Scoping and Recursion

1
Scoping and Recursion

• CSC 171 FALL 2001
• LECTURE 8

2
History Konrad Zuse

• 1935-1938
• Konrad Zuse, Berlin, developed his Z-1 computer
  using relays
• After the war, he reconstructed his Z-4 machine
  at the University of Zurich
• Founded a computer company absorbed into Siemens
  Corporation.

3
Scope the life cycle of variables

• What is the output? And why?
• public class myClass
• static int x 4
• public static void main(String args)
• int x 5
• myMethod()
• System.out.println(x x)
• public static void myMethod() x 6

4
Why?

• Two variables exist, both named x
• Think of having two friends, both called Joe
• We distinguish Joe from Joe by use of a full name
• Joe Clarkson vs. Joe Jackson
• We can think of variables as having full names
• one is named myClass x
• One is named main x
• Consider the scope of the two variables

5
Scope

• What happens now?
• public class myClass
• static int x 4
• public static void main(String args)
• myMethod()
• System.out.println(x x)
• public static void myMethod() x 6

6
Types of Scope

• Class scope
• Methods and instance variables of a class have
  class scope
• These things exist within the of the class
• Block Scope
• Begins at identifiers declaration
• Ends at
• Local variables and method parameters have block
  scope

7
Block Scope

• What happens?
• public class myClass
• static int x
• public static void main(String args)
• int x 5
• int y 6
• System.out.println(x x y y)

8
Scope

• What happens now?
• public class myClass
• static int x 4
• public static void main(String args)
• myMethod(x)
• System.out.println(x x)
• public static void myMethod(int x) x 6

9
Common Error

• Will it compile?
• public class myClass
• static int x 4
• public static void main(String args)
• if (x lt 5) int y 5 else int y 6
• System.out.println(x x y y)

10
Common Error

• Does this fix it?
• public class myClass
• static int x 4
• public static void main(String args)
• int y
• if (x lt 5) int y 5 else int y 6
• System.out.println(x x y y)

11
Parameter Scoping

• public class myClass
• int x 4
• public static void main(String args)
• int x myMethod1(5)
• System.out.println(x x)
• public static int myMethod1(int x)
• myMethod2(x) return x 1
• public static int myMethod2(int x) x 7

12
Parameter Scoping

• public class myClass
• int y 0
• public static void main(String args)
• int x myMethod1(5) yx
• System.out.println(x x y y)
• public static int myMethod1(int x)
• y x return myMethod2(x) 1
• public static int myMethod2(int x) yx return
  x 1

13
Recursion

• Methods can call other methods
• Methods can also call themselves
• public long factorial(long number)
• if (number lt1) return 1
• else
• return number factorial(number 1)

14
Recursion vs. Iteration

• GCD
• Euclids (300 B.C.) Algorithm
• Oldest known non-trivial algorithm
• If r is the remainder when a is divided by b
• Then the common divisors of a and b are the same
  as the common divisors of b and r

15
Examples of Euclids Alg

• GCD(206,40)
• GCD(6,4)
• GDC(4,2)
• GCD(2,0)
• 2

• GCD(154,35)
• GCD(35,14)
• GCD(14,7)
• GCD(7,0)
• 7

16
In Class Excercise

• Using Euclids algorithm write
• public int GCD(int a, int b)
• Partner up
• 10 min
• hand in on paper two names

17
Iterative version

• public static int GCD1(int a , int b)
• while (b ! 0)
• int temp ab
• a b
• b temp
• return a

18
Recursive version

• public static int GCD2(int a , int b)
• if (b 0 ) return a
• else return GCD2(b,ab)