If Lab

Lab Objectives

To gain experience in

• implementing simple decisions
• writing expressions with relational operators
• implementing repeated and nested branches
• using Boolean variables
• using Boolean operations

1. The if statement

   In Java, decisions to do or not to do something can be made by using the statement if(test expression). The test expression must be an expression that can be evaluated as either true or false. If it evaluates to true, a succeeding statement or group of statements enclosed in { . . . }, called a block statement will be executed.

   How many * will be printed when the following code is executed when n has a value of 7?

      if(n > 10)
         System.out.print("*****");
      if(n > 7)
         System.out.print("****");
      if(n > 4)
         System.out.print("***");
      if(n > 1)
         System.out.print("**");
   
      System.out.println("*");
   

   Hint: work out which boolean conditions are true first.

2. Relations and Relational Operators

   The relational operators in Java are == != < > <= and >=

   Formulate the following conditions in Java:

   x is positive

   x is zero or negative

   x is at least 10

   x is less than 10

   x and y are both zero

   x is even (hint)

3. Checking Input

   The following program reads in two points, (x₁, y₁), and (x₂, y₂) and calculates the slope of a line formed between the two points. Create and run the program. Describe what happens when the two points have the same x coordinate?

   public class Slope
   {
      public static void main(String [] args)
      {
         System.out.println("Input x coordinate of the first point:");
         double x1 = Console.readDouble();
   
         System.out.println("Input y coordinate of the first point:");
         double y1 = Console.readDouble();
   
         System.out.println("Input x coordinate of the second point:");
         double x2 = Console.readDouble();
   
         System.out.println("Input y coordinate of the second point:");
         double y2 = Console.readDouble();
   
         double slope = (y2 - y1) / (x2 - x1);
   
         System.out.println("The slope of the line between Points 1 and 2 is " + slope);
      }
   }
   

   What would happen if the coordinates were integers?

   Modify the program to dissallow a vertical line (that is, denominator = 0 or x1 having the same value as x2).

   What are the results when the both points have the same value, e.g. (4, 2)?

   What are the results when the first point is (4, 2.5) and the second point (3, 1.5) ?

4. Isosceles?

   Write a program that reads in three integers, a, b, and c, representing the lengths of the sides of a triangle and say whether the resulting triangle is isosceles or not.

   Test your program with the following values of a, b and c.

   • 3, 4, 5
   • 5, 5, 2
   • 10, 6, 6
   • 7, 7, 7

5. Triangle type

   Write a program that reads in three integers, a, b, and c, representing the lengths of the sides of a triangle. The program should then say whether the resulting triangle is isosceles, equilateral, ordinary or impossible (i.e. the three sides cannot form any triangle.)

   Test your program with the following values of a, b and c.

   • 3, 4, 5
   • 5, 5, 2
   • 7, 7, 7
   • 7, 1, 3
   • 4, 3, 3
   • 2, 2, 8

   In each case, sketch the triangle, establish what type it is and see if your program classifies it properly.

   Here is a hint.

6. Logical Operations

   Java has three logical operators, &&, || and !.

   Using these operations, express the following:

   • x and y are both positive
   • neither x nor y is positive.
   • x and y are both positive or neither of them is positive.

   Formulate the following conditions on the date that is given by the variables day and month

   • The date is in the second quarter of the year.
   • The date is the last day of the month. (Assume February always has 28 days).
   • The date is before April 15.

   Assume that day is an int between 1 and 31 inclusive and that month is an int between 1 and 12 inclusive.

7. Nested Branches (Optional)

   A) [Preamble] Extend the following code to test whether two circles - each having a fixed center point and a user-defined radius - are touching.

   public class CircleOverlap
   {
      public static void main(String [] args)
      {
         System.out.println("Radius 1:");
         double radius1 = Console.readDouble();
         double xcenter1 = 0;
         double ycenter1 = 0;
         System.out.println("Radius 2:");
         double radius2 = Console.readDouble();
         double xcenter2 = 40;
         double ycenter2 = 0;
   
         //   Your work goes here
      }
   }
   

   Here is a hint.

   B)

   If there are multiple conditions, it can happen that a conditionally executed block contains further decisions.

   Extend the code from the previous section to check if the circles are disjoint, overlapping or mutually contained.

   And here is a hint.