Computer Science Notes
Chapter 3
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Chapter 3: Selection Statements
These notes are meant to accompany Introduction to Java Programming: Fundamentals First, sixth edition by Y.
Daniel Lang.
Programming Skills in a Nutshell:
At the end of this chapter you should have the following programming skills:
1. Translate logical conditions into Boolean expressions.
2. To use simple if … else statements to control program flow.
3. To use nested if statements for multiple outcomes and conditions.
4. To use a switch statement for multiple outcomes and conditions that are based on a positive integer.
5. Here is a template that uses the key programming skills you should have at this point:
import java.util.Scanner;
// The import statement tells the compiler where to find the Scanner class,
// which is used for user input.
/**The Chap03Basics class implements an application that
* performs various mathematical tasks, and is meant to
* illustrate the basics of selection statements
* (i.e., if and switch statements).
* This is meant as an example of the material in Chapter 3 of the text
* _Introduction to Java Programming: Fundamentals First_ by Y. D. Liang
* @author Kevin Mirus
*/
public class Chap03Basics
{
/** Gives the user a menu of options of mathematical calculations.
* @param args is not used.
*/
public static void main(String[] args)
{
//Tell the user what the program does
String programPurpose = "This program performs various mathematical calculations.";
System.out.println(programPurpose);
System.out.println("\nYour menu of choices is:");
System.out.println("t: Get quiz questions from the Math Tutor.");
System.out.println("f: Get some Math Facts about a number you enter.");
System.out.println("g: Convert a number score into a com sci letter Grade.");
System.out.println("c: Compute the tax on a purchase.");
System.out.println("Enter a single letter for your menu choice");
//Declare a vhar variable for the user's menu choice.
char menuChoice;
String menuChoiceString;
//Create a Scanner object for reading in the user's input
Scanner keyboard = new Scanner(System.in);
//get the user's data
menuChoiceString = keyboard.next();
menuChoice = menuChoiceString.charAt(0);
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//perform appropriate menu item action
switch (menuChoice)
{
case 't': //Generate quiz questions from the Math Tutor
//First, do an addition quiz problem
//use the fact that the Math.random() method
//generates random floating point numbers between 0 and 1
System.out.println("\nHere come four arithmetic questions:");
int number1 = (int) (Math.random() * 10);
int number2 = (int) (Math.random() * 10);
int correctAnswer = number1 + number2;
int userAnswer;
System.out.println("What is " + number1 + " + " + number2 + " ?");
userAnswer = keyboard.nextInt();
if (userAnswer == correctAnswer)
System.out.println("You are correct!");
else
System.out.println("Sorry, the correct answer is: " + correctAnswer);
//Second, do a subtraction quiz problem
number1 = (int) (Math.random() * 10);
number2 = (int) (Math.random() * 10);
//Swap numbers if necessary so number1 is greater than number 2
if (number2 > number1)
{
int temp = number1;
number1 = number2;
number2 = temp;
}
correctAnswer = number1 - number2;
System.out.println("\nWhat is " + number1 + " - " + number2 + " ?");
userAnswer = keyboard.nextInt();
if (userAnswer == correctAnswer)
System.out.println("You are correct!");
else
System.out.println("Sorry, the correct answer is: " + correctAnswer);
//Third, do a multiplication quiz problem
number1 = (int) (Math.random() * 10);
number2 = (int) (Math.random() * 10);
correctAnswer = number1 * number2;
System.out.println("\nWhat is " + number1 + " * " + number2 + " ?");
userAnswer = keyboard.nextInt();
if (userAnswer == correctAnswer)
System.out.println("You are correct!");
else
System.out.println("Sorry, the correct answer is: " + correctAnswer);
//Fourth, do a division quiz problem
number1 = (int) (Math.random() * 10);
number2 = (int) (Math.random() * 10) * number1;
correctAnswer = number2 / number1;
System.out.println("\nWhat is " + number2 + " / " + number1 + " ?");
