4: User Input

What We Will Cover Announcements Please silence all cell phones and pagers Sit at a computer station, start your computer and login When class is over, please shut down your computer. Remember that any data or program files are lost. Continuations 4.1: Interactive Keyboard Input 4.1.1: About Keyboard Input 4.1.2: Importing Classes 4.1.3: Using System.in 4.1.4: Reading Numbers 4.1.5: Summary Exercise 4.1 4.2: Using Dialogs for User I/O 4.2.1: Introduction to Dialogs 4.2.2: Coding Dialogs 4.2.3: Exiting with System.exit() 4.2.4: Enhancing Dialogs 4.2.5: Summary Exercise 4.2 4.3: Problem Solving and Pair Programming 4.3.1: Problems and Problem-Solving 4.3.2: Algorithms 4.3.3: Pair Programming 4.3.4: Best Practices 4.3.5: Summary Exercise 4.3 4.4: Relationships, Logic and Truth 4.4.1: The Value of a Relationship 4.4.2: Comparing Characters 4.4.3: Logical Operators 4.4.4: What is Truth? 4.4.5: Summary Exercise 4.4 4.5: Selection Statements 4.5.1: Flow Of Control 4.5.2: Using if Statements 4.5.3: Using if-else Statements 4.5.4: Nested if Statements 4.5.5: Using if-else Chains 4.5.6: Testing Multiple Conditions 4.5.7: Summary Exercise 4.5 Wrap Up Due Next: A3: Conversions (9/27/04) Exercise 4 and CodeLab Lesson 4 (9/27/04) Continuations Homework Questions? A2: Compiling Expressions (9/20/04) Exercise 3 and CodeLab Lesson 3 (9/20/04) Questions from last class? Which of the following statements is false? A literal string is enclosed in double quotes. The + operator is used to join, or concatenate, two strings to make one longer string. To include a double quote inside a string, you must put an escape character in front of it. Strings are stored in either 4 or 8 bytes, depending on the precision you want. ^ top 4.1: Interactive Keyboard Input Objectives At the end of the lesson the student will be able to: Get user input from the command line Get numbers from the command line ^ top 4.1.1: About Keyboard Input For programs used infrequently, convenient to compile data into the code For instance, to calculate the average of four numbers: public class AverageOf4 { public static void main(String[] args) { double average = (32.6 + 55.2 + 67.9 + 48.6) / 4; System.out.println("Average = " + average); } } Easy to include the four numbers with the code operating on those numbers However, you must recompile the program if you change any number More convenient for users to enter data without recompiling a program This section discusses how you can get user input while a program is running ^ top 4.1.2: Importing Classes About Classes and Packages To make it easier to write programs, Java has many libraries These libraries contain prewritten code Referred to as the Java Application Programming Interface (API) Recall that all Java code is stored in classes Groups of related classes are organized into packages Some Commonly Used Packages Package Name Description java.lang Provides classes fundamental to Java. java.io Provides classes to read and write files. java.txt Provides classes to handle text, dates, and numbers java.util Provides classes to work with collections and miscellaneous utilities. javax.swing Provides classes to create graphical user interfaces and applets. Importing Classes and Packages Classes in the java.lang package are available automatically To use other classes, you must import them import packagename.ClassName; or import packagename.*; Using the '*' wildcard will import all classes in a package For example: import java.text.NumberFormat; import javax.swing.JOptionPane; import javax.swing.JFrame; import javax.swing.*; ^ top 4.1.3: Using System.in System.in is the foundation object for entering data into a program Recall that System.out is used to send data to the standard output stream Usually the video display screen Similarly, System.in is used to get data from the standard input stream Usually the keyboard Scanner reader = Scanner.create(System.in); int n = reader.nextInt(); Streams deliver data as a stream of individual data bytes to the program Java uses a number of objects to manage the streams of data To use the stream objects, we need to import the java.io package: import java.io.*; System.in is an InputStream object that has basic read methods Processes the input data by accepting one or more bytes at a time Each byte is processed as an int Allows using -1 as an end-of-file (EOF) marker Use an InputStreamReader to convert int values of InputStream to char values To program this, we "wrap" an InputStreamReader around System.in InputStreamReader isr = new InputStreamReader(System.in); Although we can now work with the char data types, easier to use strings Class BufferedReader provides a method to read one line at a time To program this, we "wrap" a BufferedReader around our InputStreamReader BufferedReader br = new BufferedReader(isr); Can now use the readLine() method to read one line at a time String data = br.readLine(); Method readLine() collects all the input until the Enter key is pressed Putting everything together into one program, we have: import java.io.