JAVA 2.3
| [IFT 102] | Introduction to Java Technologies |
Score: 50 pts (10 pts * 5)
I. Prelab Exercises (10 pts)
A. Textbook Sections 5.1-5.3
1. Rewrite each condition below in valid Java syntax (give a boolean expression):
a. x > y > z
b. x and y are both less than 0
c. neither x nor y is less than 0
d. x is equal to y but not equal to z
2. Suppose gpa is a variable containing the grade point average of a student. Suppose the goal of a program is to let a student know if he/she made the Dean’s list (the gpa must be 3.5 or above). Write an if… else… statement that prints out the appropriate message (either “Congratulations—you made the Dean’s List” or “Sorry you didn’t make the Dean’sList”).
3. Complete the following program to determine the raise and new salary for an employee by adding if … else statements to compute the raise. The input to the program includes the current annual salary for the employee and a number indicating the performance rating (1=excellent, 2=good, and 3=poor). An employee with a rating of 1 will receive a 6% raise, an employee with a rating of 2 will receive a 4% raise, and one with a rating of 3 will receive a 1.5% raise.
// ************************************************************
// Salary.java
// Computes the raise and new salary for an employee
// ************************************************************
import java.util.Scanner;
public class Salary
{
public static void main (String[] args)
{
double currentSalary; // current annual salary
double rating; // performance rating
double raise; // dollar amount of the raise
Scanner scan = new Scanner(System.in);
// Get the current salary and performance rating
System.out.print (“Enter the current salary: “);
currentSalary = scan.nextDouble();
System.out.print (“Enter the performance rating: “);
rating = scan.nextDouble();
// Compute the raise — Use if … else …
// Print the results
System.out.println (“Amount of your raise: $” + raise);
System.out.println (“Your new salary: $” + currentSalary + raise);
}
}
B. Textbook Section 5.4
In a while loop, execution of a set of statements (the body of the loop) continues until the boolean expression controlling the loop (the condition) becomes false. As for an if statement, the condition must be enclosed in parentheses. For example, the loop below prints the numbers from 1 to LIMIT:
final int LIMIT = 100; // setup
int count = 1;
while (count <= LIMIT) // condition
{ // body
System.out.println(count); // — perform task
count = count + 1; // — update condition
}
There are three parts to a loop:
· The setup, or initialization. This comes before the actual loop, and is where variables are initialized in preparation for the first time through the loop.
· The condition, which is the boolean expression that controls the loop. This expression is evaluated each time through the loop. If it evaluates to true, the body of the loop is executed, and then the condition is evaluated again; if it evaluates to false, the loop terminates.
· The body of the loop. The body typically needs to do two things:
· Do some work toward the task that the loop is trying to accomplish. This might involve printing, calculation, input and output, method calls—this code can be arbitrarily complex.
· Update the condition. Something has to happen inside the loop so that the condition will eventually be false — otherwise the loop will go on forever (an infinite loop). This code can also be complex, but often it simply involves incrementing a counter or reading in a new value.
Sometimes doing the work and updating the condition are related. For example, in the loop above, the print statement is doing work, while the statement that increments count is both doing work (since the loop’s task is to print the values of count) and updating the condition (since the loop stops when count hits a certain value).
The loop above is an example of a count-controlled loop, that is, a loop that contains a counter (a variable that increases or decreases by a fixed value—usually 1—each time through the loop) and that stops when the counter reaches a certain value.
Not all loops with counters are count-controlled; consider the example below, which determines how many even numbers must be added together, starting at 2, to reach or exceed a given limit.
final int LIMIT = 16; TRACE
int count = 1; sum nextVal count
int sum = 0; — ——- —–
int nextVal = 2;
while (sum < LIMIT)
{
sum = sum + nextVal;
nextVal = nextVal + 2;
count = count + 1;
}
System.out.println(“Had to add together ” + (count-1) + ” even numbers ” +
“to reach value ” + LIMIT + “. Sum is ” + sum);
Note that although this loop counts how many times the body is executed, the condition does not depend on the value of count.
