CS100J, Spring 2001
Tues 9/25
Lecture 8
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Announcements:
+ new? responsible for www.cs.cornell.edu/Courses/cs100j/2001fa/
+ E1: put in folder (sort by last name)
+ P3 due Thurs 10/4
+ T1: on Mon 10/1 (rooms not known until Thurs)
+ Review session: Exams-->Review 1
+ MATLAB: See Announcements
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Topics:
+ control flow
+ methods
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Redundancy and clarity:
+ Statements: expression, assignment, selection, repetition
+ very powerful -> can solve many, many problems
+ problem:     spaghetti code! (thing FORTRAN)
+ solution:    pull repeated tasks into their own "chunks"
+ abstraction: chunkification, move to more abstract levels, hide "the dirt"
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Mathematical functions:
+ think function: 
  you write/enter name(inputs), and it generates output
+ without programming:
    ____
  \/ 4    => 2

  sqrt(4) => 2

  4 is the input/argument/parameter
  2 is the output/returned value

  the action of performing the function/method: INVOCATION/CALLING/

  NOTE: the returned value REPLACES the invoked function
        how? what about sqrt(4) + 3 ? 
        you would add 2 + 3 -> 5
        so, the 2 replaced sqrt(4)
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Programming functions:
+ called METHODS in Java
+ examples:
  Math.random()
  System.out.println(_stuff_)
  System.out.print(_stuff_)
  main(String[] args)
  String1.equals(String2)
  SavitchIn.readInt()
  Max.max(v1,v2)
  Max.min(v1,v2)
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Syntax:
+ how to write your own?
+ general structure:
  header {
     body
  }
+ syntax:	
  modifiers returntype name(arguments) {
      statements;
  }
+ must access a method from a class or from within a class
+ you may write methods in any order in a class -- Java will find them!
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Structure:
+ Need block statement as part of method
+ no semicolons after the last brace (})
+ indent statements (the substructure) underneath the header
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Commenting:
+ brief: describe action of method above the header
+ longer: see pp 243-244, App.4 on pg 993 for an example
  - Precondition: what's true before the method is invoked
  - Action: what the method does
  - Postcondition: what's true after the method is invoked
+ action and postcondition are sometimes the same thing
+ sometimes it's clearer to just write the action
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Arguments:
+ arguments are variables that receive values and are used in method body
+ Java is strongly typed
+ arguments is composed of series of inputs with form:
  type1 var1, type2 var2, ...
+ can also have NO arguments
  ex) System.out.println(), which prints a blank line
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Name:
+ use a legal identifier
+ name should be an action
+ lowercase for first part of name
+ ex) getDistance, computeArea
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Return type:
+ methods RETURN either a value or nothing

(1) nothing to return:
    - ex) System.out.println();
    - cannot use println() in an expression!
    - note: common source of confusion: output is NOT a returned value
      proof: try $int x = System.out.print("Hello");$
    - use $void$ keyword
    - may use $return;$ to break from a void method

(2) value to return:
    - the method computes a value and sends it back to the expression
      that invoked the method
    - the value replaces the method in the expression
    - ex) double x = Math.sqrt(4)+10;
          // x gets 2 + 10, or 12
    - Java is strongly typed, so type of returned value 
      must be the returntype
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Modifiers:
+ use $public static$ or $private static$ for now
+ why? We're putting everything in the Main Class
+ want to hold off on OOP until next week
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Statements:
+ Statements execute in method after the method is invoked
+ follow same control flow
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Example:

  public class Example1 { 
     public static void myPrint(int x) {
        System.out.println("The answer is: "+x);
     } 

     public static void main(String[] args) {
        myPrint(10);
     }
  }     

+ control flow:
  - Java starts at $main$ in Main Class (assuming you set Main Class target)
  - Java works top-down, statement-to-statement
  - Java invokes myPrint(10):
  - the value 10 is copied in x
  - the string and 10 are added together by promoting 10 to "10" 
  - the string "The answer is: 10" is printed
  - the controld returns to the point where the method was invoked
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Terminology:

+ Formal parameters:
  - variables written inside method header
  - born when method invoked
    live during method execution
    die when method ends

+ Actual parameters:
  - anything passed into a method

+ Call by value:
  - value of actual parameter copied into formal parameter
  - method can NOT change the value of an actual parameter

+ Scope:
  - the visibility of a element of code (variable, method)
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Example:

  public class Example2 { 
     public static void myPrint(String s, int x) {
        System.out.println(s+" is "+x);
     } 

     public static int myRandom(int x) {
        int y = 1;
	return (int)( Math.random()*(x+y) );
     }

     public static void main(String[] args) {
	int x = 10;
	int y = myRandom(x);
	myPrint(x);
	myPrint(y);
     }
  }     
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Example:   

  public class Example3 { 

     public static void warning(int x) {
        if (x != 0) {
	   System.out.println("Buh!");
	   return;
	else
  	   System.out.println("OK!");
     }

     public static void main(String[] args) {
        warning( (int) Math.random()*2 );
	System.out.println("yo!");
     }
  }     
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