*; public class LineInput { public static void main(String[] args) throws IOException { // set up for basic input stream InputStreamReader isr = new InputStreamReader(System.in); // need to use readLine() BufferedReader br = new BufferedReader(isr); System.out.print("Type a line and press Enter: "); String data = br.readLine(); System.out.println("You entered: " + data); } } Note that we added a line of code after the main() method header throws IOException This code is required at this point for us to use the readLine() method Will review the reason later in this lesson ^ top 4.1.4: Reading Numbers Method readLine() accepts input from the user as strings Often need to convert String values to numeric types Recall that Java provides some very handy String conversion methods We can use these methods to convert our string input into numbers Conversion to Primitive Values Return Type Conversion Method Example int Integer.parseInt(String) Integer.parseInt("1234") long Long.parseLong(String) Long.parseLong("123456") float Float.parseFloat(String) Float.parseFloat("123.45") double Double.parseDouble(String) Double.parseDouble("123.45") An Example Consider an application to sum (add) two numbers import java.io.*; public class SumOf2 { public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); System.out.print("First number: "); String data = br.readLine(); double num1 = Double.parseDouble(data); System.out.print("Second number: "); data = br.readLine(); double num2 = Double.parseDouble(data); System.out.println(num1 + " + " + num2 + " = " + (num1 + num2)); } } We setup for reading from the command line as usual InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); We read each number as a string System.out.print("First number: "); data = br.readLine(); Final step is to convert the String to a double num1 = Double.parseDouble(data); Then we can use the numbers using arithmetic operators System.out.println("" + num1 + " + " + num2 + " = " + (num1 + num2)); ^ top 4.1.5: Summary To make it easier to write programs, Java has many libraries of prewritten code You can make use of this code by importing packages import packagename.ClassName; or import packagename.*; Alternatively, you can use the fully qualified package name You can use System.in to read data from the command line To use System.in, you need to need to import the java.io.IOException class To convert bytes to chars, you wrap System.in in an InputStreamReader To buffer the input and use the readLine() method, you also wrap the stream in a BufferedReader Thus most code to read from the command line starts like: import java.io.*; public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); ... } To read a string from the command line, you use the readLine() method String input = br.readLine(); If you need a number as an input, you convert the String to a number ^ top Exercise 4.1 Start a text file named exercise4.txt. Prepare the exercise header as described in the HowTo on submitting exercises Use the following program for this exercise: import java.io.*; public class SumOf2 { public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); System.out.print("First number: "); String data = br.readLine(); double num1 = Double.parseDouble(data); System.out.print("Second number: "); data = br.readLine(); double num2 = Double.parseDouble(data); System.out.println(num1 + " + " + num2 + " = " + (num1 + num2)); } } Record the answers to the following questions in your exercise4.txt file: Enter and execute the above program. Enter the phrase "bad data" at an input prompt. Q1: What error message did you see? Remove the code throws IOException and recompile the program. Q2: What error message did the compiler give you? ^ top 4.2: Using Dialogs for User I/O Objectives At the end of the lesson the student will be able to: Collect user input in an input dialog box Show messages using a message dialog ^ top 4.2.1: Introduction to Dialogs Java provides GUI methods for user input and output Use the JOptionPane() class Static Method Description showConfirmDialog Asks a confirming question, like yes/no/cancel. showInputDialog Prompts for user text input. showMessageDialog Tell the user about something that has happened. An input dialog will display a window similar to the following: Syntax for the simplest form of the method is like: String variable = JOptionPane.showInputDialog(String) Argument String is a prompt displayed within the input dialog box For example, the following statement produces the input dialog shown String inputString = JOptionPane.showInputDialog( "Enter your first name:"); After the dialog is