# [java] What's the best practice to round a float to 2 decimals?

I'm using eclipse + Android SDK.

I need to round a float value to 2 decimals. I usually use the next "trick" using Math library.

``````float accelerometerX = accelerometerX * 100;
accelerometerX = round(accelerometerX);
Log.d("Test","" + accelerometerX/100);
``````

But I feel it is not the best way to do it.

Is there a library to do these type of operations?

This question is related to `java` `android` `math`

Let's test 3 methods:
1)

``````public static double round1(double value, int scale) {
return Math.round(value * Math.pow(10, scale)) / Math.pow(10, scale);
}
``````

2)

``````public static float round2(float number, int scale) {
int pow = 10;
for (int i = 1; i < scale; i++)
pow *= 10;
float tmp = number * pow;
return ( (float) ( (int) ((tmp - (int) tmp) >= 0.5f ? tmp + 1 : tmp) ) ) / pow;
}
``````

3)

``````public static float round3(float d, int decimalPlace) {
return BigDecimal.valueOf(d).setScale(decimalPlace, BigDecimal.ROUND_HALF_UP).floatValue();
}
``````

Number is 0.23453f
We'll test 100,000 iterations each method.

Results:
Time 1 - 18 ms
Time 2 - 1 ms
Time 3 - 378 ms

Tested on laptop
Intel i3-3310M CPU 2.4GHz

``````//by importing Decimal format we can do...

import java.util.Scanner;
import java.text.DecimalFormat;
public class Average
{
public static void main(String[] args)
{
int sub1,sub2,sub3,total;
Scanner in = new Scanner(System.in);
System.out.print("Enter Subject 1 Marks : ");
sub1 = in.nextInt();
System.out.print("Enter Subject 2 Marks : ");
sub2 = in.nextInt();
System.out.print("Enter Subject 3 Marks : ");
sub3 = in.nextInt();
total = sub1 + sub2 + sub3;
System.out.println("Total Marks of Subjects = " + total);
res = (float)total;
average = res/3;
System.out.println("Before Rounding Decimal.. Average = " +average +"%");
DecimalFormat df = new DecimalFormat("###.##");
System.out.println("After Rounding Decimal.. Average = " +df.format(average)+"%");
}
}
/* Output
Enter Subject 1 Marks : 72
Enter Subject 2 Marks : 42
Enter Subject 3 Marks : 52
Total Marks of Subjects = 166
Before Rounding Decimal.. Average = 55.333332%
After Rounding Decimal.. Average = 55.33%
*/

/* Output
Enter Subject 1 Marks : 98
Enter Subject 2 Marks : 88
Enter Subject 3 Marks : 78
Total Marks of Subjects = 264
Before Rounding Decimal.. Average = 88.0%
After Rounding Decimal.. Average = 88%
*/

/* You can Find Avrerage values in two ouputs before rounding average
And After rounding Average..*/
``````

Here is a shorter implementation comparing to @Jav_Rock's

``````   /**
* Round to certain number of decimals
*
* @param d
* @param decimalPlace the numbers of decimals
* @return
*/

public static float round(float d, int decimalPlace) {
return BigDecimal.valueOf(d).setScale(decimalPlace,BigDecimal.ROUND_HALF_UP).floatValue();
}

System.out.println(round(2.345f,2));//two decimal digits, //2.35
``````

``````double roundTwoDecimals(double d) {
DecimalFormat twoDForm = new DecimalFormat("#.##");
return Double.valueOf(twoDForm.format(d));
}
``````

Here is a simple one-line solution

``````((int) ((value + 0.005f) * 100)) / 100f
``````

I've tried to support the -ve values for @Ivan Stin excellent 2nd method. (Major credit goes to @Ivan Stin for his method)

``````public static float round(float value, int scale) {
int pow = 10;
for (int i = 1; i < scale; i++) {
pow *= 10;
}
float tmp = value * pow;
float tmpSub = tmp - (int) tmp;

return ( (float) ( (int) (
value >= 0
? (tmpSub >= 0.5f ? tmp + 1 : tmp)
: (tmpSub >= -0.5f ? tmp : tmp - 1)
) ) ) / pow;

// Below will only handles +ve values
// return ( (float) ( (int) ((tmp - (int) tmp) >= 0.5f ? tmp + 1 : tmp) ) ) / pow;
}
``````

