4: More Math and Variables

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: More Math and Variables 4.1.1: Review of Arithmetic 4.1.2: Magic Numbers 4.1.3: Assignment Variations 4.1.4: Arithmetic Precision and Range 4.1.5: Type Casting 4.1.6: Summary Exercise 4.1 4.2: Making Selections 4.2.1: Flow of Control 4.2.2: The Value of a Relationship 4.2.3: Using if Statements 4.2.4: Using if-else Statements 4.2.5: Summary Exercise 4.2 4.3: More About Selection 4.3.1: Test Conditions and Boolean Values 4.3.2: Comparing Characters 4.3.3: Nested if Statements 4.3.4: Testing Multiple Conditions 4.3.5: Summary Exercise 4.3 4.4: Testing and Debugging 4.4.1: Kinds of Programming Errors 4.4.2: Anatomy of an Error Message 4.4.3: How To Debug Syntax Errors 4.4.4: Some Error Messages and Their Common Causes 4.4.5: Summary Exercise 4.4 Wrap Up Due Next: A3: 9/5 Degrees of Separation (9/21/05) Exercise 3 and CodeLab 3 (9/21/05) A4: Conversions (9/28/05) Exercise 4 and CodeLab 4 (9/28/05) Continuations Homework Questions? A3: 9/5 Degrees of Separation (9/21/05) Exercise 3 and CodeLab 3 (9/21/05) How To Submit Homework Assignments Checking for style errors using CheckStyle 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. Pair-Programming Exercise (end of class Monday) ^ top 4.1: More Math and Variables Objectives At the end of the lesson the student will be able to: Rewrite assignment statements into shortcut alternatives Diagnose problems with arithmetic precision and range and formulate solutions Generate code to convert data from one type to another ^ top 4.1.1: Review of Arithmetic Recall that Java uses the following operators for arithmetic + for addition - for subtraction * for multiplication / for division % for modulus (remainder) The dash (minus sign) is also used for negation Arithmetic expressions are combinations of numbers, variables, constants and operators double area = 0.0, radius = 4.5; final double PI = 3.14159; area = PI * radius * radius; Operators have the same precedence as in algebra Parenthesis: ( ) Method calls Unary operators: +, - Multiplication, division, modulus: *, /, % Addition, subtraction: +, - Also recall that the results of integer division are truncated You must use modulus operator (%) to get the remainder value In addition, you use mathematical methods for more complex operations System.out.println(Math.sqrt(3.0 * 3.0)); ^ top 4.1.2: Magic Numbers Imagine that you are a programmer hired to modify a payroll program You come across the following section of code: double pay; pay = hours * 7.5 + (hours / 40) * (hours - 40) * 7.5 * 0.5; The numbers are important to the program, but what do they mean? Numbers like these are called "magic numbers" They are magic because the value or presence is unexplainable without more knowledge Often, no one knows what they mean after 3 months, including the author A programmer can often infer the meaning of numbers after reading the code carefully Much better code is to use named constants rather than literal numbers For example: final double WAGE = 7.5; final double OVERTIME_ADDER = 0.5; final int HOURS_PER_WEEK = 40; double pay; pay = hours * WAGE + (hours / HOURS_PER_WEEK) * (hours - HOURS_PER_WEEK) * WAGE * OVERTIME_ADDER; Now it is much easier to read and understand the code And see any problems or limitations Programming Style: Constant Variables and Magic Numbers Since the meaning of literal ("magic") numbers is hard to remember, you should declare constants instead final int FEET_PER_YARD = 3; final double PI = 3.14159; final double WAGE = 7.5; final double OVERTIME_ADDER = 0.5; final int HOURS_PER_WEEK = 40; Note that the name is all uppercase letters with an underscore separator This is a common coding convention that you must follow ^ top 4.1.3: Assignment Variations Unlike math, you can have the same variable on both sides of an equals sign int sum = 25; // initialize sum to 25 sum = sum + 10; // add to sum Note that the value of the variable is changed in the second line Reading variables from memory does not change them Values placed into a variable replace (overwrite) previous values Abbreviated Assignment Expressions Any statement of form: variable = variable <op> (expression); May be written in an abbreviated form: variable <op>= expression; <op> is one of: +, -, *, /, or % For example: sum = sum + 10; Can be written as: sum += 10; Incrementing and Decrementing Adding 1 to a number occurs frequently when programming count = count + 1; You can write this in an abbreviated form like: count += 1; Since it is so common, programmers wanted an even shorter form: ++count; count++; This is known as incrementing a variable Similarly, you can decrement a variable --count; count--; ^ top 4.1.4: Arithmetic Precision and Range There are only so many possible numbers that can be stored in a variable This limitation often leads to problems for the unwary Integer Overflow What happens when an integer is too