displayed, the program scans keyboard for input As the user enters data, the dialog box displays the characters Method showInputDialog returns a String data type containing user input Need to save this data in a variable for processing ^ top 4.2.2: Coding Dialogs Lets create a simple program to ask for and display a name Adapted from textbook page 65 We use all three dialog boxes: showInputDialog: ask for name showConfirmDialog: verify entry showMessageDialog: display results Complete program shown below: import javax.swing.JOptionPane; public class NameApp { public static void main(String[] args) { String inputString = JOptionPane.showInputDialog( "Enter your first name: "); int choice = JOptionPane.showConfirmDialog(null, "Are you sure?"); String message = "First name: " + inputString + "\n" + "Choice: " + choice + "\n" + "Press enter to exit."; JOptionPane.showMessageDialog(null, message); System.exit(0); } } Input with showInputDialog() Use the showInputDialog method of JOptionPane to get text input from users In the above example, we created the first dialog like this: String inputString = JOptionPane.showInputDialog( "Enter your first name:"); Essentially a single line of code Input with showConfirmDialog() Use the showConfirmDialog method of JOptionPane to get text input from users In the above example, we created the first dialog like this: JOptionPane.showConfirmDialog(null, "Are you sure?"); Output with showMessageDialog() Use the showMessageDialog to show the user a message From the above example: String message = "First name: " + inputString + "\n\n" + "Press enter to exit."; JOptionPane.showMessageDialog(null, message); Note that you can use both showInputDialog() and showConfirmDialog() for output as well ^ top 4.2.3: Exiting with System.exit() When a GUI is used, a thread is started To terminate this thread and allow the program to end, you use System.exit Otherwise, the user must use Ctrl+C keys to exit the program System.exit(0); Passing the number 0 to the exit() method means that the program exited normally. You must call System.exit(0) when using GUIs for your program to exit correctly ^ top 4.2.4: Enhancing Dialogs Commonly-Used Arguments in JOptionPane Methods Argument Description parentComponent An object representing the frame of the dialog box and controls its placement on the screen. If null is used, the dialog is centered on the screen. messageString A descriptive message displayed in the dialog box. titleString A title typically placed in the title bar above the dialog box. messageTypeInt Indicates the type of icon to display in the dialog box. Use one of the fields of JOptionPane to choose the icon. Examples of Other Methods for Displaying Dialog Boxes showInputDialog can accept four arguments JOptionPane.showInputDialog(parentComponent, messageString, titleString, messageTypeInt); To display a different dialog without an input field: JOptionPane.showMessageDialog(parent, message); If the parent component is unknown, you can use null for that argument JOptionPane.showMessageDialog(null, "Hello, world!"); Icons provided by JOptionPane One can include the following icons (or not) in the dialogs Icon Message Type Description JOptionPane.ERROR_MESSAGE Displays an error icon. JOptionPane.INFORMATION_MESSAGE Displays an information icon. JOptionPane.WARNING_MESSAGE Displays an warning icon. JOptionPane.QUESTION_MESSAGE Displays an question icon.   JOptionPane.PLAIN_MESSAGE Does not display an icon. Further Information How to Make Dialogs ^ top 4.2.5: Summary JOptionPane class provides several methods to display GUI dialogs Must import the JOptionPane class to use these methods import javax.swing.JOptionPane; JOptionPane has three commonly-used methods for dialog boxes Static Method Description showConfirmDialog Asks a confirming question, like yes/no/cancel. showInputDialog Prompts for user text input. showMessageDialog Tell the user about something that has happened. You can enhance dialogs with titles and icons Must use the System.exit() method to end programs that use GUIs ^ top Exercise 4.2 Modify and rename your program from Exercise 3.3 to use dialogs rather than the command line. Collect user input using showInputDialog() and display the results using showMessageDialog(). Save your modified program along with this weeks exercises. ^ top 4.3: Problem Solving and Pair Programming Objectives At the end of the lesson the student will be able to: Describe the purpose of programming Create simple algorithms Discuss the pros and cons of pair programming ^ top 4.3.1: Problems and Problem-Solving Recall that programming is about solving problems using a computer program When you first start computer programming, you may think that the hard part is translating your ideas into code This is definitely not the case The most difficult part is finding a solution