Below are the tests cases I've tried. Please let me know if this is not addressing any other cases.

``````@Test
public void testFloatRound() {
// +ve values
Assert.assertEquals(0F, NumberUtils.round(0F), 0);
Assert.assertEquals(1F, NumberUtils.round(1F), 0);
Assert.assertEquals(23.46F, NumberUtils.round(23.4567F), 0);
Assert.assertEquals(23.45F, NumberUtils.round(23.4547F), 0D);
Assert.assertEquals(1.00F, NumberUtils.round(0.49999999999999994F + 0.5F), 0);
Assert.assertEquals(123.12F, NumberUtils.round(123.123F), 0);
Assert.assertEquals(0.12F, NumberUtils.round(0.123F), 0);
Assert.assertEquals(0.55F, NumberUtils.round(0.55F), 0);
Assert.assertEquals(0.55F, NumberUtils.round(0.554F), 0);
Assert.assertEquals(0.56F, NumberUtils.round(0.556F), 0);
Assert.assertEquals(123.13F, NumberUtils.round(123.126F), 0);
Assert.assertEquals(123.15F, NumberUtils.round(123.15F), 0);
Assert.assertEquals(123.17F, NumberUtils.round(123.1666F), 0);
Assert.assertEquals(123.46F, NumberUtils.round(123.4567F), 0);
Assert.assertEquals(123.87F, NumberUtils.round(123.8711F), 0);
Assert.assertEquals(123.15F, NumberUtils.round(123.15123F), 0);
Assert.assertEquals(123.89F, NumberUtils.round(123.8909F), 0);
Assert.assertEquals(124.00F, NumberUtils.round(123.9999F), 0);
Assert.assertEquals(123.70F, NumberUtils.round(123.7F), 0);
Assert.assertEquals(123.56F, NumberUtils.round(123.555F), 0);
Assert.assertEquals(123.00F, NumberUtils.round(123.00F), 0);
Assert.assertEquals(123.50F, NumberUtils.round(123.50F), 0);
Assert.assertEquals(123.93F, NumberUtils.round(123.93F), 0);
Assert.assertEquals(123.93F, NumberUtils.round(123.9312F), 0);
Assert.assertEquals(123.94F, NumberUtils.round(123.9351F), 0);
Assert.assertEquals(123.94F, NumberUtils.round(123.9350F), 0);
Assert.assertEquals(123.94F, NumberUtils.round(123.93501F), 0);
Assert.assertEquals(99.99F, NumberUtils.round(99.99F), 0);
Assert.assertEquals(100.00F, NumberUtils.round(99.999F), 0);
Assert.assertEquals(100.00F, NumberUtils.round(99.9999F), 0);

// -ve values
Assert.assertEquals(-123.94F, NumberUtils.round(-123.93501F), 0);
Assert.assertEquals(-123.00F, NumberUtils.round(-123.001F), 0);
Assert.assertEquals(-0.94F, NumberUtils.round(-0.93501F), 0);
Assert.assertEquals(-1F, NumberUtils.round(-1F), 0);
Assert.assertEquals(-0.50F, NumberUtils.round(-0.50F), 0);
Assert.assertEquals(-0.55F, NumberUtils.round(-0.55F), 0);
Assert.assertEquals(-0.55F, NumberUtils.round(-0.554F), 0);
Assert.assertEquals(-0.56F, NumberUtils.round(-0.556F), 0);
Assert.assertEquals(-0.12F, NumberUtils.round(-0.1234F), 0);
Assert.assertEquals(-0.12F, NumberUtils.round(-0.123456789F), 0);
Assert.assertEquals(-0.13F, NumberUtils.round(-0.129F), 0);
Assert.assertEquals(-99.99F, NumberUtils.round(-99.99F), 0);
Assert.assertEquals(-100.00F, NumberUtils.round(-99.999F), 0);
Assert.assertEquals(-100.00F, NumberUtils.round(-99.9999F), 0);
}
``````