Question

I am writing a function round:
static float round(float number, precision){}

The function should work like this: round(12.3456f, 3) = 12.345

My definition of function is like this:

public static float round(float value, int precision) {
float result;
if(precision <= 0){
    throw new RuntimeException("Precision can not be zero or less");
}

int number = (int) power(10,precision);
value = value * number;
result = (float)Math.round(value)/number;

return result;
} 

But the issue is that, my unit test case for this function doesn’t pass,

 public void mathTestNew() {
    assertEquals("MathTest",12.341,OOTBFunctions.round(12.3416f,3));
 }

The result is
junit.framework.AssertionFailedError: MathTest expected:<12.341> but was:<12.342>

I am not sure how to overcome this error. I am not sure if BigDecimal will help me in this.

Answer 1

If you did want to use BigDecimal:

public static float round(float value, int precision) {
    if (precision <= 0) {
        throw new IllegalArgumentException("Precision cannot be zero or less.");
    }
    BigDecimal decimal = BigDecimal.valueOf(value);
    return decimal.setScale(precision, RoundingMode.FLOOR).floatValue();
}

You may lose accuracy when converting from BigDecimal to float, so if accuracy is a must, do not convert; keep the value as a BigDecimal.

As mentioned in other answers, float is an approximation of a base 10 number. The following demonstrates just that:

System.out.println(BigDecimal.valueOf(12.3416f)); // outputs 12.34160041809082
System.out.println(new BigDecimal("12.3416"));    // outputs 12.3416