to the problem After finding a solution, translating your solution to code is a routine, almost mechanical, task Defining a Problem You first start solving a problem by understanding its requirements Understanding the requirements often includes analysis of: Data inputs: files, user interaction Data manipulation of data: computations, selection Data output: output device, formats If the problem is complex, you divide it into sub-problems Each sub-problem is then analyzed to understand its requirements At the end, you should write (or type) a definition of the problem, or sub-problems, in your own words ^ top 4.3.2: Algorithms After you define a problem, you write out a sequence of steps to solve the problem These steps should be written in a natural language such as English or Español Instructions like these are referred to as an algorithm Algorithm -- a sequence of instructions to solve a problem. Total cost of Items Problem: Total the cost for a list of items. Algorithm: Write the number 0 on a blackboard (or other media) Do the following for each item on the list: Add the cost of the item to the number on the board Replace the old number by this sum Announce that the answer is the number written on the board Play Tic-Tac-Toe Problem: Play the game of tic-tac-toe. Algorithm: Select 2 players Draw a big # Player 1: draw an X in one of the squares Player 2: draws an O in some square Continue alternating between the players until either: Someone has three letters in a row or diagonal All the squares are filled. Observations about Algorithms Notice that the sequence of steps you follow in a algorithm is often important Also, notice that algorithms often require making one or more decisions In addition, algorithms often contain repetition: steps that are repeated ^ top 4.3.3: Pair Programming Defining problems and developing algorithms on your own is often difficult You may work with others, though it is best to try to define a problem and develop an algorithm on your own first What you cannot do is show other people your homework solutions or code See Working Together for more information However, when you translate the algorithms into code, you often run into syntax errors that are difficult to see It would be nice to get help from another human in these cases However, the instructor prohibits asking most people from looking at your code What is the solution to this problem? Pair Programming for Homework Assignments Defining Pair Programming Pair programming is two people work together on one computer at the same time Exactly two people: not one nor three or more Exactly one computer: not two or more The driver operates the keyboard and mouse The navigator actively participates in the programming Analyzes the design and code to prevent errors Also in charge of using printed reference materials like textbooks Each person "drives" about half the time Physically get up and move positions when switching roles At most 25% of your time is spent working alone Any work done alone is reviewed by the other person The object is to work together and to learn from each other You cannot divide the work into two pieces with each partner working on a separate piece Why Pair Program? Students who pair program report: Higher confidence in a program solution More satisfaction with programming Instructors report higher retention rates More Information The Impact of Pair Programming on Student Performance, Perception and Persistence ^ top 4.3.4: Best Practices You may choose any other student in this class for a partner Both of your names appear on the assignment When choosing partners and working together, certain practices help you perform better Pair-programmers are more successful when they have similar experience levels Need to find a partner with whom you can easily exchange ideas Getting Along When working with other people, you need to get along with them For an easy 10-minute review of how to get along, see All I Really Need to Know about Pair Programming I Learned In Kindergarten More Information Experimenting with Pair Programming in the Classroom Guidelines for the Use of Pair Programming in a Freshman Programming Class All I Really Need to Know about Pair Programming I Learned In Kindergarten ^ top 4.3.5: Summary Programming is about solving problems using a computer program To develop a computer program, you start by defining the problem(s) to solve Once a problem is well defined, you develop an algorithm to solve it An algorithm is a sequence of instructions to solve a problem Since defining problems, developing algorithms and implementing algorithms is hard, you can work with a partner But only if you follow the rules of pair programming Use some care when choosing a pair-programming partner to maximize your sucess In addition, both you and your partner should read: All I Really