Hey there, why does `0.42.rem(0.14)` return `0.13...

# mathematics

[nino]

Hey there, why does 0.42.rem(0.14) return 0.13999999999999996 and how can I fix it?

[nino]

Intellij even suggests me to change the expression 0.42.rem(0.14) == 0.0 to false

[nino]

but
0.28.rem(0.14) == 0.0 is true

[andylamax]

floating point arithmetics 😔

[altavir]

It is called https://en.wikipedia.org/wiki/IEEE_754. The only way to avoid it is to use BigDecimal.

[nino]

BigDecimal(0.42).rem(BigDecimal(0.14)) doesn't solve the problem

[altavir]

Because it uses limited MathContext by default. It could be done, but I need to remember how. In general, you can't check equality of floating point numbers, the correct way is to do it like this:

require( abs(0.42.rem(0.14)) < 1e-4))

You can add an extension to do that

[altavir]

https://pl.kotl.in/qOAIWX6XZ here is how it looks with BigDecimal. But it is not recommended.

[nino]

both solutions work. Thanks!

[nino]

I still don't understand why 0.28.rem(0.14) == 0.0 is true though

[nino]

and also 0.56.rem(0.14) == 0.0

[altavir]

I think it is the problem of the latest bit flip. The problem is that due to floating precision 0.42 is slightly smaller than 3*0.14 so you get almost 0.14 as a reminder.

[altavir]

println(0.42 >= 3*0.14)

[altavir]

it is false

[nino]

thanks @altavir

[Andrew]

BigDecimal works but you need to use the string or .valueOf constructor to avoid any floating point issues

BigDecimal.valueOf(0.42).rem(BigDecimal.valueOf(0.14))