userAnswer = keyboard.nextInt();
if (userAnswer == correctAnswer)
System.out.println("You are correct!");
else
System.out.println("Sorry, the correct answer is: " + correctAnswer);
break;
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case 'f': //Generate some Math Facts about a number the user enters
//Get user's number
System.out.println("\nEnter an integer, please.");
number1 = keyboard.nextInt();
//Determine if number is even or odd
boolean isEven = (number1 % 2) == 0;
if (isEven)
System.out.println(number1 + " is even.");
else
System.out.println(number1 + " is odd.");
//Print the absolute value of the number
System.out.println("Here is the absolute value of your number:");
if (number1 < 0)
System.out.println("|" + number1 + "| = " + (-number1));
else
System.out.println("|" + number1 + "| = " + number1);
//Print the absolute value of the number
System.out.println("Here are some factors of your number:");
if (number1%2 == 0)
System.out.println(number1 + " is divisible by 2.");
if (number1%3 == 0)
System.out.println(number1 + " is divisible by 3.");
if (number1%5 == 0)
System.out.println(number1 + " is divisible by 5.");
if (number1%7 == 0)
System.out.println(number1 + " is divisible by 7.");
if (number1%11 == 0)
System.out.println(number1 + " is divisible by 11.");
if (Math.abs(number1) > 121)
System.out.println(number1 + " may have prime factors larger than 11, " +
"but this program can not determine them.");
break;
case 'g':
//Convert a number score into a com sci letter Grade
System.out.println("\nEnter your computer science number grade.");
double score = keyboard.nextDouble();
String letterGrade;
//A
//AB
//B
//BC
//C
//D
//F
92 % - 100 %
88 % - 91 %
82 % - 87 %
78 % - 81 %
72 % - 77 %
65 % - 71 %
0 % - 64 %
if (score >= 91.5)
letterGrade = "A";
else if (score >= 87.5)
letterGrade = "AB";
else if (score >= 81.5)
letterGrade = "B";
else if (score >= 77.5)
letterGrade = "BC";
else if (score >= 71.5)
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letterGrade = "C";
else if (score >= 64.5)
letterGrade = "D";
else
letterGrade = "F";
System.out.println("A score of " + score + " earns a letter grade of " + letterGrade);
break;
case 'c':
//Compute the tax on a purchase
System.out.println("\nPlease enter the purchase price:");
double price = keyboard.nextDouble();
System.out.println("Please enter the tax rate as a decimal:");
double taxRate = keyboard.nextDouble();
double totalPrice = price * (1 + taxRate);
System.out.println("The total price as an unformatted number is: $" + totalPrice);
System.out.printf("The total price as an formatted number is: $%4.2f\n", totalPrice);
break;
default:
System.out.println("That was not a valid menu choice.");
}
System.out.println("\n\nHere are some miscellaneous applications from Chapter 3:");
boolean lovePackers = true;
boolean loveWisconsin = true;
System.out.println("Type true if you love the Packers, ");
System.out.println("or type false if you dislike the Packers.");
lovePackers = keyboard.nextBoolean();
System.out.println("Type true if you love Wisconsin, ");
System.out.println("or type false if you dislike Wisconsin.");
loveWisconsin = keyboard.nextBoolean();
if (loveWisconsin == true && lovePackers == true)
System.out.println("You must be a native Wisconsinite.");
else if (loveWisconsin == true && lovePackers == false)
System.out.println("You must be a Wisconsin immigrant.");
else if (loveWisconsin == false && lovePackers == true)
System.out.println("You must be from Minnesota.");
else if (loveWisconsin == false && lovePackers == false)
System.out.println("You must be from Illinois.");
//Exclusive or:
if (loveWisconsin ^ lovePackers)
System.out.println("That's good enough for me.");
//Longhand exclusive or:
if ((loveWisconsin || lovePackers) && !(loveWisconsin && lovePackers))
System.out.println("That's good enough for me.");
System.out.println("\n\nHere are some more examples of the printf() method:");
//The following line produces an output where the format specifier %6.2f
//will be replaced with the
//floating point number 1.5 written
//using 6 spaces and 2 digits after the decimal point
//(and followed by a newline).
