Program Elements. Notes. Exercises CHAPTER 3

Transcription

1 CHAPTER 3 Program Elements Notes Chapter 3 introduces the program elements that are the underlying constructs used inside an object to define the services it contributes to a program. Without these constructs, classes, objects, and their methods cannot be made concrete. Chapter 2 introduced the abstract concepts of classes, objects and methods. Chapter 3 introduces a minimum amount of detail to be able to write useful programs. Chapter 3 establishes this detail so that Chapter 4 can lay a strong foundation, both abstract and concrete, for object-oriented programming. In this way, object-oriented programming is introduced early in the book, but in a way that is very tangible. In addition, this chapter also explores the basic activities that a programmer should go through when developing software. These activities form the cornerstone of high-quality software development and represent the first step towards a disciplined development process. We feel strongly that these issues and activities should be introduced early and often to students. Avoiding these activities only encourages students to form bad habits and practices that have to be corrected later. The Average example is implemented twice in order to demonstrate to students that alternative implementations should be considered. Exercises 3-9 What is the result of the following expressions when length equals 8, width equals 3, and height equals 2.25? Assume length and width are integer variables, and height is a floating point variable. a) length / width 8 / 3 = 2 b) length / height 8 / 2.55 = c) length % width 8 % 3 = 2 d) width % length 3 % 8 = 3 e) length * width + height 8 * = f) length + width * height * 2.25 = Give an equivalent expression for the following: a) total < sum 13

2 sum > total b) MAX <= highest highest >= MAX c) intensity > threshold threshold < intensity 3-11 Explain three reasons to use constants in a program instead of literals. Not Provided 3-12 Java is strongly typed. What does that mean? Not Provided 3-13 What is wrong with the following code fragment? Using braces and correct indentation, rewrite it so that it produces correct output. if (total == MAX) if (total < sum) System.out.println ("total equals MAX and total is less than sum"); else System.out.println ("total is not equal to MAX"); The else goes with the second if, not the first if as the indentation indicates. Use braces after first if will correct the problem. if (total == MAX) { if (total < sum) total equals MAN and total is less than sum"); else total is not equal to MAX"); 3-14 What is wrong with the following code fragment? Will this code compile if it is part of a valid program? Explain completely. if (length = MIN_LENGTH) System.out.println ("The length cannot be reduced further."); The if statement needs to be changed to: if (length == MIN_LENGTH) System.out.println ("The length cannot be reduced further."); so that the statement will check for equality. The code will not compile. Unlike other languages, the condition of an if statement must evaluate to a boolean value. 14 Chapter 3 Program Elements

3 3-15 What is wrong with the following code fragment? What are three distinct ways it could be changed to remove the flaw? count = 50; while (count >= 0) { System.out.println (count); count = count + 1; The code is an infinite loop. It could be avoided by initializing count to a negative value such that the condition is initially false, or by changing the condition such that it is initially false, or by changing the increment of count to a decrement. The third option is probably preferred, otherwise the loop has no purpose. Programming Projects 3-16 Write a program which reads an integer value and prints the sum of all even integers between 2 and the input value, inclusive. Print an error message if the input value is less than 2. Prompt accordingly. ******************************************************************* Evens.java Programming Project Application Authors: Lewis and Loftus Classes: Evens ******************************************************************* import java.io.*; public class Evens { =============================================================== Calculate and print sum of all even numbers up to and including the number entered by the user. =============================================================== public static void main(string[] args) throws IOException { int sum = 0; int count = 2; int number; BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); 15

4 Prompt user for an input value and store it System.out.print("Enter a number with a value of at least 2: "); System.out.flush(); number = Integer.parseInt (stdin.readline()); The program executes if the input value is at least 2. If not, the error message in the else statement is printed. if (number >= 2) { while(count <= number) { sum = sum + count; count = count + 2; The sum of all even numbers up to " + number + " is " + sum); else Minimum value of 2 required."); method main class Evens 3-17 Write a program which reads an integer value between 0 and 100 (inclusive), representing the amount of a purchase in cents. Produce an error message if the input value is not in that range. If the input is valid, determine the amount of change that would be received from one dollar, and print the number of quarters, dimes, nickels, and pennies that should be returned. Maximize the coins with the highest value. Follow the format below. The user input is shown in boldface type. Enter the purchase amount [0-100]: 36 Your change of 64 cents is given as: 2 Quarters 1 Dimes 0 Nickels 4 Pennies Hint: 64 / 25 equals 2, and 64 % 25 equals 14. ******************************************************************* Change.java Programming Project Application 16 Chapter 3 Program Elements

5 Authors: Lewis and Loftus Classes: Change ******************************************************************* import java.io.*; public class Change { ========================================================= Calculate and report the change given for a user- supplied purchase amount ========================================================= public static void main(string[] args) throws IOException { int change, purchase; int quarters, nickels, dimes, pennies; BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); Prompt user for an input value and store it System.out.print("Enter the purchase amount [0-100]: "); System.out.flush(); purchase = Integer.parseInt (stdin.readline()); The program executes if the input value is between 0 & 100. If not an error message is printed. if (purchase >= 0) { if (purchase <= 100) { Calculate and print the amount of change change = purchase; System.out.print("Your change of " + change); cents is given as:"); Calculate the coins to be returned Note that this changes the value of the variable "change." 17

