5. Numeric Processing Examples

5.1. Example: Rounding to Two Decimal Places

As an example of how to use Math class methods, let's consider the problem of rounding numbers. When dealing with applications that involve monetary values – dollars and cents – it is often necessary to round a calculated result to two decimal places. For example, suppose a program computes the value of a certificate of deposit (CD) to be 75.19999. Before we output this result, we would want to round it to two decimal places – to 75.20. The following algorithm can be used to accomplish this:  Algorithm design

1. Multiply the number by 100, giving 7519.9999.
2. Add 0.5 to the number giving 7520.4999.
3. Drop the fractional part giving 7520
4. Divide the result by 100, giving 75.20 

Step 3 of this algorithm can be done using the Math.floor(R) method, which rounds its real argument, R, to the largest integer not less than R (from Table 5.11). If the number to be rounded is stored in the double variable R, then the following expression will round R to two decimal places:

R = Math.floor(R * 100.0 + 0.5) / 100.0;

Alternatively, we could use the Math.round() method (Table 5.11). This method rounds a floating-point value to the nearest integer. For example, Math.round(65.3333) rounds to 65 and Math.round(65.6666) rounds to 66. The following expression uses it to round to two decimal places:

R = Math.floor(R * 100.0 + 0.5) / 100.0;

Note that it is important here to divide by 100.0 and not by 100. Otherwise, the division will give an integer result and we'll lose the two decimal places.


JAVA DEBUGGING TIP

Division. Using the correct type of literal in division operations is necessary to ensure that you get the correct type of result.