1 Gentle Introduction to Programming Session 2: Functions

2 Admin Upcoming Sunday: practical Schreiber Computer lab #4, 9-11, Scala / Eclipse installation status?

3 Review General terms in CS & programming Scala variables & types operators if / while Compiler / Interpreter Eclipse / Scala shell Ohad’s talk

4 Today More Scala: loops var / val Strings & API Defining variables (type inference) Variable scope Importance of style Guest lecture by Oded Magger (10:10) Home work review Functions Home work

5 Loops Used to repeat the same instructions until a stop criterion is met for, while loops: while ( ) for ( )

6 Example - while // factorial println( “ Enter a non-negative number ” ) var n : Int = Console.readInt() var fact : Int = 1 var i : Int = 1 while (i <= n) { fact = fact * i i = i + 1 } println(n + “ ! = “ + fact)

7 Example - for // factorial println( “ Enter a non-negative number ” ) var n : Int = Console.readInt() var fact : Int = 1 for (i <- 1 to n) { fact = fact * i } println( “ n! = “ + fact)

8 Examples at Factorial.scala

9 Example - Prime // Find whether a number is a prime println( “ Enter a non-negative number ” ) var n : Int = Console.readInt() var j : Int = 2 var stop : Boolean = false while ((j <= n/2) && !stop) { stop = (n%j == 0) j = j + 1 } if (!stop) { println(n) } Prime.scala

10 When to use for/while? Some applications are more natural to for, and others to while for is more suited when something is performed a predefined number of times, or when iterating over a list of objects while is more suited if the number of iterations is not known in advance (e.g., asking for legal input from a user)

11 Infinite Loops What are they? Beware of them

12 Scala’s Control Strctures All of Scala’s control structures result in some value: Example: println(if (price > 100) “expensive” else “cheap”)

13 Var versus Val val – “final” variable / imutable Example:

14 Strings val s : String = “ABC” A sequence of characters Have additional functionality (expressed by methods)

15 Application Programming Interface (API) How could I know that String has a method toLowerCase? How about checking the Scala API? Oops…

16 Java and Scala Scala is fully compatible with Java Actually, Scala is compiled into Java Bytecode And here is the String API (finally…)

17 Type Inference Scala’s ability to figure out types you leave off

18 Variable Scope output MultiTable.scala

19 Programming Style Comments: //, /* */ Indentation (cntl + i in eclipse) Meaningful variables names Why is it important?

20 Today More Scala: loops var / val Strings & API Defining variables (type inference) Variable scope Importance of style Guest lecture by Oded Magger (10:10) Home work review Functions Home work

21 Exercise 0 Write your first “Hello World!” program in Scala: Use the interpreter In Eclipse Make sure it compiles and executes properly Congratulations! Do you want us to do it together? Hello.scala

22 Fibonacci Fibonacci series 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 Definition: fib(0) = 0 fib(1) = 1 fib(n) = fib(n-1) + fib(n-2) en.wikipedia.org/wiki/Fibonacci_number

23 Exercise 1 Write a program that receives from the user an integer n > 0 and prints to the screen the n th Fibonacci number. a.Use a “ while ” loop b.Use a “ for ” loop

24 Solution ? Fibonacci.scala

25 Command Line Arguments In Eclipse

26 Command Line Arguments In Eclipse

27 Functions A group of declarations and statements that is assigned a name Effectively, a named statement block Usually has a value A sub-program When we write our program we always define a function named main Inside main we can call other functions Which can themselves use other functions, and so on…

28 Today More Scala: loops var / val Strings & API Defining variables (type inference) Variable scope Importance of style Guest lecture by Oded Magger (10:10) Home work review Functions Home work

29 What are They Good For? Generalize a repeated set of instructions We don’t have to keep writing the same thing over and over Solve bugs once… They can break your problem down into smaller sub-tasks Easier to solve complex problems They make a program much easier to read and maintain Abstraction – we don’t have to know how a function is implemented to use it

30 In Short Why do we need functions? Code reusability Modularity Abstraction

31 Function Definition in Scala

32 Example

33 Example

34 Function Output The last statement defines returned value to the calling function The type of the value returned must be the same as the return-type defined for the function (or a ‘lower’ type) If no value is to be returned, the return-type of the function should be set to ‘Unit’, and a unit value is returned

35 Other Types of Functions Method – member of some object (e.g., String – toLowerCase) Local functions: functions nested within functions Function literals (First-class functions), e.g: (x:Int) => x+1 Why do we need it anyway? Function values (e.g., val f = (x:Int => x+1))

36 Example: Nested Functions PrimesNested.Scala

37 There are many ways to define functions in Scala FuncDef.scala

38 Example ((x : Int) => x * 2)(a) ((x : Int) => x * 2)(5) ((x : Int) => x * 2) 55 10

39 Today More Scala: loops var / val Strings & API Defining variables (type inference) Variable scope Importance of style Guest lecture by Oded Magger (10:10) Home work review Functions Home work

40 Exercise – Integer Division Input: Two integers – A and B Output: How many times A contains B (it is the result of the integer division A/B) Do not use the operators ‘/’, ‘*’ Solution:

41 Exercise – Power of Two Input: integer A Output: is there an integer N such that A == 2^N? Solution:

42 Write a program that prints an upside-down half triangle of *. The height of the pyramid is the input. ***** *** ** **** * Exercise – Triangle Printout