6 quarters = (change / 25); change = change % 25; dimes = (change / 10); change = change % 10; nickels = (change / 5); change = change % 5; pennies = change; Output the result. " + quarters + " Quarter"); " + dimes + " Dimes"); " + nickels + " Nickels"); " + pennies + " Pennies"); else Purchase must between 0 and 100 cents."); else Purchase must between 0 and 100 cents."); method main class Change 3-18 In Problem 3-17, the coins were always expressed as plural even if there was only one (1 Dimes). Modify your answer to print the singular form of the word for each coin when only 1 is used. ******************************************************************* Change2.java Programming Project Application Authors: Lewis and Loftus Classes: Change2 ******************************************************************* import java.io.*; public class Change2 { ========================================================= Calculate and report the change given for a user supplied purchase amount ========================================================= public static void main(string[] args) throws IOException { 18 Chapter 3 Program Elements

7 int change, purchase; int quarters, nickels, dimes, pennies; BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); Prompt user for an input value and store it System.out.print("Enter the purchase amount [0-100]: "); System.out.flush(); purchase = Integer.parseInt (stdin.readline()); The program executes if the input value is between 0 & 100. If not an error message is printed. if (purchase >= 0) { if (purchase <= 100) { Calculate and print the amount of change change = purchase; System.out.print("Your change of " + change); cents is given as:"); Calculate the coins to be returned Note that this changes the value of the variable "change." quarters = (change / 25); change = change % 25; dimes = (change / 10); change = change % 10; nickels = (change / 5); change = change % 5; pennies = change; Output the result. if (quarters >= 2) else if (quarters == 1) if (dimes >= 2) " + quarters + " Quarters"); " + quarters + " Quarter"); " + dimes + " Dimes"); 19

8 else if (dimes == 1) " + dimes + " Dime"); if (nickels >= 2) else if (nickels == 1) " + nickels + " Nickels"); " + nickels + " Nickel"); if (pennies >= 2) else if (pennies == 1) " + pennies + " Pennies"); " + pennies + " Penny"); else Purchase must between 0 and 100 cents."); else Purchase must between 0 and 100 cents."); method main class Change Modify the answer to Problem 3-17 to continue processing input values until a sentinel value of -1 is entered. Change the prompt accordingly and do not print an error message when the sentinel value is entered. ******************************************************************* Change3.java Programming Project Application Authors: Lewis and Loftus Classes: Change3 ******************************************************************* import java.io.*; public class Change3 { static public int SENTINEL = -1; ========================================================= 20 Chapter 3 Program Elements

9 Calculate and report the change given for a user supplied purchase amount ========================================================= public static void main(string[] args) throws IOException { int change, purchase = 0; int quarters, nickels, dimes, pennies; boolean invalid; BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); while (purchase!= SENTINEL) { invalid = true; Prompt user for an input value and store it System.out.print("Enter the purchase amount [0-100]: "); System.out.flush(); purchase = Integer.parseInt (stdin.readline()); The program executes if the input value is between 0 & 100. If not an error message is printed. if (purchase >= 0) { if (purchase <= 100) { invalid = false; Calculate and print the amount of change change = purchase; System.out.print("Your change of " + change); cents is given as:"); Calculate the coins to be returned Note that this changes the value of the variable "change." quarters = (change / 25); change = change % 25; dimes = (change / 10); change = change % 10; nickels = (change / 5); 21

10 change = change % 5; pennies = change; Output the result. if (quarters >= 2) else if (quarters == 1) if (dimes >= 2) else if (dimes == 1) if (nickels >= 2) else if (nickels == 1) if (pennies >= 2) else if (pennies == 1) " + quarters + " Quarters"); " + quarters + " Quarter"); " + dimes + " Dimes"); " + dimes + " Dime"); " + nickels + " Nickels"); " + nickels + " Nickel"); " + pennies + " Pennies"); " + pennies + " Penny"); if (invalid) Purchase must between 0 and 100 cents."); method main class Change Modify the Football_Choice program so that it does not ask the receive / kickoff question unless the user wins the coin toss. Not Provided 3-21 Modify the Average program to validate the grades entered to make sure they are in the range 0 to 100, inclusive. Print an error message if a grade is not valid, then continue to collect grades. Continue to use the sentinel value to indicate the end of the input, but do not print an error message when it is entered. Do not count an invalid grade or include it as part of the running sum. ******************************************************************* 22 Chapter 3 Program Elements

