strange transpose times

main.rs

use std::time;

fn get_test_matrix() -> Vec<Vec<f64>> {
    let mut x: Vec<Vec<f64>> = vec![vec![0.; 790]; 500000];
    for i in 0..x.len() {
        for j in 0..x[0].len() {
            x[i][j] = fastrand::f64() * 500.;
        }
    }
    x
}

fn tb(a: &Vec<Vec<f64>>) -> Vec<Vec<f64>> {
    let mut b: Vec<Vec<f64>> = vec![vec![0.; a.len()]; a[0].len()];
    for j in 0..a[0].len() {
        for i in 0..a.len() {
            b[j][i] = a[i][j];
        }
    }
    b
}

fn ta(a: &Vec<Vec<f64>>) -> Vec<Vec<f64>> {
    let mut b: Vec<Vec<f64>> = vec![Vec::with_capacity(a.len()); a[0].len()];
    for r in a {
        for i in 0..r.len() {
            b[i].push(r[i]);
        }
    }
    b
}

fn main() {
    let a = get_test_matrix();
    let b = get_test_matrix();
    let time_a = time::Instant::now();
    ta(&a);
    println!("ta -> {}s", time_a.elapsed().as_secs());
    let time_b = time::Instant::now();
    tb(&b);
    println!("tb -> {}s", time_b.elapsed().as_secs());
}