Section 2

Agenda

• Bug of the Day
• Review exercises
• Java exercises

Getting started

1. Create a folder named section2 for your work on these exercises. You can find instructions for doing so here.

2. In VS Code, select the File->Open Folder or File->Open menu option, and use the resulting dialog box to find and open your section2 folder. The name of the folder should appear in the Explorer pane on the left-hand side of the VS Code window.

   Important: In general, you should not use File->Open File to open a specific file in VS Code. Rather, you should create a folder for the files that belong to a given assignment or section, and you should use File->Open Folder or File->Open to open the folder.

Bug of the Day

Consider the following Java code fragment, which is supposed to calculate the area of a triangle with base width b and height h.

double b = 3.0;
double h = 1.5;
double area = 1 / 2 * b * h;

When running this code, we can see that the area variable is given the value 0.0, which is incorrect. Identify the logic error in this code and suggest a fix.

Review exercises

Evaluating expressions and reasoning about operators

Recall from lecture that an expression is a combination of literals and operators that can be evaluated to obtain a final value. For example, the expression 2 * 3 evaluates to 6.

1. Show how the expression 1 + 2 * 3 - 4 / 2 is evaluated, one operation at a time. ¹

2. Show how the expression (3 + 2) * 2 - 4 is evaluated, one operation at a time. ²

Recall from lecture that a data type is a classification of a value in memory. Integers have the int data type, and the double type is often used for floating-point numbers. Prior to today, we’ve used string literals, which have the String type.

Each data type has its own set of operators. In Java, the operator + is said to be “overloaded” because it works with many different types. For example, with two numbers it can be used for addition. With two strings, + can be used for string concatenation.

3. Fill in the two tables below with the type of the result of evaluating an expression with the two specified operand types. The type of the first operand is given by the row and the type of the second operand is given by the column. ³

   +	int	double	String
   int	int
   double		double
   String			String

   /	int	double
   int
   double

Java exercises

Try to work together with at least one other person to solve the following problems.

Tracing variable assignments

4. Show how the values of a, b, c, and d change as each assignment and re-assignment statement is executed. Show the values of the variables after the execution of each line. ⁴

   int a = 0;
   int b = 1;
   int c = 3;
   int d = b * c + 3;
   a = 2 * c + b;
   b = b + 1;
   d = d % b;
   

   Fill in the following table:

   Statement	a	b	c	d
   int a = 0;
   int b = 1;
   int c = 3;
   int d = b * c + 3;
   a = 2 * c + b;
   b = b + 1;
   d = d % b;

5. Create a table that traces the values of the variables. ⁵

   int e = 5;
   int f = 10;
   int g = f;
   e = e + 1;
   f = f - 1;
   g = g + e;
   g = g / e;
   

6. Create a table that traces the values of the variables. ⁶

   int i = 4;
   int j = 2;
   int k = 2;
   i = i + 1;
   i = i * j * k;
   j = i / j / k;
   k = i % (k + 1);
   

7. For each expression, calculate the result of evaluating it. Be sure to remember the rules for the / operator we reviewed. Note: you can check your answers by entering the original expressions into VSCode. ⁷

   1. (12 / 5) + (8 % 3)
   2. (42 % 5) + (16 / 3.0)
   3. ((50 / 9) / 2.0) + (200 / (5.0 / 2))
   4. (int)5.9 + 1.9
   5. (double)(5.9 + 1.9)
   6. (int)2.5 * (double)2.5
   7. (int)2.5 * (int)2.5
   8. (double)((int) 3.14159)

Programming exercises

8. What will happen when you try to compile the following program? Form a hypothesis about what will happen before you copy and paste the program into a new Java file. If you get an error, how can you fix it? Assume that the correct output is 3.14159. ⁸

   public class Pi {
       public static void main(String[] args) {
           int pi;
           pi = 3.14159;
           System.out.println(pi);
       }
   }
   

9. Copy and paste the following program into a new Java file and alter it so that it computes and outputs the average of the values in the variables a, b, c, d, and e. Note: The computed average must be precise. ⁹

   public class Average {
       public static void main(String[] args) {
           int a = 12;
           int b = 55;
           int c = 11;
           int d = 30;
           int e = 19;
   
           // put your code here
   
           System.out.println(average);
       }
   }
   

10. Create a complete Java program in a class named Birthday that declares four variables and assigns appropriate values to them. Use the following values. ¹⁰

    • your birthday month (1-12)
    • your birthday day (1-31)
    • the birthday month of another student sitting near you (1-12)
    • the birthday day of the same student near you (1-31)

    Ask others in the class for their name and the proper numbers to store in the variables for their birthday. Then, add one or more statements to produce output in this format using your four variables:

    My birthday is 6/15, and Libby's is 1/28.
    

Last updated on June 26, 2025.