Rust's f32 formatting seems to follow inconsistent rounding rules when rounding to a fixed output precision.
fn main() {
for i in 0..=10 {
let nf = (i as f32) + 0.5f32;
println!("{:4} -> {:2.0} ; {:5} -> {:3.0}", nf, nf, -nf, -nf);
}
}Output (Playground):
0.5 -> 1 ; -0.5 -> -1
1.5 -> 2 ; -1.5 -> -2
2.5 -> 2 ; -2.5 -> -2
3.5 -> 4 ; -3.5 -> -4
4.5 -> 4 ; -4.5 -> -4
5.5 -> 6 ; -5.5 -> -6
6.5 -> 6 ; -6.5 -> -6
7.5 -> 8 ; -7.5 -> -8
8.5 -> 8 ; -8.5 -> -8
9.5 -> 10 ; -9.5 -> -10
10.5 -> 10 ; -10.5 -> -10
I get this output on Linux x86-64 with both rustc 1.42.0 (b8cedc0 2020-03-09) and rustc 1.44.0-nightly (f509b26 2020-03-18). I have not tested on other systems, and have not yet tested with f64.
Most inputs are rounded following the rule of rounding half to the nearest integer, but 0.5 and -0.5 are rounded away from zero instead.
When rounding 0.5 to an integer both 0 or 1 are correct outputs, so this may not be a bug. However it seems strange that the round-half-to-even rule should be followed for 1.5, 2.5, and up while a different rule is followed for 0.5.
By comparison, my Python and C versions followed round-half-to-even consistently (rounding 0.5 to 0), though I don't know that they provide any guarantee to follow that rule.
Cross-reference: I originally posted about this on the user forum. Rounding mode for fixed precision float Display format?
Rust's f32 formatting seems to follow inconsistent rounding rules when rounding to a fixed output precision.
Output (Playground):
I get this output on Linux x86-64 with both rustc 1.42.0 (b8cedc0 2020-03-09) and rustc 1.44.0-nightly (f509b26 2020-03-18). I have not tested on other systems, and have not yet tested with f64.
Most inputs are rounded following the rule of rounding half to the nearest integer, but 0.5 and -0.5 are rounded away from zero instead.
When rounding 0.5 to an integer both 0 or 1 are correct outputs, so this may not be a bug. However it seems strange that the round-half-to-even rule should be followed for 1.5, 2.5, and up while a different rule is followed for 0.5.
By comparison, my Python and C versions followed round-half-to-even consistently (rounding 0.5 to 0), though I don't know that they provide any guarantee to follow that rule.
Cross-reference: I originally posted about this on the user forum. Rounding mode for fixed precision float Display format?