big for its type? System.out.print("Number too big: "); System.out.println(2147483647 + 1); System.out.print("Number too small: "); System.out.println(-2147483648 - 1); Wraps around from the highest number to the lowest Must be careful that your program will not go beyond the range of its data types Floating-Point Precision and Range Floating-point numbers are not exact representations of real numbers Rounding errors occur in repeated calculations Type double has about twice the precision of type float However, even type double can have rounding errors System.out.println(.8F + .1F); System.out.println(.7 + .1 + .1); When floating point numbers get too large, they are set to infinity System.out.println(1.7E308 + 1.7E308); Similarly, when too small they are set to 0.0 System.out.println(4E-324 - 3E-324); The Moral Integer and floating-point data types work well most of the time However, if you work with large positive or negative numbers, you must be sure you do not exceed the range of the data type Also, floating-point numbers have limited precision When math operations are performed repeatedly, they can become less precise You must be careful of precision when using floating-point numbers ^ top 4.1.5: Type Casting Cast: change the data type of the returned value of an expression Recall that different data types are stored in different forms Sometimes you need to change from one form to another Example: arithmetic adding a double and an int value Example: you want to truncate the remainder of a calculation Java will automatically cast some data types to another byte => short => int => long => float => double / char =/ Also known as implicit casting May not know how to convert some values May not cast how the programmer wants Programmers can also explicitly cast data types Recall that casting is done automatically (implicitly) when a narrower-type is assigned to a broader-type Explicit casting changes the data type for a single use of the variable Precede the variable name with the new data type in parentheses: (dataType) variableName For example: int n; double x = 2.5; n = (int) x; System.out.println("x = " + x); System.out.println("n = " + n); Value of x is converted from type double to integer before assigning the value to n Note that the data type is changed only for the single use of the returned value Required Casting Explicit casting is required to assign a broader type to a narrower type ILLEGAL: Implicit casting to a narrower data type int n; double x = 2.5; n = x; // illegal in java Illegal since x is double, n is an int, and double is a broader data type than int Note that you can use an explicit cast even when an implicit one will be done automatically Truncation When Casting Floating-Point to Integer type Converting (casting) a floating-point to integer type does not round -- it truncates Fractional part is lost (discarded, ignored, thrown away) For example: int n; double x = 3.99999; n = (int) x; // x truncated System.out.println("x = " + x); System.out.println("n = " + n); Value of n is 3 Casting a char to an int Casting a char value to int produces the ASCII/Unicode value For example, what would the following display? char answer = 'Y'; System.out.println(answer); System.out.println((int) answer); Answer: >Y >89 ^ top 4.1.6: Summary Simple assignment statements have a variable, equals sign and an expression. variable = expression; The expression is computed before the assignment Java has assignment variations of the form: variable <op>= expression; In addition, you can add to or subtract from variables the value one using increment (++) and decrement (--) operators count++; count--; Primitive variables have limitations in range and precision If your program assigns values too high or too low, integers wrap around Floating-point numbers usually have enough range However, they often suffer from lack of precision System.out.println(.8F + .1F); System.out.println(.7 + .1 + .1); Type double has about twice the precision of type float When floating point numbers get too large, they are set to inf Similarly, when too small they are set to 0.0 Java will automatically cast variables of narrow types to wider types byte => short => int => long => float => double / char =/ You must use an explicit cast when assigning a broader type to a narrower type. You can explicitly cast values of an expression to a different type. Check Yourself How many ways can you add the integer 1 to a variable named index? List each way. What happens when you increase the value of an integer number beyond its range? How many digits that can be accurately stored in a float? double? What type of error occurs in the following expression? System.out.println(.7 + .1 + .1); Which is a valid typecast? a(int) int:a; (int)a; to(int, a); How do you truncate the decimal part of double variable? ^ top Exercise 4.1 The following code does not display the correct value of degreesC. 1 2 3 4 5 6 7 8 9 10 import java.util.