like List("p", "a", "r", "t"); the "p" is not visible from myList

• myShorterList looks like List("r", "t")

• myList has not been changed--it is immutable

head) returns the first element in the list
list.tail (or

list tail) returns a list with the first element removed
value :: list returns a list with value appended to the front
list.isEmpty (or list isEmpty ) tests whether the list is empty
Some slow (linear time) operations
list(i) returns the ith element (starting from 0) of the list
list.last (or list last) returns the last element in the list
list.init (or list init) returns a list with the last element removed
This involves making a complete copy of the list
list.length (or list length) returns the number of elements in the list
list.reverse (or list reverse) returns a new list with the elements in reverse order
In practice, the slow operations are hardly ever needed

for (i

println(myList(i)) } }
What is the time complexity of this method?
def printList2(myList: List[Any]) { if(! myList.isEmpty) { // the dot is required here println(myList head) printList2(myList tail) } }
What is the time complexity of this method?

All elements have the same type
However, scala> "abc" :: List(1,

2, 3) res15: List[Any] = List(abc, 1, 2, 3)
The newly-created list has a type which is the least upper bound
An empty list has “nothing” in it
scala> List() res16: List[Nothing] = List()
The “name” of the empty list is Nil
scala> Nil res17: scala.collection.immutable.Nil.type = List()
Lists are built from Nil and the :: operator (which is right-associative)
scala> 1 :: 2 :: 3 :: Nil res18: List[Int] = List(1, 2, 3)
scala> 1 :: (2 :: (3 :: Nil)) res19: List[Int] = List(1, 2, 3)

the basic formula for working with a list:
if the

list is empty return some initial value (often an empty list) else process the head recur with the tail
def printList2(myList: List[Any]) { if(! myList.isEmpty) { println(myList head) printList2(myList tail) } }

printList2(myList: List[Any]) { if(! myList.isEmpty) {

println(myList head) printList2(myList tail) } }
Here it is with pattern matching:
def printList3(myList: List[Any]) { myList match { case h :: t => println(myList head) printList3(myList tail) case _ => } }

of a List, returning a new List
scala> List(1, 2,

3, 4) map (n => 10 * n) res0: List[Int] = List(10, 20, 30, 40)
The result list doesn’t have to be of the same type
scala> List(1, 2, 3, 4) map (n => n % 2 == 0) res1: List[Boolean] = List(false, true, false, true)
Since an element of the list is the only parameter to the function, and it’s only used once, you can abbreviate the function
scala> List(1, 2, 3, 4) map (10 * _ + 6) res2: List[Int] = List(16, 26, 36, 46)
Of course, you don’t have to use a literal function; you can use any previously defined function (yours or Scala’s)
scala> List(-1, 2, -3, 4) map (_ abs) res3: List[Int] = List(1, 2, 3, 4)

nesting)
scala> val nested = List(List(1, 2, 3), List(4, 5)) nested:

List[List[Int]] = List(List(1, 2, 3), List(4, 5))
scala> nested flatten res0: List[Int] = List(1, 2, 3, 4, 5)
flatMap is like map, but the function given to flatMap is expected to return a list of values; the resultant list of lists is then “flattened”
Syntax:
def map[B](f: (A) => B): List[B]
def flatMap[B](f: (A) => Traversable[B]): List[B]
Example:
scala> val greeting = List("Hello".toList, "from".toList, "Scala".toList) greeting: List[List[Char]] = List(List(H, e, l, l, o), List(f, r, o, m), List(S, c, a, l, a))
scala> greeting map (word => word.toList) res2: List[List[Char]] = List(List(H, e, l, l, o), List(f, r, o, m), List(S, c, a, l, a)) scala> greeting flatMap (word => word.toList)
res3: List[Char] = List(H, e, l, l, o, f, r, o, m, S, c, a, l, a)