Not all loops have counters. For example, if the task in the loop above were simply to add together even numbers until the sum reached a certain limit and then print the sum (as opposed to printing the number of things added together), there would be no need for the counter. Similarly, the loop below sums integers input by the user and prints the sum; it contains no counter.
int sum = 0; //setup
String keepGoing = “y”;
int nextVal;
while (keepGoing.equals(“y”) || keepGoing.equals(“Y”))
{
System.out.print(“Enter the next integer: “); //do work
nextVal = scan.nextInt();
sum = sum + nextVal;
System.out.println(“Type y or Y to keep going”); //update condition
keepGoing = scan.next();
}
System.out.println(“The sum of your integers is ” + sum);
Exercises
1. In the first loop above, the println statement comes before the value of count is incremented. What would happen if you reversed the order of these statements so that count was incremented before its value was printed? Would the loop still print the same values? Explain.
2. Consider the second loop above.
a. Trace this loop, that is, in the table next to the code show values for variables nextVal, sum and count at each iteration. Then show what the code prints.
b. Note that when the loop terminates, the number of even numbers added together before reaching the limit is count-1, not count. How could you modify the code so that when the loop terminates, the number of things added together is simply count?
3. Write a while loop that will print “I love computer science!!” 100 times. Is this loop count-controlled?
4. Add a counter to the third example loop above (the one that reads and sums integers input by the user). After the loop, print the number of integers read as well as the sum. Just note your changes on the example code. Is your loop now count-controlled?
5. The code below is supposed to print the integers from 10 to 1 backwards. What is wrong with it? (Hint: there are two problems!) Correct the code so it does the right thing.
count = 10;
while (count >= 0)
{
System.out.println(count);
count = count + 1;
}
II. Two Meanings of Plus (10 pts)
In Java, the symbol + can be used to add numbers or to concatenate strings. This part illustrates both uses.
When using a string literal (a sequence of characters enclosed in double quotation marks) in Java the complete string must fit on one line. The following is NOT legal (it would result in a compile-time error).
System.out.println (“It is NOT okay to go to the next line
in a LONG string!!!”);
The solution is to break the long string up into two shorter strings that are joined using the concatenation operator (which is the + symbol). This is discussed in Section 2.1 in the text. So the following would be legal
System.out.println (“It is OKAY to break a long string into ” +
“parts and join them with a + symbol.”);
So, when working with strings the + symbol means to concatenate the strings (join them). BUT, when working with numbers the + means what it has always meant—add!
1. Observing the Behavior of +
To see the behavior of + in different settings do the following:
a. Study the program below, which is in file PlusTest.java.
// ************************************************************
// PlusTest.java
//
// Demonstrate the different behaviors of the + operator
// ************************************************************
public class PlusTest
{
// ————————————————-
// main prints some expressions using the + operator
// ————————————————-
public static void main (String[] args)
{
System.out.println (“This is a long string that is the ” +
“concatenation of two shorter strings.”);
System.out.println (“The first computer was invented about” + 55+
“years ago.”);
System.out.println (“8 plus 5 is ” + 8 + 5);
System.out.println (“8 plus 5 is ” + (8 + 5)) ;
System.out.println (8 + 5 + ” equals 8 plus 5.”);
}
}
b. Save PlusTest.java to your directory.
c. Compile and run the program. For each of the last three output statements (the ones dealing with 8 plus 5) write down what was printed. Now for each explain why the computer printed what it did given that the following rules are used for +. Write out complete explanations.
· If both operands are numbers + is treated as ordinary addition. (NOTE: in the expression a + b the a and b are called the operands.)
· If at least one operand is a string the other operand is converted to a string and + is the concatenation operator.
· If an expression contains more than one operation expressions inside parentheses are evaluated first. If there are no parentheses the expression is evaluated left to right.
d. The statement about when the computer was invented is too scrunched up. How should that be fixed?
2. Writing Your Own Program With +
Now write a complete Java program that prints out the following sentence:
Ten robins plus 13 canaries is 23 birds.
Your program must use only one statement that invokes the println method. It must use the + operator both to do arithmetic and string concatenation.
