To Notes

CSC 212 -- 1/29/08

Review Questions

1. What is wrong?

   public void Coin( )
   {
       flip( );
   }
   

   Ans: A constructor cannot be declared with the return value void or any other return value.

2. What is wrong?

   > javac Coin.java
   > java Coin.java
   

   Ans: (1) The second line should be java Coin. (2) The Coin class does not contain a main method, so java Coin will not work. The calls should be:

   > javac Coin.java TestCoin.java
   > java Coin
   

3. Rewrite this flip method so that it does not use an if statement:

   public void flip( )
   {
       if (Math.random( ) >= 0.5)
           headsUp = true;
       else
           headsUp = false;
   }
   

   Ans:

   public void flip( )
   {
       headsUp = Math.random( ) >= 0.5;
   }
   

4. Draw a reference diagram for the following code:

   Person p = new Person("Alice", 'F', 23);
   Person q = new Person("Shawn", 'M', 19);
   q = p;
   

   Ans: The variable p contains an arrow that points to the object with instance variables Alice, F, 23. The variable q contains an arrow that points to the object with instance variables Shawn, M, 19. When the statement q = p is executed, the arrow for q that points to the object Shawn is changed to point to the object Alice.

5. Write a main method for the Coin class that flips a coin 10 million times, keeping track of the number of heads obtained.

   Ans:

   public static void main(String[ ])
   q = p;
   

6. Write a main method for the Coin class that keeps track of the longest run of heads obtained.

7. What is wrong with this for loop that adds up the odd numbers between 1 and 100?

   sum = 0;
   for(i = 1, i != 100, i += 2);
       sum + = i;
   

Common Bugs for the Clock Lab Exercise

• Bad capitalization of keywords or variable names, for example spelling Switch instead of switch.

• Redeclaration of instance variables. In constructor:

  int sec = theSec;
  

• Using = instead of ==.

• Misplaced semicolon:

  if (sec == 60);
  {
      sec = 0;
      min++;
  }
  

• Bad indentation makes code harder to understand.

Arrays

• An array is used to hold multiple copies of elements of the same type, for example, int, double, String, Person, etc.

• Array Declaration

  double[ ] x;
  
  

• Array Instantiation

  x = new double[50];
  
  

• Array Population

  x[31] = 452.376;
  
  

• Array Access

  v = x[31];
  
  

• Write code to create the following arrays, Include code to verify that the arrays were created properly.
  1. An array of doubles: 3.54, 5.30, 7.39, 9.01.

  2. An array of String: "dog", "cat", "mouse".

  3. An array of Person objects:
     ("Alice", 'F', 23), ("Jerry", 'M', 21), ("Judy", 'F', 19).

• See the TestArrays Example:   TestArrays.java   Person.java

Searching an Array

• The SearchPersonArray Example uses a for loop to search an array for the Person object with a given name:
  SearchPersonArray.java   Person.java

• Write a program that finds the minimum value and corresponding index of the following int array:

  { 67, 23, 57, 93, 91, 45, 29 }
  

• Answer:   FindMin.java

Sorting

• Example:   Use the following code to sort this array of three elements: { 8, 5, 3 }

  if (a[0] > a[1])
  {
      temp = a[0];
      a[0] = a[1];
      a[1] = temp;
  }
  if (a[1] > a[2])
  {
      temp = a[1];
      a[1] = a[2];
      a[2] = temp;
  }
  if (a[0] > a[1])
  {
      temp = a[0];
      a[0] = a[1];
      a[1] = temp;
  }
  

• Look at the InsertionSort Example:   InsertionSort.java

• Trace the InsertionSort sorting algorithm using this table:

  i	j	a[0]	a[1]	a[2]	a[3]	a[4]	temp
  		45	34	76	53	40

  Add more rows in the table as needed.