Need to Know about Pair Programming I Learned In Kindergarten ^ top Exercise 4.3 Label this exercise: Exercise 4.3 Submit all exercises for this lesson in one file unless instructed otherwise Complete the following and record the answers to any questions in your exercise4.txt file. Specifications We will count off and divide into separate groups Within your group, exchange email addresses so that you can send the answer to this exercise to each other. As a group, take 10 minutes to define the problem and describe an algorithm for getting from this classroom to Watsonville ^ top 4.4: Relationships, Logic and Truth Objectives At the end of the lesson the student will be able to: Use relational expressions to produce boolean values Use logical operators to code complex conditions ^ top 4.4.1: The Value of a Relationship In addition to arithmetical operations, all computers can compare numbers Many decisions can be reduced to choosing between two numbers Comparing numbers can make computers seem "intelligent" Expressions that compare operands are called relational expressions Also called conditions Simple relational expressions have one relational operator comparing two values A relational expression always evaluates to either true or false The relational operators are: Math Name Java Example Result = Equal to == 5 == 10 2 == 2 false true ≠ Not equal to != 5 != 10 2 != 2 true false < Less than < 5 < 10 5 < 5 5 < 2 true false false ≤ Less than or equal to <= 5 <= 10 5 <= 5 5 <= 2 true true false > Greater than > 5 > 10 5 > 5 5 > 2 false false true ≥ Greater than or equal to >= 5 >= 10 5 >= 5 5 >= 2 false true true You may use relational operators with integer, floating-point and character data ^ top 4.4.2: Comparing Characters Character data can be evaluated using relational operators For these comparisons, char values are automatically coerced to int values Letters nearer to the start of the alphabet have lower numerical values Thus a numerical comparison can decide the alphabetical order of characters For example: Expression Result 'A' > 'C' false 'D' <= 'Z' true 'E' == 'F' false 'G' >= 'M' false 'B' != 'C' true ^ top 4.4.3: Logical Operators Sometimes you want to consider several relational expressions at once Logical operators allow you to consider multiple cases and to change conditions Use ! operator to reverse value of expression Use && to AND two or more conditions Use || to OR two or more conditions ! Operator If expr is... Then ! expr is... Example Result true false !true false false true !(5 < 2) true && operator returns true only if expressions on both sides are true && Operator If expr1 is... And expr2 is... Then expr1 && expr2 is... Example Result true true true 5 < 10 && 5 > 2 true true false false 5 < 10 && 5 < 2 false false true false 5 > 10 && 5 > 2 false false false false 5 > 10 && 5 < 2 false || operator returns true if either expression is true || Operator If expr1 is... || expr2 is... Then expr1 || expr2 is... Example Result true true true 5 < 10 || 5 > 2 true true false true 5 < 10 || 5 < 2 true false true true 5 > 10 || 5 > 2 true false false false 5 > 10 || 5 < 2 false Common Mistake with Logical Expressions Logical expressions often read like "normal" English. However, Java syntax requires more detail. For example we might say in English: A equals B or C, but the corresponding boolean expression is A == B || A == C ^ top 4.4.4: What is Truth? "What is truth?" may seem more appropriate for philosophy than programming However, in programming we have to decide this question often As we saw, computers evaluate conditions to either true or false. To get a computer to make a decision, we have to reduce questions to either true or false We will explore computer decision making in the next section ^ top 4.4.5: Summary Many decisions can be reduced to choosing between two numbers Comparing numbers can make computers seem "intelligent" Relational expressions include: == != < <= > >= Similarly, you can compare letters since letters are stored as numbers Used to alphabetize letters and words in a computer program Relational expressions always evaluate to either true or false When you need to compare several conditions, you use logical operators ! && || For example: 5 > 10 || 5 > 2 ^ top Exercise 4.4 With a partner, if needed, take 10 minutes to complete the following: Start a text file named exercise4.txt. Prepare the exercise header as described in the HowTo on submitting exercises Label this exercise: Exercise 4.4 Submit all exercises for this lesson in one file unless instructed otherwise Specifications For the values: int a = 5, b = 2, c = 4, d = 6; Record the value of each of the following relational expressions in your exercise4.txt file. a > b a != b d % b == c % b a * c != d * b b % c * a != a * b As a final part of this exercise, run the following