System.out.print("Price is: $%6.2f");
System.out.printf("Price is: $%6.2f\n", 1.5);
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System.out.print("%5d");
System.out.printf("%5d\n", 123);
System.out.print("%5x");
System.out.printf("%5x\n", 123);
System.out.print("%5o");
System.out.printf("%5o\n", 123);
System.out.print("%6.2f");
System.out.printf("%6.2f\n", 123.);
System.out.print("%10.2e");
System.out.printf("%10.2e\n", 123.);
System.out.print("%6.2g");
System.out.printf("%6.2g\n", 123.);
System.out.print("%6s");
System.out.printf("%6s\n", "123");
System.out.print("%-5dhello");
System.out.printf("%-5dhello\n", 123);
System.out.print("%06d");
System.out.printf("%06d\n", 123);
System.out.print("%+6d");
System.out.printf("%+6d\n", 123);
System.out.print("%(6d");
System.out.printf("%(6d\n", -123);
System.out.print("%10d");
System.out.printf("%10d\n", 123000);
//Tell the user that the program is done.
System.out.println("\nProgram Chap03Basics has terminated.");
}//end of main()
}//end of class Chap03Basics
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Computer Science Notes
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/* *******************************************************************
* PROGRAM OUTPUT
* *******************************************************************
This program performs various mathematical calculations.
Your menu of choices is:
t: Get quiz questions from the Math Tutor.
f: Get some Math Facts about a number you enter.
g: Convert a number score into a com sci letter Grade.
c: Compute the tax on a purchase.
Enter a single letter for your menu choice
t
Here come four arithmetic questions:
What is 0 + 0 ?
5
Sorry, the correct answer is: 0
What is 5 - 2 ?
3
You are correct!
What is 5 * 8 ?
41
Sorry, the correct answer is: 40
What is 36 / 9 ?
4
You are correct!
Here are some miscellaneous applications from Chapter 3:
Type true if you love the Packers,
or type false if you dislike the Packers.
true
Type true if you love Wisconsin,
or type false if you dislike Wisconsin.
true
You must be a native Wisconsinite.
Here are some more examples of the printf() method:
Price is: $%6.2fPrice is: $ 1.50
%5d 123
%5x 7b
%5o 173
%6.2f123.00
%10.2e 1.23e+02
%6.2g1.2e+02
%6s 123
%-5dhello123 hello
%06d000123
%+6d +123
%(6d (123)
%10d 123000
Program Chap03Basics has terminated.
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/* *******************************************************************
* PROGRAM OUTPUT
* *******************************************************************
This program performs various mathematical calculations.
Your menu of choices is:
t: Get quiz questions from the Math Tutor.
f: Get some Math Facts about a number you enter.
g: Convert a number score into a com sci letter Grade.
c: Compute the tax on a purchase.
Enter a single letter for your menu choice
f
Enter an integer, please.
-360
-360 is even.
Here is the absolute value of your number:
|-360| = 360
Here are some factors of your number:
-360 is divisible by 2.
-360 is divisible by 3.
-360 is divisible by 5.
-360 may have prime factors larger than 11, but this program can not determine them.
Here are some miscellaneous applications from Chapter 3:
Type true if you love the Packers,
or type false if you dislike the Packers.
true
Type true if you love Wisconsin,
or type false if you dislike Wisconsin.
false
You must be from Minnesota.
That's good enough for me.
That's good enough for me.
…
* ********************************************************************
*/
Computer Science Notes
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/* *******************************************************************
* PROGRAM OUTPUT
* *******************************************************************
This program performs various mathematical calculations.
Your menu of choices is:
t: Get quiz questions from the Math Tutor.
f: Get some Math Facts about a number you enter.
g: Convert a number score into a com sci letter Grade.
c: Compute the tax on a purchase.
Enter a single letter for your menu choice
g
Enter your computer science number grade.
78
A score of 78.0 earns a letter grade of BC
Here are some miscellaneous applications from Chapter 3:
Type true if you love the Packers,
or type false if you dislike the Packers.
false
Type true if you love Wisconsin,
or type false if you dislike Wisconsin.
true
You must be a Wisconsin immigrant.
That's good enough for me.
That's good enough for me.
…
* ********************************************************************
*/
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Computer Science Notes
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/* *******************************************************************
* PROGRAM OUTPUT
* *******************************************************************
This program performs various mathematical calculations.
Your menu of choices is:
t: Get quiz questions from the Math Tutor.
f: Get some Math Facts about a number you enter.
g: Convert a number score into a com sci letter Grade.
c: Compute the tax on a purchase.
