Java Modulo creating wrong results (with examples)?

Asked Mar 02 '18 at 14:59

Active Mar 02 '18 at 15:22

Viewed 35 times

i try to calculate whether the single dosages of a therapy are in accordance with the total dosage.

I created a little java tool.

For this i included a small check:

System.out.println(total);
System.out.println(single);
System.out.println(total % single);
if(total % single == 0.0) {
System.out.println("test");
}

Now. Here are a coule of results for some of my data points. And i just dont understand it:

Two examples where it works as intended.

Total: 60
Single: 7.5
Result: 0.0

Total: 30.0
Single: 3.0
Result: 0.0

Modulo gives out wrong results? Shouldnt it also be 0.0?

Total: 37.0
Single: 3.7
Result: 3.6999999999999984

Any explanations for this?

edited Mar 02 '18 at 15:22

asked Mar 02 '18 at 14:59

Donselm

Are gesamt and einzel supposed to be total and single? – Carcigenicate Mar 02 '18 at 15:00
And `40 % 13.5` *is* 13. What's the problem? – Carcigenicate Mar 02 '18 at 15:01
`40` is `(2 * 13.5) + 13` , you have the expected result of modulo . – Arnaud Mar 02 '18 at 15:01
1

The last result is likely due to double precision – phflack Mar 02 '18 at 15:02
Ahh ye exactly, i translated it into english. Ok and your right about the 40%13.5. Those are exactly the cases i want to find. But the last case with 37.0 and 3.7. Shouldnt it be 0.0? What is this double precision your talking about? Isnt modulo wrong in this case? – Donselm Mar 02 '18 at 15:21
Best to check if it's within a small value of the goal, so in this case you could check `37.0 % 3.7 < 0.001 || 37.0 % 3.7 > 3.699` – phflack Mar 02 '18 at 15:27
@Carcigenicate On java 1.8.0_161, `System.out.println(37.0 % 3.7);` prints `3.6999999999999984` – phflack Mar 02 '18 at 15:32
@phflack The result should be `0`, so it's off by `3.6999999999999984`. – 001 Mar 02 '18 at 15:35
@JohnnyMopp But in the realm of mod, it's actually off by `0.0000000000000016` – phflack Mar 02 '18 at 15:35

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