program and answer these questions: Given that the variables a and b have already been declared and assigned values, write an expression that evaluates to true if a is greater than or equal to zero and b is negative Given that the variables a and b have already been declared and assigned values, write an expression that evaluates to true if a is positive (including zero) or b is negative (excluding zero). Given the boolean variable isValid has already been declared and assigned a value of either true or false, write an assignment statement that reverses (toggles) the value. public class TruthTable { public static void main(String[] args) { boolean p, q; System.out.println("p\tq\t!p\tp && q\tp || q"); System.out.println("-\t-\t--\t------\t------"); p = true; q = true; System.out.print(p + "\t" + q + "\t" + !p); System.out.println("\t" + (p && q) + "\t" + (p || q)); p = true; q = false; System.out.print(p + "\t" + q + "\t" + !p); System.out.println("\t" + (p && q) + "\t" + (p || q)); p = false; q = true; System.out.print(p + "\t" + q + "\t" + !p); System.out.println("\t" + (p && q) + "\t" + (p || q)); p = false; q = false; System.out.print(p + "\t" + q + "\t" + !p); System.out.println("\t" + (p && q) + "\t" + (p || q)); } } ^ top 4.5: Selection Statements Objectives At the end of the lesson the student will be able to: Choose appropriate selection statements Code selection statements ^ top 4.5.1: Flow Of Control Term flow 0f control refers to the order a program executes instructions All programs can be written with three control flow elements: Sequence - continue with the next instruction Sequence is the default operation Selection - a choice between at least two options Either execute some instruction based on a condition Or continue with the next instruction if if-else if-else if-else if- ... -else Repetition - a loop (repeat a block of code) At the end of the loop: Either go back and repeat the block of code Or continue with the next instruction after the block while We will start with selection statements ^ top 4.5.2: Using if Statements Simplest selection statement is an if statement Executes a section of code only if a condition is true if (condition) { // execute statements only if true } If the condition is not true, the computer skips the code The condition can be a relational or logical expression For clarity, write the if on a different line than the statements Also, indent the statements that are executed For Example What do you notice about the following code? import javax.swing.JOptionPane; public class GuessingGame { public static void main(String[] args) { int guess = 0; String input = ""; String output = ""; String msg = "I'm thinking of a number between" + " 1 and 10.\nCan you guess it?"; input = JOptionPane.showInputDialog(msg); guess = Integer.parseInt(input); if (guess == 7) { output = "*** Correct! ***"; JOptionPane.showMessageDialog(null, output); } System.exit(0); } } About those Curly Braces Technically, the if statement affects only the single statement that follows We use curly braces to make the following statement into a block This allows us to put any number of statements within the block Though curly braces are not always required, the best practice is to include them ^ top 4.5.3: Using if-else Statements Statement if-else expands the selection statement If a condition is true then do this otherwise it is false so do something else General syntax: if (condition) { statement1; } else { statement2; } For clarity, if and else are written on different lines than the statements Also, the statements are indented For Example What do you notice about the following code? import javax.swing.JOptionPane; public class GuessingGame2 { public static void main(String[] args) { int guess = 0; String input = ""; String output = ""; String msg = "I'm thinking of a number between" + " 1 and 10.\nCan you guess it?"; input = JOptionPane.showInputDialog(msg); guess = Integer.parseInt(input); if (guess == 7) { output = "*** Correct! ***"; } else { output = "Sorry, that is not correct."; } JOptionPane.showMessageDialog(null, output); System.exit(0); } } Programming Style: Placement of Braces Different practices for placing curly braces in a compound statement Many programmers prefer placing curly braces on separate lines However, Java standard is to place the opening brace on the same line as shown above In practice, you should use the style dictated by your company's policy Or your professor's instructions For the acceptable styles for this course see: Curly Braces ^ top 4.5.4: Nested if Statements You can include if statements within other if statements Each inner if statement is evaluated only if outer condition is met For Example What do you notice about the following code? import javax.swing.JOptionPane; public class TempGuide { public static void main(String[] args) { String