获取多个整数用户输入并将其存储在 Vec 中的最有效方法是什么<i32> ?

Question

I was trying to use rust for competitive coding and I was wondering what is the most efficient way of storing user input in a Vec. I have come up with a method but I am afraid that it is slow and redundant.

Here is my code:

use std::io;

fn main() {
  let mut input = String::new();
  io::stdin().read_line(&mut input).expect("cant read line");
  let input:Vec<&str> = input.split(" ").collect();
  let input:Vec<String> = input.iter().map(|x| x.to_string()).collect();
  let input:Vec<i32> = input.iter().map(|x| x.trim().parse().unwrap()).collect();
  println!("{:?}", input);
}

PS: I am new to rust.

Answer 1

I see those ways of improving performance of the code:

1. Although not really relevant for std::io::stdin() , std::io::BufReader may have great effect for reading eg from std::fs::File . Buffer capacity can also matter.
2. Using locked stdin: let si = std::io::stdin(); let si = si.locked(); let si = std::io::stdin(); let si = si.locked();
3. Avoiding allocations by keeping vectors around and using extend_from_iter instead of collect , if the code reads multiple line (unlike in the sample you posted in the question).
4. Maybe avoiding temporary vectors alltogether and just chaining Iterator operations together. Or using a loop like for line in input.split(...) { ... } . It may affect performance in both ways - you need to experiment to find out.
5. Avoiding to_string() and just storing reference to input buffer (which can also be used to parse() into i32 . Note that this may invite famous Rust borrowing and lifetimes complexity.
6. Maybe finding some fast SIMD-enhanced string to int parser instead of libstd's parse() .
7. Maybe streaming the result to algorithm instead of collecting everything to a Vec first. This can be beneficial especially if multiple threads can be used. For performance, you would still likely need to send data in chunks, not by one single i32 .

Answer 2

Yeah, there are some changes you can make that will make your code more precise, simple and faster.

A better code :

use std::io;

fn main() {
  let mut input = String::new();
  io::stdin().read_line(&mut input).unwrap();

  let input: Vec<i32> = input.split_whitespace().map(|x| x.parse().unwrap()).collect();

  println!("{:?}", input);
}

Explanation

1. The input.split_whitespace() returns an iterator containing elements that are seperated by any kind of whitespace including line breaks. This saves the time used in spliting by just one whitespace input.split(" ") and iterating over again with a .trim() method on each string slice to remove any surronding whitespaces. (You can also checkout input.split_ascii_whitespace() , if you want to restrict the split over ascii whitespaces).

2. There was no need for the code input.iter().map(|x| x.to_string()).collect() , since you can call also call a .trim() method on a string slice.

This saves some time in both the runtime and coding process, since the .collect() method is only used once and there was just one iteration.