Create a String from the part of the vector

Question

I want to create a String from the part of the vector.

My code:

fn main() {
 let v = vec!['a', 'b', 'c', 'd'];
 let mut res = String::new(); // expect to have "cd"
 let start_pos: usize = 2;
 for i in start_pos..v.len() {
 res.push(v[i]);
 }
 println!("{}", res); // "cd"
}

Is it possible to do the same shorter?

• Toby Speight: This isn't Code Golf, so we prefer "better" to "shorter" (sometimes shorter is better, sometimes not).

Answer 1

You could convert the Vec into an iterator, and write this in fluent style.

fn main() {
 let v = vec!['a', 'b', 'c', 'd'];
 let res : String = v
 .into_iter()
 .skip(2)
 .collect();
 println!("{}", res); // "cd"
}

Instead of skip(2) on the iterator, you could convert a slice of the Vec:

fn main() {
 let v = vec!['a', 'b', 'c', 'd'];
 let res : String = v[2..]
 .into_iter()
 .collect();
 println!("{}", res); // "cd"
}

I find this very readable, and it generates fast code. One advantage it has is that res no longer needs to be mut, but can be a static single assignment. To achieve that with the loop implementation, you would need to move the loop into a helper, or "phi," function.

In real-world code, a container like v, whose data are compile-time constants and never modified, would be a const or static array, not a Vec.

Recall that a Rust String holds UTF-8 bytes, not an array of UCS-4 char. There’s a function to move a Vec<u8> that holds valid UTF-8 bytes into a String: String::from_utf8. It’s likely that your input will be in UTF-8 as well, so if you can avoid the overhead of converting between UCS-4 and UTF-8 or vice versa, that will be far more efficient.