I've got `-inf` when calculating on rust: what is `-inf`?

what is -inf?

It's negative infinity as defined in the IEEE floating point standard. It's the result of dividing negative floating point numbers by zero.

However, when I tried the following code, it didn't print true, instead it gave me an error.

fn main() {
    println!("{}", (-10. / 0.) == -inf);
}

Rust does not define a global variable or keyword named inf. So you can't use inf unless you first define (or import from somewhere) a variable with that name. -inf is just the string you get when converting negative infinity to a string. It's not a valid floating point literal by itself.

What value should I give instead of -inf to get true printed out?

You could use the constants INFINITY or NEG_INFINITY from the given floating point type, such as -10. / 0. == std::f64::NEG_INFINITY or -10. / 0. == -std::f64::INFINITY. Or you could define let inf = 1. / 0.;, then (-10. / 0.) == -inf would be true. Or you could just directly write -10. / 0. == -1. / 0. or any other expression that results in negative infinity.

What type is -inf?

The type of -inf is the same as that of the floating point numbers that you divided to get -inf. If you divide two f32s, you'll get an f32, and if you divide two f64s, you'll get an f64. If the type of the operands hasn't been decided yet, the type of the result will be {float} until a more concrete type has been decided.

Note that this isn't any different than any other case of dividing two floating point numbers (or adding them, multiplying them etc.). In other words, x / y where y is 0. has the exact same type as x / y where y is any other floating point number.

As stated by sepp2k, -inf is not valid Rust syntax. To check for infinite values, try f64::is_finite() or comparing to constants std::f64::INFINITY and std::f64::NEG_INFINITY.

fn main() {
    let result: f64 = -10. / 0.;
    println!("{}", result.is_infinite()); // true
    println!("{}", result == std::f64::INFINITY); // false
    println!("{}", result == std::f64::NEG_INFINITY); // true
}