*; public class FahrenheitConverter { public static void main(String[] args) { double degreesC, degreesF; degreesF = 212; degreesC = 5 / 9 * (degreesF - 32); System.out.println(degreesC); } } Specifications Save the code as FahrenheitConverter.java Use a cast to correct the code and obtain the correct output. Submit your program along with your other exercises for this lesson. ^ top 4.2: Making Selections Objectives At the end of the lesson the student will be able to: Formulate relational expressions to produce boolean values Generate selection statements ^ top 4.2.1: Flow of Control The term flow of control refers to the order in which a computer executes instructions All programs can be written with just three control-flow elements: Sequence - continue with the next instruction All programs continue with the next instruction by default To make the program do something else, you code either a selection or repetition (looping) statement 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 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 for do-while while We will first consider selection statements ^ top 4.2.2: The Value of a Relationship To make a selection or repetition statement, you need to code a test condition The computer uses the condition to decide whether or not to execute the statement One way to code a test condition is to use a relational expression Relational Expressions 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 numbers are called conditions or relational expressions Simple conditions have one relational operator comparing two values A condition 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 any numerical data ^ top 4.2.3: Using if Statements The simplest selection statement is an if statement This statement executes a section of code only if a condition is true If the test condition is false, the computer skips the code if (condition) { // execute statements only if true } For clarity: Write the if on a different line than the executed block Indent statements that are executed if the condition evaluates to true For Example What is new about the following code? What is the test condition? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 import java.util.*; public class GuessingGame { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("I'm thinking of a number " + " between 1 and 10.\nCan you guess it? "); int guess = input.nextInt(); if (guess == 7) { System.out.println("*** Correct! ***"); } } } About those Curly Braces Technically, the if statement affects only the single statement that follows We use curly braces to make that one statement into a block of statements This allows us to put any number of statements within the block Curly braces are not always required, but the best practice is to always include them ^ top 4.2.4: Using if-else Statements Sometimes we want to choose between two actions If a condition is true then do this Otherwise it is false so do something else General syntax: if (condition) { // evaluates to true // execute statements only if true } else { // execute statements only if false } Note that there is no test condition for the else clause The decision on which set of statements to use depends on only one condition For clarity, write the if and else on different lines than the other statements Also, indent the other statements For Example What do you notice that is different about the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 import java.util.*; public class GuessingGame2 { public static void main(String[] args) { Scanner input = new Scanner(System.in); int guess = 0; System.out.print("I'm thinking of a number " + " between 1 and 10.\nCan you guess it? "); guess = input.nextInt(); if (guess == 7) { System.out.println("*** Correct! ***"); } else { System.out.println("Sorry, that is wrong."); } } } 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 4.2.5: Summary Flow of control refers to the order in which a computer executes statements There are only three types of flow of control Sequence: continue with the next instruction Selection: a choice between at least two options Repetition: repeat a block of code (loop) To make a selection or repetition statement, you need to code a test condition The computer uses the condition to decide whether or not to execute the statement One way to code a test condition is to use a relation expression Relational operators include: == != < <= > >= Selection is a choice between at least two options Selection statements include: if if-else Check Yourself What is meant by the term "flow of control"? What is the default flow-of-control" operation after a statement finishes executing? What two values can be returned from a relational expressions. What is the value of x after the following code segment? int x = 5; if (x > 3) { x = x - 2; } else { x = x + 2; } ^ top Exercise 4.2 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 4.2 Specifications For the values: int a = 5, b = 2, c = 4, d = 6; Record the in your exercise4.txt file which of the following conditions are true and which are false. a > b a != b d % b == c % b a * c != d * b b % c * a != a * b As a final part of this exercise, write a short program that asks the user for a test score. If the score is greater than or equal to 60 then print, "You passed!"; otherwise print, "Sorry, you failed" Name this program TestScorer.java and submit it with the lesson 4 exercises. You may use the following code to get started. import java.util.