pep-material: comparison progs/lecture1.scala

equal deleted inserted replaced

-:d0caa12ab8d8
+:45557ad18ea6
 println(lst.mkString("\n"))
 // some methods take more than one argument
 println(lst.mkString("[", ",", "]"))
 // Conversion methods
 //====================
 List(1,2,3,1).toString
 List(1,2,3,1).toSet
 "hello".toList
 1.toDouble
+// useful list methods
+List(1,2,3,4).length
 List(1,2,3,4).reverse
+List(1,2,3,4).max
+List(1,2,3,4).min
+List(1,2,3,4).sum
+List(1,2,3,4).take(2).sum
+List(1,2,3,4).drop(2).sum
+List(1,2,3,4,3).indexOf(3)
+"1,2,3,4,5".split(",").toList
 // Types
 //=======
 /* Scala is a strongly typed language
 t._4
 // Hello World
 //=============
-// show an example of a stand-alone scala file
+// an example of a stand-alone scala file;
-// remind that in the course work you are asked a
+// in the coursework students must submit
-// plain scala "work-sheet"
+// plain scala "work-sheets"
+object Hello extends App {
+println("hello world")
+}
+// can be called with
+//
+// $> scalac hello-world.scala
+// $> scala Hello
+//
+// $> java -cp /usr/local/src/scala/lib/scala-library.jar:. Hello
 // Function Definitions
 //======================
 // Assert/Testing
 //================
+assert(gcd(48, 18) == 6)
+assert(gcd(48, 18) == 5, "The gcd test failed")
 // For-Comprehensions (not For-Loops)
 //====================================
 for (n <- (1 to 10).toList) yield square(n)
 m <- (1 to 10).toList) yield m * n
 mult_table.sliding(10,10).mkString("\n")
+// with if-predicates
+for (n <- (1 to 3).toList;
+m <- (1 to 3).toList;
+if (n + m) % 2 == 0) yield (n, m)
 // with patterns
 for ((m, n) <- List((1, 4), (2, 3), (3, 2), (4, 1))) yield m + n
 for (p <- List((1, 4), (2, 3), (3, 2), (4, 1))) yield p._1 + p._2
+// with only a side-effect (no list is produced)
+// has no "yield"
+for (n <- (1 to 10)) println(n)
+// concurrency (ONLY WORKS IN 2.11.8)
+for (n <- (1 to 10)) println(n)
+for (n <- (1 to 10).par) println(n)
+// for testing time
+def time_needed[T](i: Int, code: => T) = {
+val start = System.nanoTime()
+for (j <- 1 to i) code
+val end = System.nanoTime()
+((end - start) / i / 1.0e9) + " secs"
+}
+val list = (1 to 1000000).toList
+time_needed(10, for (n <- list) yield n + 42)
+time_needed(10, for (n <- list.par) yield n + 42)
 // Webpages
 //==========
+import io.Source
+val url = """http://www.inf.kcl.ac.uk/staff/urbanc/"""
+Source.fromURL(url)("ISO-8859-1").mkString
+def price_lookup(symbol: String): String = {
+val url = "http://finance.yahoo.com/d/quotes.csv?s=" + symbol + "&f=snl1"
+Source.fromURL(url).mkString.drop(1).dropRight(2)
+}
+price_lookup("GOOG")
+price_lookup("AAPL")
+val companies =
+List("GOOG", "AAPL", "MSFT", "IBM", "FB", "YHOO", "AMZN", "BIDU")
+for (s <- companies.par) println(price_lookup(s))
+// A Web Crawler
+//===============
+import io.Source
+import scala.util.matching.Regex
+import scala.util._
+// gets the first 10K of a web-page
+def get_page(url: String) : String = {
+Try(Source.fromURL(url)("ISO-8859-1").take(10000).mkString).
+getOrElse { println(s"  Problem with: $url"); ""}
+}
+// regex for URLs
+val http_pattern = """"https?://[^"]*"""".r
+// drops the first and last character from a string
+def unquote(s: String) = s.drop(1).dropRight(1)
+def get_all_URLs(page: String): Set[String] =
+http_pattern.findAllIn(page).map(unquote).toSet
+// naive version of crawl - searches until a given depth,
+// visits pages potentially more than once
+def crawl(url: String, n: Int): Unit = {
+if (n == 0) ()
+else {
+println(s"Visiting: $n $url")
+for (u <- get_all_URLs(get_page(url))) crawl(u, n - 1)
+}
+}
+// some starting URLs for the crawler
+val startURL = """http://www.inf.kcl.ac.uk/staff/urbanc"""
+//val startURL = """http://www.inf.kcl.ac.uk/staff/mcburney"""
+crawl(startURL, 2)
+// Adding your own methods to Strings
+//====================================
+// imagine you want to implement an additional
+// method to strings, like
+//
+//     "HAL".increment
+//
+// you can avoid ugly fudges, like a MyString
+// class by using implicit conversions
+implicit class MyString(s: String) {
+def increment = for (c <- s) yield (c + 1).toChar
+}
+"HAL".increment
+// Further Information
+//=====================
+// Scala download
+//
+//  http://www.scala-lang.org
+// Eclipse for Scala
+//
+//  http://scala-ide.org/download/sdk.html
+// library docs
+//
+//  http://www.scala-lang.org/api/current/
+// tutorials
+//
+//  http://docs.scala-lang.org/tutorials/

changeset 32	45557ad18ea6
parent 26	a7afc2540a88
child 33	b6cb302633eb