III. Tracking Sales (10 pts)
File Sales.java contains a Java program that prompts for and reads in the sales for each of 5 salespeople in a company. It then prints out the id and amount of sales for each salesperson and the total sales. Study the code, then compile and run the program to see how it works. Now modify the program as follows:
1. Compute and print the average sale. (You can compute this directly from the total; no loop is necessary.)
2. Find and print the maximum sale. Print both the id of the salesperson with the max sale and the amount of the sale, e.g., “Salesperson 3 had the highest sale with $4500.” Note that you don’t need another loop for this; you can do it in the same loop where the values are read and the sum is computed.
3. Do the same for the minimum sale.
4. After the list, sum, average, max and min have been printed, ask the user to enter a value. Then print the id of each salesperson who exceeded that amount, and the amount of their sales. Also print the total number of salespeople whose sales exceeded the value entered.
5. The salespeople are objecting to having an id of 0—no one wants that designation. Modify your program so that the ids run from 1-5 instead of 0-4. Do not modify the array—just make the information for salesperson 1 reside in array location 0, and so on.
6. Instead of always reading in 5 sales amounts, at the beginning ask the user for the number of sales people and then create an array that is just the right size. The program can then proceed as before.
// ***************************************************************
// Sales.java
//
// Reads in and stores sales for each of 5 salespeople. Displays
// sales entered by salesperson id and total sales for all salespeople.
//
// ***************************************************************
import java.util.Scanner;
public class Sales
{
public static void main(String[] args)
{
final int SALESPEOPLE = 5;
int[] sales = new int[SALESPEOPLE];
int sum;
Scanner scan = new Scanner(System.in);
for (int i=0; i<sales.length; i++)
{
System.out.print(“Enter sales for salesperson ” + i + “: “);
sales[i] = scan.nextInt();
}
System.out.println(“\nSalesperson Sales”);
System.out.println(” —————— “);
sum = 0;
for (int i=0; i<sales.length; i++)
{
System.out.println(” ” + i + ” ” + sales[i]);
sum += sales[i];
}
System.out.println(“\nTotal sales: ” + sum);
}
}
IV. User and Item Average Ratings (20 pts: 10pts for problem analysis & 10 pts for Java code)
An online store shows a collection of products that the users can purchase. Each product (or item) has a unique ID. Similarly, each user has a unique ID. Users are allowed to rate items by giving them a value between 1 and 5, with 1 representing the worst rating and five representing the best rating.
You are given a file that contains the ratings that different users gave for various items. Each line in this file holds three values, namely, the user ID, the item ID and the rating value. For example, this line:
173 215 3
Shows that the user with ID 173 has rated the item with ID 215, and she gave that item a rating of 3. You can assume that the user IDs run in sequence starting at 0, and the same applies to the item IDs.
You are required to write a Java program to read in all the user-item ratings, compute the average rating per item, and the average rating per user, and print the average rating for each user and for each item.
Start by analyzing your problem. Your problem analysis should clearly state:
1. The input to your program
2. The output of your program
3. The different modules of your program To define your modules, think about how to divide your problem into sub-problems that each of them needs to be solved.
4. The algorithm for each module, and for the whole program. The algorithm for the program should simply list the sequence in which the various modules will be invoked.
You will find the user-item-ratings file in the assignment folder on BlackBoard. You will also find the code for reading a file, which was discussed in class, on BlackBoard.
Deliverables
1. Complete all the programming activities in sections II thru IV in this lab; then zip all your Java SOURCE CODE FILES for submission.
2. Write a lab report in Word or PDF document. The report should be named lab2.docx or lab2.pdf and must contain the following:
a. The first page should be a cover page that includes the Class Number, Lab Activity Number, Date, and Instructor’s name.
b. Answer of all the questions in section I: Prelab Exercises. No code file is required here. All the code involved, if any, must be copied and pasted in this report.
c. Provide a 1-paragraph conclusion about your experience; how long you spent completing the lab, the challenges you faced, and what you learned.
3. Upload your lab report and the zip file of your source code to Blackboard. DO NOT SUBMIT separate source files. If you do, they will be ignored.
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