*; public class TestScorer { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.println("Enter your test score: "); int score = input.nextInt(); // Enter code here } } ^ top 4.3: More About Selection Objectives At the end of the lesson the student will be able to: Construct test conditions that evaluate to a boolean value Use logical operators to assemble complex conditions Avoid some common pitfalls when creating test conditions ^ top 4.3.1: Test Conditions and Boolean Values All selection and loop statements have a test condition This test condition controls the operation of the statement You must construct test conditions so they always evaluate to a boolean value Boolean Values Boolean types have just one of two values: either true or false boolean truth = true, lies = false; Recall that an expression is part of a statement that returns a value Thus, a boolean expression is some code that returns either true or false For example: true false 2 < 5 a == b We use boolean expressions like these to control selection and looping statements ^ top 4.3.2: 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 More Information ASCII Table (7-bit) Unicode Home Page Binary Number System ^ top 4.3.3: Nested if Statements Note that you can include if statements within other if statements Each inner if statement is evaluated only if the outer condition is met This has the effect or requiring two conditions to evaluate to true before the statements of the inner if are executed For Example What do you notice that is different about the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 import java.util.*; public class TempGuide { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a temperature: "); double temp = input.nextDouble(); if (temp > 65) { if (temp > 75) { System.out.println("That's too hot!"); } else { System.out.println("That's just right."); } } else { System.out.println("That's cold!"); } } } Nesting if statements inside of if statements can be confusing when too deep Rule of thumb: no more than three deep Nesting in the else Clause You can also nest if statements in the else clause This has the effect of testing another condition if the first one evaluates to false For example, what do you notice that is different about the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 import java.util.*; public class TempGuide2 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a temperature: "); double temp = input.nextDouble(); if (temp > 75) { // first condition System.out.println("That's too hot!"); } else if (temp > 65) { // second condition System.out.println("That's just right."); } else { // default condition System.out.println("That's cold!"); } } } Note the alignment of the nested statements Instead of: if (temp > 75) { System.out.println("That's too hot!"); } else { if (temp > 65) { System.out.println("That's just right."); } else { System.out.println("That's cold!"); } } You format like this: if (temp > 75) { System.out.println("That's too hot!"); } else if (temp > 65) { System.out.println("That's just right."); } else { System.out.println("That's cold!"); } This structure clearly shows the operation of the code Also, it prevents indentations cascading to the right ^ top 4.3.4: Testing Multiple Conditions One reason to use nested if statements is for testing multiple conditions However, a better way is to combine conditions using conditional operators Conditional operators allow you to consider multiple cases and to change conditions AND Operation One such operator is "and", which is spelled && in Java For example, the following is true if temp >= 65 and temp <= 75 (temp >= 65) && (temp <= 75) When two boolean expressions are connected using && the whole expression is true only if both conditions are true However, if either condition is false the whole expression is false OR Operation Also, you can combine two boolean expressions using the "or" operator, spelled || in Java For example, the following is true if temp < 65 or temp > 75 (temp < 65) || (temp > 75) When two boolean expressions are connected using || the whole expression is true if either condition is true Only if both conditions are false is the entire expression false NOT Operation Additionally, you can negate any expression using ! operator To use it, place the entire expression in parenthesis and place the ! operator in front For example, to get a temp between 65 and 75, we can negate our previous example: !