Enter a single letter for your menu choice
c
Please enter the purchase price:
157.84
Please enter the tax rate as a decimal:
.055
The total price as an unformatted number is: $166.5212
The total price as an formatted number is: $166.52
Here are some miscellaneous applications from Chapter 3:
Type true if you love the Packers,
or type false if you dislike the Packers.
false
Type true if you love Wisconsin,
or type false if you dislike Wisconsin.
false
You must be from Illinois.
…
* ********************************************************************
*/
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/* *******************************************************************
* PROGRAM OUTPUT
* *******************************************************************
This program performs various mathematical calculations.
Your menu of choices is:
t: Get quiz questions from the Math Tutor.
f: Get some Math Facts about a number you enter.
g: Convert a number score into a com sci letter Grade.
c: Compute the tax on a purchase.
Enter a single letter for your menu choice
q
That was not a valid menu choice.
…
* ********************************************************************
*/
Book’s Statement of Skills:
1. To declare boolean type variables and write Boolean expressions. (3.2)
2. To distinguish between conditional and unconditional && and || operators. (3.2.1)
3. To use Boolean expressions to control selection statements. (3.3 – 3.5)
4. To implement selection control using if and nested if statements. (3.3)
5. To implement selection control using switch statements. (3.4)
6. To write expressions using the conditional operator. (3.5)
7. To display formatted output using the System.out.printf method and to format strings using the
String.format method. (3.6)
8. To know the rules governing operand evaluation order, operator precedence, and operator associativity.
(3.7 – 3.8)
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Section 3.1: Introduction
This chapter will teach you how to if and switch statements, which are used to which lines of code a
program executes based on user input or the values stored in variables.
Example:
Problem to solve: If the user enters a negative value for a radius of a circle, then display an error message;
otherwise compute and display the circumference.
↓ Pseudocode..
Get the user’s input for a circle radius
If (the radius is negative)
Then print an error message
Otherwise
Compute and print the circumference
↓ Real code
// Get the user’s input for a circle radius
System.out.println("Enter the radius of a circle:");
Scanner keyboard = new Scanner(System.in);
double radius = keyboard.nextDouble();
if (radius < 0.0)
System.out.println("Bad entry; you entered r = “ + radius + “, and the radius of a circle should not be negative");
else
System.out.println("For a circle of radius “ + radius + “, the circumference is: " + (2*Math.PI*radius));
Section 3.2: boolean Data Type and Operations
Comparison operators: <, <=, >, >=, ==, !=
The boolean data type contains one of two values: true or false.
Boolean operators: !, &&, ||, ^
Section 3.2.1: Unconditional vs. Conditional Boolean Operators
&& is a conditional, or short-circuit operator because if the first expression evaluates to false, the second
expression’s value is not computed.
& is an unconditional operator because both expressions’ values are computed.
Section 3.2.2: Example: Determining Leap Year
Section 3.2.3: Example: A Simple Math Learning Tool
(see sample code Chap03Basics at start of lecture notes…)
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Section 3.3: if Statements
An if statement is used to force a program to execute certain lines of code based on the truth of a given
condition.
Section 3.3.1: Simple if Statements
The if statement by itself forces a program to execute a statement only if a given condition is true.
Syntax of a simple if statement:
if (condition)
{
conditionTrueStatement(s);
}
nextStatement;


When the condition is false, the program executes nextStatement right away, thus skipping
conditionTrueStatement(s),
but when the condition is true, the program executes conditionTrueStatement(s) first, and then
nextStatement.
Example:
// Get the user’s input for a circle radius
System.out.println("Enter the radius of a circle:");
Scanner keyboard = new Scanner(System.in);
double radius keyboard.nextDouble();
double circumference;
if (radius >= 0.0)
{
circumference = 2*Math.PI*radius;
System.out.println("For a circle of radius “ + radius + “, the circumference is: " + circumference);
}
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Section 3.3.2: if … else Statements
The if .. else statement forces a program to execute one statement only if a given condition is true, and a
different statement if the condition is false.
Syntax of a simple if statement:
if (condition)
{
conditionTrueStatement(s);
}
else
{
conditionFalseStatement(s);
}
nextStatement;


When the condition is true, the program executes conditionTrueStatement(s) first, and then
nextStatement.
When the condition is false, the program executes conditionFalseStatement right away, and then
nextStatement.
