CODEWITHSUNDEEP
javarounding
Meow
Meow

Reputation: 19071

Java - How to round up float (or BigDecimal ) value by 0.5?

Seems simple question but I really suck at math and few examples online I've searched seems not working for me. (the result just return the same value as input etc)

For instance.. but its in C not Java Round to Next .05 in C

So my goal is I have %.1f format float or double or big decimal and wanting to round it up to nearest .5

example:

1.3 --> 1.5
5.5 --> 5.5
2.4 --> 2.5
3.6 --> 4.0
7.9 --> 8.0

I tried following example but didn't work :( below just output 1.3 which is original value. I wanted it to be 1.5

public class tmp {

    public static void main(String[] args) {

      double foo = 1.3;

      double mid = 20 * foo;

      System.out.println("mid " + mid);

      double out = Math.ceil(mid);

      System.out.println("out after ceil " + out);

      System.out.printf("%.1f\n", out/20.0);
    }
}

Upvotes: 7

Views: 25153

Answers (7)

Shane
Shane

Reputation: 4298

Some of the other answers round incorrectly (Math.round should be used, not Math.floor or Math.ceil), and others only work for rounding to 0.5 (which is what the question asked, yes). Here's a simple method that correctly rounds to the nearest arbitrary double, with a check to assure that it's a positive number.

public static double roundToNearest(double d, double toNearest) {
    if (toNearest <= 0) {
        throw new IllegalArgumentException(
                "toNearest must be positive, encountered " + toNearest);
    }
    return Math.round(d/toNearest) * toNearest;
}

Upvotes: -1

Shehan Simen
Shehan Simen

Reputation: 1306

The below formula does not work well for number like 2.16

public static float roundToHalf(float x) {
  return (float) (Math.ceil(x * 2) / 2);
}

The correct answer should be 2.0, but the above method gives 2.5

The correct code should be:

public static double round(float d)
{
    return 0.5 * Math.round(d * 2);
}

Upvotes: 5

Peter Lawrey
Peter Lawrey

Reputation: 533442

Without using a function, you can do

double rounded = (double)(long)(x * 2 + 0.5) / 2;

Note: this will round towards infinity.

Upvotes: 2

RealHowTo
RealHowTo

Reputation: 35372

See the Big Decimal Javadoc about why a String is used in the constructor

 public static double round(double d, int decimalPlace){
    BigDecimal bd = new BigDecimal(Double.toString(d));
    bd = bd.setScale(decimalPlace,BigDecimal.ROUND_HALF_UP);
    return bd.doubleValue();
 }

Upvotes: 4

Bert F
Bert F

Reputation: 87503

 double nearestPoint5 = Math.ceil(d * 2) / 2;

Upvotes: 5

Mike Daniels
Mike Daniels

Reputation: 8642

Here's a simple method:

public static float roundToHalf(float x) {
    return (float) (Math.ceil(x * 2) / 2);
}

This doubles the value, takes its ceiling, and cuts it back in half.

Upvotes: 20

Nick
Nick

Reputation: 572

Multiplying (and later dividing) by 2, not 20, should do the trick.

Upvotes: 9

Related Questions