((temp < 65) || (temp > 75)) Parenthesis Remember that a boolean expression in an if statement must be enclosed in parenthesis Thus, an if statement that uses an && operator with two comparisons looks like: if ((temp >= 65) && (temp <= 75)) { Also note that relational operators have a higher precedence than logical operators Thus, you can remove the inner parenthesis without affecting the meaning if (temp >= 65 && temp <= 75) { However, if using parenthesis is clearer to you then use the extra parenthesis For Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 import java.util.*; public class TempGuide3 { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a temperature: "); double temp = input.nextDouble(); if (temp >= 65 && temp <= 75) { System.out.println("That's just right."); } else { System.out.println("That's uncomfortable!"); } } } ^ top 4.3.5: Summary All selection and repetition statements have a test condition This test condition controls the operation of the statement You must construct test conditions so they always evaluate to a boolean value Boolean types have just one of two values: either true or false A boolean expression is any expression that returns either true or false You can compare letters as well as numbers since letters are stored as numbers Used to alphabetize letters and words in a computer program Nested if statements are legal but often confusing Best to avoid nesting if statements in the if clause In contrast, nesting if statements in the else clause is fine When you need to compare several conditions, you use logical operators ! && || For example: (temp >= 65) && (temp <= 75) (temp < 65) || (temp > 75) !((temp < 65) || (temp > 75)) Check Yourself What two values can a boolean variable contain? Why can you compare character data using a relational expression? What is the effect of having an if statement nested in an if statement? What is the effect of having an if statement nested in an else statement? When does an AND (&&) of two or more conditions evaluate to true? When does an OR (||) of two or more conditions evaluate to false? What is the effect of the NOT (!) operator? ^ top Exercise 4.3 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 Save the following code as GradeConverter.java Change the code to: Get a score from the users as an double number between 0 and 100 Convert the score to a letter grade Display the letter grade Submit your program along with your other exercises for this lesson. 1 2 3 4 5 6 7 8 9 10 11 12 import java.util.*; public class GradeConverter { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a score "); double score = input.nextDouble(); // Insert statements here } } ^ top 4.4: Testing and Debugging Objectives At the end of the lesson the student will be able to: Describe the three kinds of programming errors Find the syntax errors in your programs ^ top 4.4.1: Kinds of Programming Errors There are three types of programming errors Syntax Errors Run-time Errors Logic Errors Syntax Errors Violation of the "grammar" rules of the programming language For instance: forgetting a semicolon Syntax errors are discovered by the compiler You program will not compile if you have a syntax error Errors Versus Warnings If you violate a syntax rule, the compiler gives you an error message Sometime the compiler will give you are warning message instead This indicates that your code is technically correct However, your code is unusual enough that it is probably a mistake Thus, you lose points for warning messages in your programming assignments Run-time Errors Error conditions are sometimes detected by the computer at run-time For example: trying to divide by zero You program will usually end when it detects these errors How do you make sure your program does not have this type of error? Logic Errors Errors in the program's algorithm For example: subtracting rather than adding in an arithmetic formula Computer does not recognize these types of errors Makes them the most difficult to diagnose How do you make sure your program does not have this type of error? ^ top 4.4.2: Anatomy of an Error Message To debug your programs you have to read the error messages As you read them you need to extract the important information For example, this error message tells you several important things: Fiddle.java:7: unclosed string literal System.out.println("Hello, world!); ^ The name of the program Fiddle.java:7: unclosed string literal The line number of the error Fiddle.java:7: unclosed string literal The error description Fiddle.java:7: unclosed string literal The character position where the compiler became confused System.out.println("Hello, world!); ^ The most immediately useful information is the line number ^ top 4.4.3: How To Debug Syntax Errors Repeat the following process until all the compiler errors are gone. Find the first error only. You should look