Week 12 Review and Exam 2

CS 1063 Week 12: While Loops

Objectives

Write loops using the while statement

Assignments

Reading Week 12
Begin Project 3
Take Quiz 9 (Thursday, Friday and Saturday)

The `while` loop:

A while statement executes a block of code repeatedly. A termination condition controls how often the loop is executed.
The while loop is the most general kind of loop --- when in doubt, you can always use a while loop. Form:
```
while (condition)
   statement
```
If the condition is true, the statement is executed, followed by reevaluating the condition. As soon as the condition is false, the statement after the loop is executed.

Examples

Example 1: Output the numbers between 1 and 10 using a while loop. How many times will this loop execute? How many times will the condition be evaluated?

int num = 1;
while ( num <= 10 ) {
   System.out.print(num + " ");
   num++;
}
System.out.println();

Example 2: Reading an initial value so that the loop test can be evaluated the first time is called priming the loop. This loop stops when number is equal to 0. Do you know exactly how many times this loop will execute?

int sum = 0;
Scanner in = new Scanner(System.in));
System.out.print("Enter a number to be added: ");
int number = in.nextInt();          

while ( number != 0 ) {
   sum += number;
   System.out.print("Enter a number to be added: ");
   number = in.nextInt(); 
}
System.out.println("total = " + sum);

Example 3: If the user enters an invalid value, a loop can be used to print an error message and to continue reading values until a valid one is entered. This loop stops when month is between 1 and 12, inclusive. Do you know exactly how many times this loop will execute?

Scanner in = new Scanner(System.in));
System.out.print("Enter the month (1-12): ");
int month = in.nextInt();          

while ( month < 1 || month > 12 ) {
   System.out.println("That is an invalid value.");
   System.out.print("Enter the month (1-12): ");
   month = in.nextInt();          
}
System.out.println("month = " + month);

Activity 1 Bank Investment

A bank account with an initial balance of $10000 earns 5% interest. The interest is computed at the end of every year on the current balance and then deposited into the bank account. How many years does it take for the balance to reach 20,000? See the table below for an example.

Year Balance

0 10,000

1 10,500

2 11,025

3 11,576.25

4 12,155.06

5 12,762.82

Year	Balance
0	10,000
1	10,500
2	11,025
3	11,576.25
4	12,155.06
5	12,762.82

An accessor method is a method that does not change the state of an object. That is, it does not change the values stored in the instance field.

A mutator method is a method that changes the state of an object. That is, it changes one or more of the values stored in the instance field.

Write a program that includes a class Investment. The constructor takes an initial balance and a rate of interest. It will contain the typical accessor methods getBalance and getInterest and a method yearsForBalance that will return the number of years it takes to reach a target balance. Your class should also contain a method reset that takes new values for the initial balance and rate of interest as parameters and stores these values in the instance field - This method should be called from your constructor.

Your test class should:

Read the initial investment, rate and target balance from the keyboard using the Scanner class.
Instantiate an Investment object named invest using these numbers.
Print the number of years it will take for your initial investment to reach your target balance.
Read another initial investment, rate and target balance from the keyboard.
Change your instance field using the reset method.
Print the number of years it will take for this initial investment to reach the new target balance.

Activity 2 Greatest Common Divisor

Write a program that finds the greatest common divisor, or gcd, of two numbers. The gcd of two numbers x and y is the largest number that evenly divides both x and y. For example, the gcd of 12 and 42 is 6, and the gcd of 14 and 74 is 2. An example of Euclids algorithm is given below.

Example 1:

Step 1: Divide one number by the other. The order does not make a difference.
12/42 = 0 remainder 12
Step 2: Divide the divisor by the remainder from step 1.
42/12 = 3 remainder 6
Step 2 should be repeated until the remainder = 0. The last divisor is the answer.
12/6 = 2 remainder 0
Stop - the answer is 6

Example 2:

Step 1: Divide one number by the other. The order does not make a difference.
74/14 = 5 remainder 4
Step 2: Divide the divisor by the remainder from step 1.
14/4 = 3 remainder 2
Step 2 should be repeated until the remainder = 0. The current divisor is the answer.
4/2 = 2 remainder 0
Stop - the answer is 2

Write a class Gcd that has a constructor that takes two integers. You should have the usual accessor method(s). Gcd should also have a method calculateGcd that returns the gcd of the numbers. Furnish a method reset that takes new values for the numbers and stores them in the instance fields so that calculateGcd can be calledagain. Call reset from the constructor to initialize your instance field.

Your test class should do the following:

Read the two numbers from the keyboard using the BufferedReader or Scanner class.
Instantiate a new Gcd object named test with the two numbers from the keyboard
Print the gcd of the two numbers
Ask for two more numbers from the keyboard
Change your numbers using the reset method
Print the gcd of the two numbers

Activity 3: Simulations using the `Random` class

In a simulation, you repeatedly generate random numbers and use them to simulate an activity.

Setup:

Project/Folder name: Dice
New class with main method: DiceTest
New class: Dice
Type in Dice.java using method stubs with comments.
Compile Dice.java to check for any errors.
Save your project.

Public interface for the Dice class:

// Constructor
public Dice (int sides)

// accessors
// Returns the number of sides on the die
public int getSides()

// Returns the value of the last roll
public int getLastRoll()

// Outputs the state of the die
public String toString()

// modifier
// Finds and returns the value of the last roll
public int roll()

Write a test program that simulates throwing 2 Dice. You are to determine the number of throws it takes to reach a target sum. In the test class:

Instantiate two 6 sided dice d1 and d2.
Read a target sum from the keyboard.
Roll the dice and get a beginning sum.
Print your sum.
Initialize your times thrown to 1. Why 1?
Use a while loop to continue throwing the dice until you obtain the target sum.
When the loop finishes print the number of times it takes to throw the target number.