Is it possible to declare the type of the variable in Rust for loops?

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清歌不尽 2020-12-03 16:30

C++ example:

for (long i = 0; i < 101; i++) {
    //...
}

In Rust I tried:

for i: i64 in 1..100 {
    // ...
}

3条回答

星月不相逢 (楼主)

2020-12-03 17:28

If your loop variable happens to be the result of a function call that returns a generic type:

let input = ["1", "two", "3"];
for v in input.iter().map(|x| x.parse()) {
    println!("{:?}", v);
}

error[E0284]: type annotations required: cannot resolve `<_ as std::str::FromStr>::Err == _`
 --> src/main.rs:3:37
  |
3 |     for v in input.iter().map(|x| x.parse()) {
  |                                     ^^^^^

You can use a turbofish to specify the types:

for v in input.iter().map(|x| x.parse::()) {
//                                   ^^^^^^^
    println!("{:?}", v);
}

Or you can use the fully-qualified syntax:

for v in input.iter().map(|x| ::from_str(x)) {
//                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    println!("{:?}", v);
}