at the first error and ignore the rest. Often the first error causes all the rest of the errors. Reading any error beyond the first is often a waste of time. Analyze the first error. Find the line reported by the first error. This is the line where the compiler first got confused. Look at this line in your code and try to spot the problem. The error message may provide clues as to the problem. If you cannot see a problem on that line, look for a problems on lines just before the reported line. If you cannot find the error in your code, then look at the causes of common errors in the next section. Most times, you will spot the error. If you do not, then compare the first error message with those listed below. If you still cannot find your error, show it to the instructor. He will help you find and correct the problem. You can either bring it to him in person or email him. After correcting the first error, try to recompile your program. ^ top 4.4.4: Some Error Messages and Their Common Causes Instructions If you cannot see an error in your code on or before the reported line number, then read the error message Compare the error description to the list below unclosed comment This error is usually caused by a missing end of a comment (*/). For example, the following code: /** * Hello.java * Purpose: Prints a message to the screen. * * @author Ed Parrish * @version 1.0 8/30/05 public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); } } generates the following error: Hello.java:1: unclosed comment /** ^ To fix it, add the closing comment: */ ']' expected This error is usually caused by a missing square bracket (]). For example, the following code: public class HelloWorld { public static void main(String[ args) { System.out.println("Hello, world!"); } } generates the following error: HelloWorld.java:2: ']' expected public static void main(String[ args) { ^ To fix it, add the closing square bracket: ] ';' expected This error is usually caused by a semicolon (;). For example, the following code: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!") } } generates the following error: HelloWorld.java:4: ';' expected } ^ Note that the line reported for the error is off by one number. To fix the problem, add the missing semicolon: ; unclosed string literal This error is usually caused by a double quote ("). For example, the following code: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!); } } generates the following error: HelloWorld.java:3: unclosed string literal System.out.println("Hello, world!); ^ To fix the problem, add the missing double quote: " 'class' or 'interface' expected This is a general-purpose syntax error and is often caused by: An extra closing curly bracket (}) A space between the slashes of a single-line comment For example, the extra curly bracket in this code: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!"); } }} generates the following error: HelloWorld.java:5: 'class' or 'interface' expected }} ^ To fix the problem, remove the extra curly bracket: } ^ top 4.4.5: Summary There are three kinds of programming errors Syntax errors Run-time Errors Logic Errors Sometimes the compiler will issue a warning if your code is unusual but still correct You should treat this warning as an error because it probably is a logic error Syntax errors are the easiest errors to find and correct I provide some instructions for finding and correcting these errors This information is summarized in How To Debug Compiler Errors Check Yourself What are the three kinds of programming errors? What is the most useful information you can extract from an error message? What is the process for debugging syntax errors? What is the purpose of testing code that compiles without error? ^ top Exercise 4.4 Label this exercise: Exercise 4.4 Submit all exercises for today's lesson in one file unless instructed otherwise Complete the following and list the error messages you found and what you did to correct them in your exercise4.txt file. Specifications The following program was written by a person in a hurry. During the entry process a large number of errors were made. You need to find and correct the errors reported by the compiler by following the process described in section 4.4.3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 import java.util.Scanner; /** * Erroneous.java * A very erroneous program. * * @author B. A. Ware * @version 1.0 9/15/05 public class Erroneous { public static void main(String[ args) { Scanner input = now Scanner(System.in); System.out.print("Enter a message: "); String message = input.nextLine(); System.out.println("Your message: + message); } // end of main method }} / / end of Erroneous class ^ top Wrap Up Reminders Due Next: A3: 9/5 Degrees of Separation (9/21/05) Exercise 3 and CodeLab 3 (9/21/05) A4: Conversions (9/28/05) Exercise 4 and CodeLab 4 (9/28/05) 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: December 25 2005 @20:03:35