a list, returning a new list
scala> List(1, -2, 3,

-4) filter (_ > 0) res3: List[Int] = List(1, 3)
There is a corresponding (less often used) filterNot method
scala> List(1, -2, 3, -4) filterNot (_ > 0) res4: List[Int] = List(-2, -4)

to the values in a list, pairwise, starting from

the left or starting from the right
scala> val list = List(10, 1, 2, 3) list: List[Int] = List(10, 1, 2, 3)
scala> list.foldLeft(0)(_ - _) res3: Int = -16
scala> list.foldRight(0)(_ - _) res4: Int = 8
scala> ((((0 - 10) - 1) - 2) - 3) res6: Int = -16
scala> (10 - (1 - (2 - (3 - 0)))) res8: Int = 8

for loop
scala> for (ch

ch <- "abcde" is a generator; you can have more than one
scala> for { x <- 1 to 5 | y <- 10 to 30 by 10 } print((x + y) + " ") 11 21 31 12 22 32 13 23 33 14 24 34 15 25 35
The above needs braces, { }, not parentheses, ( )
You can have definitions (not the same as declarations):
scala> for (i <- 1 to 10; | j = 100) print ((i + j) + " ") 101 102 103 104 105 106 107 108 109 110
j = 100 is a definition
In this example, the semicolon preceding the definition is required
You can also have guards:
scala> for (i <- 1 to 10 | if i != 7) print(i + " ") 1 2 3 4 5 6 8 9 10

generator, and after that you can have more generators,

definitions, and guards
scala> for { i <- 1 to 5 if i % 2 == 0 | k = 100 | j <- 1 to 5 | if j * k < 450 } print((k + 10 * i + j) + " ") 121 122 123 124 141 142 143 144

yield, is ()
With a yield, the value is a

list of results (one result for each time through the loop)
The syntax is: for (sequence) yield expression
Examples:
scala> for (i <- 1 to 5) yield 10 * i res12: scala.collection.immutable.IndexedSeq[Int] = Vector(10, 20, 30, 40, 50)
scala> for (n <- List("one", "two", "three")) yield n.substring(0, 2) res2: List[java.lang.String] = List(on, tw, th)

p. 125):
val forLineLengths = for {

file <- filesHere // ‘filesHere’ is an array of files if file.getName.endsWith(".scala") line <- fileLines(file) // get an Iterator[String] trimmed = line.trim if trimmed.matches(".*for.*") } yield trimmed.length // get an Array[Int]
The above method:
gets each file from an array of files
considers only the file with the .scala extension
gets an iterator for the lines in the file
removes whitespace from the beginning and end of the line
looks for “for” within the line (using a regular expression)
counts the number of characters in the line
returns an array of line lengths of lines containing “for” in scala files

List(1, 2, 3, 4)

scala> "abc" toList res13: List[Char] = List(a,

b, c)

scala> Map("apple" -> "red", "banana" -> "yellow") toList res14: List[(java.lang.String, java.lang.String)] = List((apple,red), (banana,yellow))

scala> Set("abc", 123) toList res16: List[Any] = List(abc, 123)

scala> List(1, 2, 3) toList res17: List[Int] = List(1, 2, 3)
Also: toArray, toString, toSet, toMap

"b", "c") myList: List[java.lang.String] = List(a, b, c)
This works:
scala> val

List(x, y, z) = myList x: java.lang.String = a y: java.lang.String = b z: java.lang.String = c
But it’s pretty useless unless you know the exact number of items in the list
Here’s a better way:
scala> val hd :: tl = myList hd: java.lang.String = a tl: List[java.lang.String] = List(b, c)

def main(args: Array[String]) { println(translate("Scala is a wonderful language !")) }
def translate(english: String) = {
val dictionary = Map("a" -> "ein", "is" -> "ist", "language" -> "Sprache", "wonderful" -> "wunderbar")
def lookup(word: String) = { if (dictionary contains word) dictionary(word) else word }
(english.split(" ") map (lookup(_))).mkString(" ") }
}
Output: Scala ist ein wunderbar Sprache !