Using Math.round to round to one decimal place?

Question

I have these two variables

double num = 540.512
double sum = 1978.8

Then I did this expression

double total = Math.round((num/ sum * 100) * 10) / 10;

but I end up with 27.0.

In fact I have many other variables and when I use them in the expression I always get a 0 in the tenth's place.

Possible duplicate of [How to round a number to n decimal places in Java](http://stackoverflow.com/questions/153724/how-to-round-a-number-to-n-decimal-places-in-java) — José, Sep 24 '16 at 04:53

score 113 · Answer 1 · answered Mar 05 '14 at 01:43

113

Helpful method I created a while ago...

private static double round (double value, int precision) {
    int scale = (int) Math.pow(10, precision);
    return (double) Math.round(value * scale) / scale;
}

answered Mar 05 '14 at 01:43

jpdymond

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Works for me! double pi = 3.14159265; round(pi, 1) --> 3.1 round(pi, 4) --> 3.1416 – MAbraham1 Jul 27 '17 at 13:24
1

@LeoDroidcoder Actually, the expected result for `round(16.46, 1);` is `16.5` – NBTX Dec 24 '17 at 06:42

score 29 · Answer 2 · answered Mar 05 '14 at 04:00

29

try this

for example

DecimalFormat df = new DecimalFormat("#.##");
df.format(55.544545);

output:

55.54

answered Mar 05 '14 at 04:00

user3381763

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This works, but it is much slower than using Math.round(double a) – jpdymond Mar 05 '14 at 12:52
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This won't round the number. It just truncates to two decimals. – dguay Oct 16 '17 at 12:41
4

This answer should be removed, as it does not address the actual question. This solution does not round. – basZero Aug 02 '18 at 14:00

score 26 · Answer 3 · edited Jun 01 '20 at 09:56

26

Double toBeTruncated = new Double("2.25");

Double truncatedDouble = new BigDecimal(toBeTruncated).setScale(1, BigDecimal.ROUND_HALF_UP).doubleValue();

it will return 2.3

edited Jun 01 '20 at 09:56

Minas Mina

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answered Feb 04 '15 at 10:19

Sankar

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BTW, you probably don't want a variable name with `truncated` in it, since that has a different connotation than `round`. How about `toBeRounded` and `roundedDouble`? I mention this, because my first thought in seeing your answer was "Why is he truncating, when rounding was asked for?" – ToolmakerSteve Aug 28 '15 at 01:20

score 24 · Answer 4 · answered Mar 05 '14 at 01:38

24

The Math.round method returns a long (or an int if you pass in a float), and Java's integer division is the culprit. Cast it back to a double, or use a double literal when dividing by 10. Either:

double total = (double) Math.round((num / sum * 100) * 10) / 10;

or

double total = Math.round((num / sum * 100) * 10) / 10.0;

Then you should get

27.3

answered Mar 05 '14 at 01:38

rgettman

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1+ Another option is not to do any rounding at all, but instead only round the String display of the number. – Hovercraft Full Of Eels Mar 05 '14 at 01:42
@HovercraftFullOfEels That's not rounding then, just cutting off precision no matter of the tendency. – Willi Mentzel Oct 13 '14 at 05:30
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@haywire: Actually, using methods like System.out.printf() or String.format() can take care of the rounding for you. It doesn't just cut off precision, see my answer. – Micah Stairs Aug 18 '15 at 14:07
@MicahStairs sure they can, but that does not seem what HovercraftFullOfEels meant – Willi Mentzel Aug 18 '15 at 15:43
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@haywire, sounds to me like exactly what was meant. What other interpretation is there of *round* the string display? He didn't say *truncate*, did he? – ToolmakerSteve Aug 28 '15 at 01:18
@haywire yap, my bad :) misunderstood it – Willi Mentzel Aug 28 '15 at 08:48
Does `sum` refer to precision in your answer? – Shimmy Weitzhandler Nov 13 '18 at 06:32

Mifeet · Answer 5 · 2016-08-15T08:14:12.617

If you need this and similar operations more often, it may be more convenient to find the right library instead of implementing it yourself.

Here are one-liners solving your question from Apache Commons Math using Precision, Colt using Functions, and Weka using Utils:

double value = 540.512 / 1978.8 * 100;
// Apache commons math
double rounded1 = Precision.round(value, 1);
double rounded2 = Precision.round(value, 1, BigDecimal.ROUND_HALF_UP);
// Colt
double rounded3 = Functions.round(0.1).apply(value)
// Weka
double rounded4 = Utils.roundDouble(value, 1)

Maven dependencies:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-math3</artifactId>
    <version>3.5</version>
</dependency>

<dependency>
    <groupId>colt</groupId>
    <artifactId>colt</artifactId>
    <version>1.2.0</version>
</dependency>

<dependency>
    <groupId>nz.ac.waikato.cms.weka</groupId>
    <artifactId>weka-stable</artifactId>
    <version>3.6.12</version>
</dependency>

score 7 · Answer 6 · edited Mar 24 '21 at 22:41

7

Double number = new Double("5.25");
Double tDouble = 
  new BigDecimal(number).setScale(1, BigDecimal.ROUND_HALF_UP).doubleValue();

this will return 5.3

edited Mar 24 '21 at 22:41

Youri

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answered Feb 15 '18 at 10:13

sankar ganesh

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`BigDecimal.ROUND_HALF_UP` is deprecated from Java 9+. Use `RoundingMode.HALF_UP` in preference. – Ashley Frieze Mar 10 '22 at 12:52

score 5 · Answer 7 · answered Aug 18 '15 at 14:11

5

A neat alternative that is much more readable in my opinion, however, arguably a tad less efficient due to the conversions between double and String:

double num = 540.512;
double sum = 1978.8;

// NOTE: This does take care of rounding
String str = String.format("%.1f", (num/sum) * 100.0);

If you want the answer as a double, you could of course convert it back:

double ans = Double.parseDouble(str);

answered Aug 18 '15 at 14:11

Micah Stairs

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Conversion to and from String is much more than `a tad` less efficient. If the question really was about `display to one decimal digit` (and it might be, not clear), then you've got a great answer. But if that is not the desire, this isn't a good path. Stick to doing numeric operations on numbers, unless the desired result is a string. – ToolmakerSteve Aug 28 '15 at 01:24

Dan Alboteanu · Answer 8 · 2018-02-15T19:53:09.130

5

DecimalFormat decimalFormat = new DecimalFormat(".#");
String result = decimalFormat.format(12.763);                // -->  12.7

edited Feb 15 '18 at 19:53

answered Jan 10 '18 at 09:17

Dan Alboteanu

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Since the question is about rounding, shouldn't this show 12.8? – Bill K Apr 04 '22 at 22:41

score 4 · Answer 9 · 2017-12-31T17:09:06.093

4

Your method is right, all you have to do is add a .0 after both the tens and it will fix your problem!

double example = Math.round((187/35) * 10.0) / 10.0;

The output would be:

5.3

edited Dec 31 '17 at 17:09

answered Dec 29 '17 at 22:00

I put the example up – Dec 31 '17 at 17:09

Using Math.round to round to one decimal place?

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