Code shared from the Rust Playground

playground.rs

use std::time::Instant;

fn main() {
    let mut a: Vec<i32> = vec![1; 10000];
    let mut b: Vec<i32> = vec![1; 10000];
    
    let t = Instant::now();
    let mut nv = Vec::with_capacity(10000);
    for i in a.iter() {
        nv.push(*i * 2);
    }
    println!("{:?}", t.elapsed().as_nanos());
    let t1 = Instant::now();
    b.iter().map(|e| *e * 2).collect::<Vec<i32>>();
    println!("{:?}", t1.elapsed().as_nanos());
}