CS 1063  Week 10:  Decisions

Objectives

• Group statements into blocks
• Compare integers, doubles, strings, and objects
• Program conditions using Boolean operators and variables

Assignments

• Reading Week 10
• Lab 7 Due (Monday)
• Begin Lab 8
• Take Quiz 8 (Thursday, Friday and Saturday)

String Functions

A string is a sequence of characters.  Strings are object of the String class.

Given the following string:

String message = "Hello, World!"

1. You can compute the length of a string with the length method.

   int n = message.length();  // n = 13
   

2. Strings can be concatenated, that is, put end to end to yield a new longer string.  String concatencation is denoted by the + operator.

   String name = "Dave";
   String message = "Hello, " + name;  // message = Hello, Dave
   

3. Whenever one of the arguments of the + operator is a string, the other argument is converted to a string.

   String a = "Agent";
   int n = 7;
   String bond = a + n;  // bond = "Agent7"
   double total = 3.70;
   System.out.println("The total is " + total);  // outputs "The total is 3.70"
   

4. Use the substring method to extract a part of a string.

   String greeting = "Hello, World!";
   String sub = greeting.substring(0,5);  // sub = "Hello"
   sub = greeting.substring(7,12);  // sub = "World"
   sub = greeting.substring(7);  // sub = "World!"
   

Comparing Strings

What does the following code do?

• if ( string1.equals(string2) ) ...

• if ( string1 == string2 ) ...    // not useful

If two strings are not identical to each other, you still may want to know the relationship between them.

Boolean Operators

• A && B is true only if both A and B are true.

• A || B is true if either A or B are true.

Truth table

Example: Print out a message when y >= x >= 0.

Answer:

if ( y >= x && x >= 0 )
   System.out.println("y >= x >= 0");

Example:

if ( single )
   if ( gender == 'M' )
      if ( age >= 18 && age <= 26 )
         System.out.println("This person qualifies for our special offer");

Answer:

if ( single && gender == 'M' && age >= 18 && age <= 26 )
   System.out.println("This person qualifies for our special offer");

Example: If grade is outside the interval [0,100], indicate an error.  Otherwise output the grade.

Answer:

Version 1: Code using ||:

if ( grade < 0 || grade > 100 )
   System.out.println("illegal grade"); 
else
   System.out.println("Grade is " + grade); 

Version 2: Code using &&:

if ( grade >= 0 && grade <= 100 )
   System.out.println("Grade is " + grade); 
else
   System.out.println("illegal grade");

Activity 1

• Write an if statement that prints the message "The number is valid" if the variable grade is within the range 0 through 100.
• Write an if statement that prints the message "The number is valid" if the variable temperature is within the range -50 through 150.
• Write an if statement that prints the message "The number is not valid" if the variable hours is outside the range 0 through 80.

Activity 2:  The BankCharges Class

A bank charges $10 per month plus the following check fees for a commercial checking account.

$.10 each for less than 20 checks

$.08 each for 20-39 checks

$.06 each for 40-59 checks

$.04 each for 60 or more checks

The bank also charges an extra $15 if the balance of the account falls below $400 (before any check fees are applied).  Design a class that stores the beginning balance of an account and the number of checks written.  It should also have a method that returns the bank's service fees for the month.

Public Interface for the BankCharges

/* Initializes the instance's fields. */
public BankCharges(double balance, int numChecks)

/* Returns the current balance for this account */
public double getBalance()

/* Returns the number of checks written on the account */
public int getNumChecks()

/* Determines the service fees for this account by calling
   the private method calculateServiceFees.
   Subtract these fees from the balance.
*/
public void updateBalance()

/* Returns the service fees and balance as a String with labels */
public String toString()

/* This method is a private method to help in determining the
   new balance after the service fees are deducted.
   It is not part of the public interface but is useful in other methods.
*/

/* This method determines the service fees for this account and
   returns this fee.  Be sure to include the $10 monthly charge and
   the $15 if the balance is below $400.
*/
private double calculateServiceFees()

Testing your BankCharges class

• Ask the user to enter the current balance for this account from the keyboard.  (double)
• Ask the user to enter the number of checks written during this month from the keyboard.  (int)
• Instantiate a BankCharges object named account1 using the input from above.
• Print the balance and number of checks with an appropriate label using the two accessor methods.
• Update the balance.
• Print the balance and number of checks using the toString method.

De Morgan's Law

Precedence of operators

The following table gives the operator precedence from highest (performed first) to lowest (performed last)

Comparing characters

Besides comparing numbers, you can compare characters using the relational operators.  The order of char comparisons is based on the Unicode position of each character (see Appendix B).

Short-circuit evaluation of boolean expressions

Short-circuit evaluation means that in a multiple clause boolean expression, evaluation stops as soon as the answer is determined.

Examples:

true || X is always true so there is no point finding X.

Similarly false && Y is always false regardless of Y's value.

Example 1: possible division by zero

if ( scoreTotal/numScores > 90.0 )
   System.out.println( "Excellent!  Very good work ");

Possible solutions:

nested ifs:

if ( numScores > 0 ) {
   if ( scoreTotal / numScores > 90.0 ) {
      System.out.println("Excellent!  Very good work ");
   }
}

short-circuit evaluation using &&:

if ( numScores > 0 && scoreTotal / numScores > 90.0 )
   System.out.println("Excellent!  Very good work ");

In both solutions, the division only occurs if numScores is greater than 0.

Activity 3  The LeapYear Class

A year with 366 days is called a leap year.  A year is a leap year if it is divisible by 4 (for example, 1980).  However, since the introduction of the Gregorian calendar, a year is not a leap year if it is divisible by 100 (for example, 1900); however, it is a leap year if it is divisible by 400 (for example, 2000).  Write a program that asks the user for a year and computes whether that year is a leap year.  Implement a class LeapYear with a method boolean isLeapYear().  Include a method setYear(int year) that replaces the current year.  Test your class with the dates mentioned in the problem.