Example:
// Get the user’s input for a circle radius
System.out.println("Enter the radius of a circle:");
Scanner keyboard = new Scanner(System.in);
double radius = keyboard.nextDouble();
double circumference;
if (radius < 0.0)
{
System.out.println("The radius you entered was r = “ + radius);
System.out.println("Bad entry; please enter a positive radius");
}
else
{
circumference = 2*Math.PI*radius;
System.out.println("For a circle of radius “ + radius + “, the circumference is: " + circumference);
}
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Section 3.3.3: Nested if Statements
Nested if statements are used when you want to force the program to execute one of many different statements
based on the Boolean values of one or more expressions.
Syntax of a simple if statement: Use the syntax of an if … else statement, with the else statements containing
additional if … else statements.
Example:
Compute the letter grade of a numerical score according to the following grading scale:
A
92 % - 100 %
AB 88 % - 91 %
B
82 % - 87 %
BC
78 % - 81 %
C
72 % - 77 %
D
65 % - 71 %
F
0 % - 64 %
//Convert a number score into a letter Grade
System.out.println("Enter a numerical score.");
Scanner keyboard = new Scanner(System.in);
double score = keyboard.nextDouble();
String letterGrade;
if (score >= 91.5)
letterGrade = "A";
else if (score >= 87.5)
letterGrade = "AB";
else if (score >= 81.5)
letterGrade = "B";
else if (score >= 77.5)
letterGrade = "BC";
else if (score >= 71.5)
letterGrade = "C";
else if (score >= 64.5)
letterGrade = "D";
else
letterGrade = "F";
System.out.println("A score of " + score + " earns a letter grade of " + letterGrade);
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Example:
Write the Wisconsite/Packer messages using nested if statements instead of compound boolean expressions.
Scanner keyboard = new Scanner(System.in);
boolean lovePackers = true;
boolean loveWisconsin = true;
System.out.println("Type true if you love the Packers, ");
System.out.println("or type false if you don’t");
lovePackers = keyboard.nextBoolean();
System.out.println("Type true if you love Wisconsin, ");
System.out.println("or type false if you don’t");
loveWisconsin = keyboard.nextBoolean();
Scanner keyboard = new Scanner(System.in);
boolean lovePackers = true;
boolean loveWisconsin = true;
System.out.println("Type true if you love the Packers, ");
System.out.println("or type false if you don’t");
lovePackers = keyboard.nextBoolean();
System.out.println("Type true if you love Wisconsin, ");
System.out.println("or type false if you don’t");
loveWisconsin = keyboard.nextBoolean();
if (loveWisconsin == true && lovePackers == true)
System.out.println("You must be a native Wisconsinite.");
else if (loveWisconsin == true && lovePackers == false)
System.out.println("You must be a Wisconsin
immigrant.");
if (loveWisconsin == true)
if (lovePackers == true)
System.out.println("You must be a native.");
else
System.out.println("You must be an immigrant.");
else if (loveWisconsin == false && lovePackers == true)
System.out.println("You must be from Minnesota.");
else if (loveWisconsin == false && lovePackers == false)
System.out.println("You must be from Illinois.");
else
if (lovePackers == true)
System.out.println("You must be from Minnesota.");
else
System.out.println("You must be from Illinois.");
//Exclusive or:
if (loveWisconsin ^ lovePackers)
System.out.println("That's good enough for me.");
//Exclusive or:
if (loveWisconsin)
if (!lovePackers)
System.out.println("That's good enough for me.");
else;
else
if (lovePackers)
System.out.println("That's good enough for me.");
Section 3.3.4: Example: Computing Taxes
Section 3.3.5: Example: An Improved Math Learning Tool
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Section 3.4: switch Statements
switch statements are used when you want to force the program to execute one of many different statements
based on the integer values of a single integer expression.
Syntax of a simple if statement:
switch (integerExpression)
{
case value1:
statement(s)1;
break;
case value2:
statement(s)2;
break;
…
case valueN:
statement(s)N;
break;
default:
statement(s)Default;
}
nextStatement;




If the integerExpression evaluates to the number value1, the program executes statement(s)1, and
then skips to nextStatement when the break statement is encountered. If the break statement were
omitted by accident, the program would execute statement(s)2 next.
