How to increase a float to the next multiple of 0.5

Question

I would like to increase my float value to the first multiple of 0.5 after it.

For example, if I have: float x = 3.397; I would like it to become: x = 3.5;. Or, if I have: float x = 7.895; it should become x = 8.0;.

Yes, your requirement is clear. What is not clear is what problems you have or what question you want to ask. — Seelenvirtuose, Oct 21 '20 at 15:38
@Seelenvirtuose I do not know how to do what I would like to be done — ThatsSamu, Oct 21 '20 at 15:41
What should be the result of 0.5? of large values in the range where xxxx.5 is not representable? — chux - Reinstate Monica, Oct 21 '20 at 15:58
lots of duplicates: [round BigDecimal to nearest 5 cents](https://stackoverflow.com/q/2106615/995714), [How can I round up to the nearest multiple of the specified number?](https://stackoverflow.com/q/38116385/995714), [Rounding up a number to nearest multiple of 5](https://stackoverflow.com/q/9303604/995714), [Rounding numbers to specific multiples](https://stackoverflow.com/q/10456564/995714)... — phuclv, Oct 22 '20 at 16:07

Joop Eggen · Accepted Answer · 2020-10-22T12:37:48.520

1

Quite simple, but in every detail errors are hidden.

float nextHalf(float x) {
    // CORRECTED THIS: long n = Math.round(x * 2.0) + 1;
    return (float) (Math.ceil(x * 2.0) / 2);
}

With corrections due to @chux-ReinstateMonica.

As rounding methods round/floor/ceil target integer units of 1, for 0.5 one must first scale the numbers by a factor 2 and afterwards divide by 2.

Of course because of the incorporated exponent in a floating point number at some large numbers the difference between successive numbers is no longer below 1.0 so then

nextHalf(x) == nextHalf(x + 1.0)

edited Oct 22 '20 at 12:37

answered Oct 21 '20 at 15:58

Joop Eggen

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@chux-ReinstateMonica my fault half heartedly reading the question. Corrected as no other answer – Joop Eggen Oct 22 '20 at 07:14
With `ceil()`, which returns `double`, `long n` is unnecessarily restrictive. Perhaps `double n` and avoid the "float might overflow long" problem? – chux - Reinstate Monica Oct 22 '20 at 10:28

How to increase a float to the next multiple of 0.5

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