Building expressions using variables

We can build expressions using variables just as we did with numeric values. Remember that we have both arithmetic and relational operators to work with. Here’s a code segment with two variables of type double (Note: The class and main method definitions are missing. You would need to add those to run this example):

double x = 2.3;

double y = 3.2;

System.out.println(x + y);

System.out.println(x != y);

System.out.println(2 * x < y);

The output of the code segment above would be (try to reason it out first):

5.5

true

false

Our next example will compute the volume of a cone given the radius of it base and its height and then display the result. A simple Web search will reveal that the formula for the volume is: Ok great. Now what? When we’re faced with such a task it’s often important to first identify what variables we’re going to need. Luckily we can look for the formula to provide us with a hint – a big hint. Each of the variables in the formula corresponds to a variable in our program! Since the volume must be a real number, we declare all the variables to be of type double. Here’s the full program:

public class ComputeConeVolume

{

public static void main(String[] args)

{

double pi = 3.141592654;

double radius = 10.0;

double height = 5.0;

double volume;

volume = 1.0/3.0 * pi * radius * radius * height;

// don’t worry about the next statement wrapping around

System.out.println(“The volume of a cone with radius “ + radius + “ and height “ + height + “ is “ + volume);

}

}

The first four lines in main are the declarations, followed by the expression to compute the volume, and finally a statement to display the result. You can see that the program computes the volume of a particular cone, one with a radius of 10 and a height of 5. If you were interested in a cone with different dimensions, you would need to modify the initial values and the recompile and run the program. This seems to be a big limitation, but fortunately we’ll be able to overcome this in the next unit when we look at how to obtain input from the user.