The same pattern follows for other values of integerExpression.
If integerExpression doesn’t evaluate to any of the values, then the statement(s)Default get executed.
Notice that parentheses for block statements are not needed after each case…
Example:
//perform appropriate menu item action
switch (menuChoice)
{
case 't':
//Generate quiz questions from the Math Tutor
…
break;
case 'f':
//Generate some Math Facts about a number the user enters
…
break;
case 'g':
//Convert a number score into a com sci letter Grade
…
break;
case 'c':
//Compute the tax on a purchase
…
break;
default:
System.out.println("That was not a valid menu choice.");
}
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Section 3.5: Conditional Expressions
 A shorthand way to assign one of two values to a variable based on a condition
 Syntax of a conditional expression: variable = booleanExpression ? expression1 : expression2;
Traditional way to compute an absolute value:
A conditional expression that does the same thing
// Get the user’s input for a number
System.out.println("Enter a number:");
Scanner keyboard = new Scanner(System.in);
double x = keyboard.nextDouble();
// Get the user’s input for a number
System.out.println("Enter a number:");
Scanner keyboard = new Scanner(System.in);
double x = keyboard.nextDouble();
double abs_x;
double abs_x;
if (x < 0.0)
abs_x = -x;
else
abs_x = x;
abs_x = (x < 0) ? –x : x;
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Section 3.6: Formatting Console Output and Strings
The printf method of the PrintStream class (in the java.io package) allows you to display the values stored in
variables in any of several format. The formats you can control include the number of columns used, the
number of places after the decimal displayed, and how negative numbers are displayed. The way the printf
method works is to replace the variable you want to display in a string with a format specifier (a code that starts
with the % character and ends with a letter) that tells the JVM how to display the variable. The variable goes
after the string that it is supposed to appear in. Format specifiers may include flags that give additional format
information.
Example:
//the following line produces an output where the format specifier %6.2f will be replaced with the
//floating point number 1.5 written using 6 spaces and 2 digits after the decimal point (and followed by a
//newline)
System.out.printf("Price is: $%6.2f\n", 1.517);
//sample output: Price is: $ 1.50
Format Types
Code
d
x
o
f
e
g
s
b
c
Type
Decimal integer
Example
System.out.printf("%5d", 123);
//output: 123
Hexadecimal integer
System.out.printf("%5x", 123);
//output: 7B
Octal integer
System.out.printf("%5o", 123);
//output: 173
Fixed floating point
System.out.printf("%6.2f", 123.);
//output:123.00
Exponential floating point
System.out.printf("%10.2e", 123.);
//output: 1.23e+1
General floating point (printf decides whether to use f System.out.printf("%6.2g", 123);
or e)
//output:123.00
String
System.out.printf("%6s", “123”);
//output: 123
Boolean value
System.out.printf("%b", lovePackers);
//output: true
Character value
System.out.printf("%c", menuChoice);
//output: g
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Format Flags
Flag
-
Meaning
Left justifiation
0
Show leading zeros
+
Show a + sign for positive numbers
(
Enclose negative numbers in parentheses
,
Show decimal separators
Example
System.out.printf("%-5dhello", 123);
//output:123 hello
System.out.printf("%06d", 123);
//output:000123
System.out.printf("%+6d", 123);
//output: +123
System.out.printf("%(6d", -123);
//output: (123)
System.out.printf("%10d", 123000);
//output: 123,000
Note: to access this functionality use Java version 5.0 or higher.
The format method of the String class also recognizes format specifiers.
Example:
String outputString = String.format("%-5dhello", 123);
System.out.println(outputString);
//output:123 hello
Section 3.7: Operator Precedence and Associativity
Precedence
Highest Precedence
Lowest Precedence
Operator Precedence Chart
Operator
var++ and var-- (Postfix operators)
+,  (Unary operators)
++var and --var (Prefix operators)
(type) (casting)
! (Not)
*, /, %
+,  (Binary addition and subtraction)
<, <=, >, >= (Comparison)
& (Unconditional and)
^ (Exclusive or)
| (Unconditional or)
&& (Conditional and)
|| (Conditional or)
=, +=, -=, *=, /=, %= (Assignment operators)
Section 3.8: Operand Evaluation Order
 From left to right, according to the operator precedence