Sunday, June 16, 2013

How would you round a double value to certain decimal Precision and Scale ?

How would you round a double value to certain decimal Precision and Scale ?

Firstly let us understand the difference between Precision and Scale. If the number is 9232.129394, then
Precision
represents the number of number of significant digits to which a number is calculated i.e. 4 digits (9232)
Scale
represents the number of digits to the right of the decimal point i.e. 6 in above case (.129394)
Some other examples are,
Precision 4, scale 2: 99.99
Precision 10, scale 0: 9999999999
Precision 8, scale 3: 99999.999
Precision 5, scale -3: 99999000
No one wants to loose the precision of the number as it will change the value by large amount. If you still want to loose the precision simply divide the number by 10 to the power precision.
There are multiple ways in Java to round the double value to certain scale, as mentioned in the below example
BigDecimal rounding in Java
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import java.math.BigDecimal;
import java.math.RoundingMode;
import java.text.DecimalFormat;
public class RoundDouble {
    public double round1(double input, int scale) {
        BigDecimal bigDecimal = new BigDecimal(input).setScale(scale, RoundingMode.HALF_EVEN);
        return bigDecimal.doubleValue();
    }
    public double round2(double input) {
        return Math.round(input * 100) / 100.0d;
    }
    public double round3(double input) {
        DecimalFormat df = new DecimalFormat("#.00");
        return Double.parseDouble(df.format(input));
    }
    public static void main(String[] args) {
        RoundDouble rd = new RoundDouble();
        System.out.println(rd.round1(9232.129394d, 2));
        System.out.println(rd.round2(9232.129394d));
        System.out.println(rd.round3(9232.129394d));
    }
}
The first method of rounding using BigDecimal should be preferred in most scenarios.
RoundingMode.HALF_EVEN
Rounding mode to round towards the "nearest neighbor" unless both neighbors are equidistant, in which case, round towards the even neighbor. Behaves as for RoundingMode.HALF_UP if the digit to the left of the discarded fraction is odd; behaves as for RoundingMode.HALF_DOWN if it’s even. Note that this is the rounding mode that statistically minimizes cumulative error when applied repeatedly over a sequence of calculations. It is sometimes known as "Banker’s rounding," and is chiefly used in the USA. This rounding mode is analogous to the rounding policy used for float and double arithmetic in Java.

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