Why does .parse() convert “42” to f64 but fails co

问题:

Why and how does this line work:

let guess: f64 = "42".parse().expect("Not a number!");

But this does not?

let guess: i32 = "42.0".parse().expect("Not a number!");

Resulting in:

thread 'main' panicked at 'Not a number!: ParseIntError { kind: InvalidDigit }'

What is the correct way to parse "float" &str to integer?

Update:

I found this to work:

let guess: i32 = "42.0".parse::<f64>().expect("Not a number!") as i32;

However I don't understand the mechanics of how it works and if it is the correct way to do it?

回答1:

What you're calling is actually let guess: i32 = "42.0".parse::<i32>();.

However, "42.0" is not a correct representation of an i32.

According to the documentation:

Because parse is so general, it can cause problems with type inference. As such, parse is one of the few times you'll see the syntax affectionately known as the 'turbofish': ::<>. This helps the inference algorithm understand specifically which type you're trying to parse into.

The correct solution which you've already found is indeed to hint at the parser that the string is the representation of a float:

"42.0".parse::<f64>()