Document and/or allow truncation/rounding when casting from float to int? about num-traits HOT 4 CLOSED

If it is a float that it is being cast to, I don't want to do any rounding.

This isn't generally possible to guarantee either. f64 to f32 definitely has to round, but even integer to float will round if the value is greater than the mantissa bits can represent.

In any case, it's not possible to control rounding with the current API. A new rounding API would need to be proposed.

from num-traits.

Doc improvements are welcome in PRs.

it may be worth letting the caller dictate the rounding behavior,

That sounds like a whole new API! Maybe you could implement your own wrapper with different floating point behavior, and I found an example of that in a different crate with az::Round. Another option I found is numeric_cast with multiple traits available.

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I'm currently working with num_traits inside of a crate I don't own (see linked PR), and it is using ToPrimitive in order to cast between f32 to either an integer or a float (specified only as an impl Primitive). If it is a float that it is being cast to, I don't want to do any rounding. While the library could change to use a different trait for their numeric types, it's a bit invasive of a change, and it feels natural that conversion between primitives should be able to control their rounding mode.

For the specific issue, there's workarounds, but I think it was worth asking here since it seems to be the most natural place to change it. If you don't think it should be added though, that's fine

from num-traits.

ah yes, currently I'm doing f32 -> Generic which would not require rounding.

Sounds good. I don't think I'll propose such an API myself, so for now I'll just close this.

from num-traits.