Prelab Exercises

Prelab Exercises
Sections 2.3 - 2.4

What is the difference between a variable and a constant?
A variable is an identifier that refers to a memory location used to hold data values that can change as the program executes. A constant, on the other hand, is an identifier that holds a data value that cannot change.
Explain what each of the lines below does. Be sure to indicate how each is different from the others.
1. int x;
  Declares a variable named x that will hold an integer value (type int). Memory is allocated for the variable but the value is undefined.
2. int x = 3;
  Declares a variable named x that will hold a value of type int and stores the initial value of 3 in the variable.
3. x = 3;
  Stores the value of 3 in the variable x.

The following program reads three integers and prints the average. Fill in the blanks so that it will work correctly.


// *******************************************************************
//   Average.java
//
//   Read three integers from the user and print their average
// *******************************************************************

import java.util.Scanner;

public class Average
{
  public static void main(String[] args)
  {
    int val1, val2, val3;
    double average;
    Scanner scan = new Scanner(System.in);

    // get three values from user
    System.out.println("Please enter three integers and " +
                       "I will compute their average");

    val1 = scan.nextInt();

    val2 = scan.nextInt();

    val3 = scan.nextInt();

    //compute the average

    average = (double)(val1 + val2 + val3)/3;

    //print the average

    System.out.println ("The average is " + average);
  }
}

Given the declarations below, find the result of each expression.
```
int a = 3, b = 10, c = 7;
double w = 12.9, y = 3.2;
```
1. a + b * c = 73
2. a - b - c = -14
3. a / b = 0
4. b / a = 3
5. a - b / c = 2
6. w / y = 4.03125
7. y / w = 0.248062...
8. a + w / b = 4.29
9. a % b / y = 3 / 3.2 = 0.9375
10. b % a = 1
11. w % y = 0.1

Carefully study the following program and find and correct all of the syntax errors.

// File:     Errors.java
// Purpose:  A program with lots of syntax errors
//           Correct all of the errors (STUDY the program carefully!!)

import java.util.Scanner;    // No # symbol in an import statement

// Class name must match file name including case and the convention is
// to capitalize class names.  Hence the class name should be Errors not
// errors.
public class Errors
{
   public static void main (String[] args)
   {
       String Name;  // Name of the user -- two slashes for a comment
       int number;
       int numSq;
       Scanner scan = new Scanner(System.in);

       System.out.print ("Enter your name, please: "); // missing semicolon
       Name = scan.next();   // Name is a String not an int

       System.out.print ("What is your favorite number? "); //Missing "
       number = scan.nextInt();

       numSq = number * number;

       System.out.println (Name + ", the square of your number is " +
                  numSq);    // Missing the concatenation operator +
                             // and wrong variable name
   } // Missing a closing brace
}

Trace the execution of the following program assuming the input stream contains the numbers 10, 3, and 14.3. Use a table that shows the value of each variable at each step. Also show the output (exactly as it would be printed).


    // FILE:  Trace.java
    // PURPOSE:  An exercise in tracing a program and understanding
    //           assignment statements and expressions.

    import java.util.Scanner;
    
    public class Trace
    {
       public static void main (String[] args)
       {
          int one, two, three;
          double what;
	  Scanner scan = new Scanner(System.in);

          System.out.print ("Enter two integers: ");
          one = scan.nextInt();
          two = scan.nextInt();
      
          System.out.print("Enter a floating point number: ");
          what = scan.nextDouble();

          three = 4 * one + 5 * two;
          two = 2 * one;
          System.out.println ("one " + two + ":" + three);

          one = 46 / 5 * 2 + 19 % 4;
          three = one + two;
          what = (what + 2.5) / 2;
          System.out.println (what + " is what!");
       }
    }

         Output                                 Variables
                                         one    two   three    what
      ---------------------------------------------------------------
      Enter two integers:	         10      3
      Enter a floating point number:                           14.3
                                                        55
                                                20
      one 20:55

                                         21
                                                        41
                                                               8.4
      8.4 is what!

Prelab Exercises Sections 2.3 - 2.4

Prelab Exercises
Sections 2.3 - 2.4