pep-material: comparison progs/lecture1.scala

equal deleted inserted replaced

-:b192bc772613
+:335079d938aa
 // Collections
 //=============
 List(1,2,3,1)
 Set(1,2,3,1)
 // ranges
 1 to 10
 if (a == b) println("Equal") else println("Unequal")
 Set(1,2,3) == Set(3,1,2)
 List(1,2,3) == List(3,1,2)
-val n1 = 3 + 7
-val n2 = 5 + 5
-n1 == n2
 // this applies to "concrete" values...pretty much everything;
 // but for example you cannot compare functions (later)
 //==================
 println("test")
 val tst = "This is a " ++ "test"
 print(tst)
 println(tst)
 val lst = List(1,2,3,1)
 //====================
 List(1,2,3,1).toString
 List(1,2,3,1).toSet
+"hello".toList
 "hello".toList.tail
 1.toDouble
-1L
+1L  // a long
-// a long
+1F  // a float
 // useful list methods
 List(1,2,3,4).length
 List(1,2,3,4).reverse
 t._4
 List(("one", 1), ("two", 2), ("three", 3))
 // Function Definitions
 //======================
 def incr(x: Int) : Int = x + 1
 def double(x: Int) : Int = x + x
 // If-Conditionals
 //=================
 // - Scala does not have a then-keyword
-// - !both if-else branches need to be present!
+// - !!both if-else branches need to be present!!
 def fact(n: Int) : Int =
 if (n == 0) 1 else n * fact(n - 1)
 fact(5)
 if (n == 0) 1 else x * power(x, n - 1)
 power(5, 5)
-// Option type
-//=============
-//in Java if something unusually happens, you return null or something
-//
-//in Scala you use Options instead
-//   - if the value is present, you use Some(value)
-//   - if no value is present, you use None
-List(7,2,3,4,5,6).find(_ < 4)
-List(5,6,7,8,9).find(_ < 4)
-// error handling with Options (no exceptions)
-//
-//  Try(something).getOrElse(what_to_do_in_case_of_an_exception)
-//
-import scala.util._
-import io.Source
-val my_url = "https://nms.kcl.ac.uk/christian.urban/"
-Source.fromURL(my_url).mkString
-Try(Source.fromURL(my_url).mkString).getOrElse("")
-Try(Some(Source.fromURL(my_url).mkString)).getOrElse(None)
-// the same for files
-Try(Some(Source.fromFile("text.txt").mkString)).getOrElse(None)
-// how to implement a function for reading something from files...
-def get_contents(name: String) : List[String] =
-Source.fromFile(name).getLines.toList
-get_contents("test.txt")
-// slightly better - return Nil
-def get_contents(name: String) : List[String] =
-Try(Source.fromFile(name).getLines.toList).getOrElse(List())
-get_contents("text.txt")
-// much better - you record in the type that things can go wrong
-def get_contents(name: String) : Option[List[String]] =
-Try(Some(Source.fromFile(name).getLines.toList)).getOrElse(None)
-get_contents("text.txt")
-get_contents("test.txt")
-// String Interpolations
-//=======================
-val n = 3
-println("The square of " + n + " is " + square(n) + ".")
-println(s"The square of ${n} is ${square(n)}.")
-// helpful for debugging purposes
-//
-//         "The most effective debugging tool is still careful thought,
-//          coupled with judiciously placed print statements."
-//                   — Brian W. Kernighan, in Unix for Beginners (1979)
-def gcd_db(a: Int, b: Int) : Int = {
-println(s"Function called with ${a} and ${b}.")
-if (b == 0) a else gcd_db(b, a % b)
-}
-gcd_db(48, 18)
-// Asserts/Testing
-//=================
-assert(gcd(48, 18) == 6)
-assert(gcd(48, 18) == 5, "The gcd test failed")
 // For-Comprehensions (not For-Loops)
 //====================================
 // BTW: a roundabout way of printing out a list, say
 val lst = ('a' to 'm').toList
 for (i <- (0 until lst.length)) println(lst(i))
 // Why not just? Why making your life so complicated?
 for (c <- lst) println(c)
 // Aside: concurrency
 // scala -Yrepl-class-based -cp scala-parallel-collections_2.13-0.2.0.jar
 for (n <- (1 to 10)) println(n)
 println(cnt)
 }
 test()
+// Option type
+//=============
+//in Java if something unusually happens, you return null or something
+//
+//in Scala you use Options instead
+//   - if the value is present, you use Some(value)
+//   - if no value is present, you use None
+List(7,2,3,4,5,6).find(_ < 4)
+List(5,6,7,8,9).find(_ < 4)
+// error handling with Options (no exceptions)
+//
+//  Try(something).getOrElse(what_to_do_in_case_of_an_exception)
+//
+import scala.util._
+import io.Source
+val my_url = "https://nms.kcl.ac.uk/christian.urban/"
+Source.fromURL(my_url).mkString
+Try(Source.fromURL(my_url).mkString).getOrElse("")
+Try(Some(Source.fromURL(my_url).mkString)).getOrElse(None)
+// the same for files
+Try(Some(Source.fromFile("text.txt").mkString)).getOrElse(None)
+// how to implement a function for reading something from files...
+def get_contents(name: String) : List[String] =
+Source.fromFile(name).getLines.toList
+get_contents("test.txt")
+// slightly better - return Nil
+def get_contents(name: String) : List[String] =
+Try(Source.fromFile(name).getLines.toList).getOrElse(List())
+get_contents("text.txt")
+// much better - you record in the type that things can go wrong
+def get_contents(name: String) : Option[List[String]] =
+Try(Some(Source.fromFile(name).getLines.toList)).getOrElse(None)
+get_contents("text.txt")
+get_contents("test.txt")
 // Further Information
 //=====================
 // The Scala homepage and general information is at

changeset 310	335079d938aa
parent 308	e86add5a6961
child 313	dea46bdfd648