msg = "Enter a temperature"; String input = JOptionPane.showInputDialog(msg); double temp = Double.parseDouble(input); String output; if (temp > 65) { if (temp > 75) { output = "That's too hot!"; } else { output = "That's just right."; } } else { output = "That's cold!"; } JOptionPane.showMessageDialog(null, output); System.exit(0); } } Nested conditionals can be confusing if too deep Rule of thumb: no more than three deep "Dangling Else" Problem Potential problem with nested if statements is the dangling else problem if (condition1) if (condition2) Statement1; else Statement2; Indentation is misleading and should be: if (condition1) if (condition2) Statement1; else Statement2; Common mistake is not matching else with the correct if Rule: else always matches the last unmatched if Avoid this problem by using curly braces to specify the grouping if (condition1) { if (condition2) { Statement1; } else { Statement2; } } ^ top 4.5.5: Using if-else Chains Often confusing when if statements are nested within other if statements Should avoid this type of construction However, a very useful construction is an if ... else if statement An if statement is nested inside an else statement Tests conditions sequentially until the first true condition is found The first true condition executes the code within the block Skips the remainder of the if-else chain Often used to create a menu For Example What do you notice about the following code? import javax.swing.JOptionPane; public class TempGuide2 { public static void main(String[] args) { String msg = "Enter a temperature"; String input = JOptionPane.showInputDialog(msg); double temp = Double.parseDouble(input); String output; if (temp > 75) { // first condition output = "That's too hot!"; } else if (temp > 65) { // second condition output = "That's just right."; } else { // default condition output = "That's cold!"; } JOptionPane.showMessageDialog(null, output); System.exit(0); } } Note the arrangement of if-else statements ^ top 4.5.6: Testing Multiple Conditions With logical operators (&&, ||, !), you can test for multiple conditions For example, you can create condition that displays only if two conditions are true What do you notice about the following code? import javax.swing.JOptionPane; public class TempGuide3 { public static void main(String[] args) { String msg = "Enter a temperature"; String input = JOptionPane.showInputDialog(msg); double temp = Double.parseDouble(input); String output; if (temp >= 65 && temp <= 75) { output = "That's just right."; } else { output = "That's uncomfortable!"; } JOptionPane.showMessageDialog(null, output); System.exit(0); } } How could we get the same result using a logical OR operation? ^ top 4.5.7: Summary Selection is a choice between at least two options Selection statements include: if if-else if-else if-else if- ... - else Single statements are expanded into compound statements using blocks Nested if statements are legal but often confusing Which if does an else match up with? Use curly braces to clarify Better yet, avoid using nested if statements In contrast, chains of if-else statements are very useful Programs execute the code when the first condition matches Often used to create menus ^ top Exercise 4.5 Label this exercise: Exercise 4.5 Complete the following and record the answer to any questions in your exercise4.txt file. Specifications A student's letter grade is calculated according to the following table: Numerical Grade Letter Grade greater than or equal to 90 A less than 90 but greater than or equal to 80 B less than 80 but greater than or equal to 70 C less than 70 but greater than or equal to 60 D less than 60 F Write a program that accepts a students numerical grade, converts the grade to an equivalent letter grade, and displays the letter grade. Q1: How do you make sure that only one condition is selected? Q2: How do you avoid the dangling-else problem? Submit your final program along with your exercise4.txt file. You can use the following code to get started. import javax.swing.JOptionPane; public class GradeConverter { public static void main(String[] args) { String msg = "Enter a score"; String input = JOptionPane.showInputDialog(msg); double score = Double.parseDouble(input); String output = ""; // Insert statements here JOptionPane.showMessageDialog(null, output); System.exit(0); } } ^ top Wrap Up Reminders Due Next Class: A3: Conversions (9/27/04) Exercise 4 and CodeLab Lesson 4 (9/27/04) When class is over, please shut down your computer You may complete unfinished exercises at the end of the class or at any time before the next class. Instructions on submitting exercises are available from the HowTo's page. ^ top Home | WebCT | Announcements | Schedule | Expectations | Course info Help | FAQ's | HowTo's | Links Last Updated: September 20 2004 @12:22:37