Math.round and String.format functions does not round up correctly for all float values

Question

I tried Math.round and String.format function to round float/double values.

1) Using Math.round function

Math.round(floatValue* 10.0) / 10.0;

For floatValue 40.55, result is 40.6 //Correct & Expected result.

For floatValue 30.05, result is 30.0 //Incorrect result. Expected result: 30.1

2) Using String.format function

String.format("%.1f", floatValue);

For floatValue 40.55 result is 40.6 //Correct & Expected result.

For floatValue 59.65 result is 59.6 //Incorrect result. Expected result: 59.7

Why same function is behaving differently for different float values.

@ΦXocę웃Пepeúpaツ I would expect 30.1 though, there is an inconsistency if both methods are sometimes rounding up and sometimes rounding down — Michael Dodd, Feb 05 '19 at 08:02
Sorry for the wrong value. Expected value is 30.1.(Wronly typed while preparing quesion) — Tejas Shelke, Feb 05 '19 at 08:04
@Sweeper 40.55 -> 40.6 is a round up. 30.05 -> 30.0 is a round down. — Michael Dodd, Feb 05 '19 at 08:06
You can't use `Math.round()` to round floats to *N* decimal places. Explanation here: https://stackoverflow.com/a/12684082/207421. — user207421, Feb 05 '19 at 08:07
For floatValue 40.55, result is 40.6 //Rounding up. For floatValue 30.05, result is 30.0 //Not rounding. — Tejas Shelke, Feb 05 '19 at 08:07

Scary Wombat · Answer 1 · 2019-02-05T23:43:02.680

0

In can be visualized with

    float fv = 30.05f;
    System.out.println((fv));
    System.out.println((fv * 10.0));
    System.out.println(Math.round(fv * 10.0));
    System.out.println(Math.round(fv * 10.0) / 10.0);

result

30.05
300.49999237060547
300
30.0

As you can see 30.05 is actually stored as 30.49999237060547 so that is why it appears that it is rounding down. See the more concise explaination by @EricPostpischi in the comments.

See https://rules.sonarsource.com/java/tag/misra/RSPEC-1244

and

What's wrong with using == to compare floats in Java?

edited Feb 05 '19 at 23:43

answered Feb 05 '19 at 08:07

Scary Wombat

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Yeah, I also found that [30.05f * 10.0 = 300.49999237060547]. That's the issue. I made it to work by changing code Math.round(this * 10.0f) / 10.0f. – Tejas Shelke Feb 05 '19 at 09:39
30.05 is not stored as 300.49999237060547. First, that is off by a factor of 10, Second, it is not correct to say that one number is stored as another number. If somebody assigned −3 to an `unsigned char`, so that 253 were in the `unsigned char`, we would not say that −3 is “stored as” 253. We recognize that −3 is converted to `unsigned char`, and the result is a different number, 253. Similarly, when `30.05` is read from source code, it is converted to 30.049999237060546875, and then 30.049999237060546875 is stored in `fv`. It is a different number and is **not** a representation of 30.05. – Eric Postpischil Feb 05 '19 at 15:09
@EricPostpischil Thank you for posting out my typo and also for explaining the problem in a lot better style than I did. – Scary Wombat Feb 05 '19 at 23:44

Math.round and String.format functions does not round up correctly for all float values

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