11 Average3.java Programming Project Application Authors: Lewis and Loftus Classes: Average3 ******************************************************************* import java.io.*; class Average3 { static final int SENTINEL = -1; ============================================================ Modified version of Average program to validate the grades entered. Process until the sentinel value is entered. ============================================================ public static void main (String[] args) throws IOException { int max = 0, count = 0, grade = 9999; float sum = 0, average; boolean invalid; BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in)); Read and process all grades while (grade!= SENTINEL) { invalid = true; Prompt user for grades System.out.print ("Enter a grade between 0 & 100 (-1 to quit): "); System.out.flush(); grade = Integer.parseInt (stdin.readline()); if (grade >= 0) if (grade <= 100) { invalid = false; count = count + 1; sum = sum + grade; grade is in range 23

12 if (grade > max) max = grade; If input value was out of range, print error message if (invalid) { System.out.print ("Grade between 0 & 100 required "); System.out.println ("(or -1 to exit)."); if (count == 0) System.out.println ("No valid grades were entered."); else { average = sum / count; System.out.println(); System.out.println ("Total number of students: " + count); System.out.println ("Average grade: " + average); System.out.println ("Highest grade: " + max); method main class Average Modify the Average program to determine and produce the minimum grade in addition to the maximum. ******************************************************************* Average4.java Programming Project Application Authors: Lewis and Loftus Classes: Average4 ******************************************************************* import java.io.*; class Average4 { static final int SENTINEL = -1; ============================================================ 24 Chapter 3 Program Elements

13 Modified version of Average program to validate the grades entered, process until the sentinel value is entered, and record and print min grade. ============================================================ public static void main (String[] args) throws IOException { int max = 0, min = 101; int count = 0, grade = 101; float sum = 0, average; boolean invalid; BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in)); Read and process all grades while (grade!= SENTINEL) { invalid = true; Prompt user for grades System.out.print ("Enter a grade between 0 & 100 (-1 to quit): "); System.out.flush(); grade = Integer.parseInt (stdin.readline()); if (grade >= 0) if (grade <= 100) { invalid = false; count = count + 1; sum = sum + grade; if (grade > max) max = grade; if (grade < min) min = grade; grade is in range If input value was out of range, print error message if (invalid) { System.out.print ("Grade between 0 & 100 required "); System.out.println ("(or -1 to exit)."); if (count == 0) System.out.println ("No valid grades were entered."); 25

14 else { average = sum / count; System.out.println(); System.out.println ("Total number of students: " + count); System.out.println ("Average grade: " + average); System.out.println ("Highest grade: " + max); System.out.println ("Lowest grade: " + min); method main class Average Modify the Average program to read and process exactly 17 grades. ******************************************************************* Average5.java Programming Project Application Authors: Lewis and Loftus Classes: Average5 ******************************************************************* import java.io.*; class Average5 { static final int STUDENTS = 17; ============================================================ Modified version of Average program to validate the grades entered. Process 17 students exactly. ============================================================ public static void main (String[] args) throws IOException { int max = 0, min = 101; int count = 0, grade = 101; float sum = 0, average; boolean invalid; BufferedReader stdin = new BufferedReader 26 Chapter 3 Program Elements

15 (new InputStreamReader (System.in)); Read and process all grades while (count < STUDENTS ) { invalid = true; Prompt user for grades System.out.print ("Enter a grade for student #" + (count+1) + ":"); System.out.flush(); grade = Integer.parseInt (stdin.readline()); if (grade >= 0) if (grade <= 100) { invalid = false; count = count + 1; sum = sum + grade; if (grade > max) max = grade; if (grade < min) min = grade; grade is in range If input value was out of range, print error message if (invalid) { System.out.print ("Grade between 0 & 100 required."); average = sum / count; System.out.println(); System.out.println ("Total number of students: " + count); System.out.println ("Average grade: " + average); System.out.println ("Highest grade: " + max); System.out.println ("Lowest grade: " + min); method main class Average5 27

16 3-24 Modify the Average program to prompt for and read the exact number of students in the class, then loop until exactly that many valid grades have been read. ******************************************************************* Average6.java Programming Project Application Authors: Lewis and Loftus Classes: Average6 ******************************************************************* import java.io.*; class Average6 { ============================================================ Modified version of Average program to validate the grades entered. Process n students exactly. ============================================================ public static void main (String[] args) throws IOException { int max = 0, min = 101; int count = 0, grade = 101; float sum = 0, average; int number_students; boolean invalid; BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in)); Prompt user for number of grades System.out.print("Enter a the number of scores to be processed: "); System.out.flush(); number_students = Integer.parseInt (stdin.readline()); Read and process all grades while (count < number_students ) { invalid = true; Prompt user for grades System.out.print ("Enter a grade for student #" + (count+1) + ":"); 28 Chapter 3 Program Elements

17 System.out.flush(); grade = Integer.parseInt (stdin.readline()); if (grade >= 0) if (grade <= 100) { invalid = false; count = count + 1; sum = sum + grade; if (grade > max) max = grade; if (grade < min) min = grade; grade is in range If input value was out of range, print error message if (invalid) { System.out.print ("Grade between 0 & 100 required."); average = sum / count; System.out.println(); System.out.println ("Total number of students: " + count); System.out.println ("Average grade: " + average); System.out.println ("Highest grade: " + max); System.out.println ("Lowest grade: " + min); method main class Average6 29

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