Add a (pseudo) random number

Right now the secret number is neither secret nor random. If we were not using Repl.it, we could do this:

extern crate rand;
use rand::Rng;
let secret_number = rand::thread_rng().gen_range(1, 101);

But Repl.it does not allow us to use external crates! So let's find a workaround using time and math:

use std::time::{SystemTime, UNIX_EPOCH};
let nanos = SystemTime::now()
.duration_since(UNIX_EPOCH)
.unwrap()
.subsec_nanos();
let secret_number = nanos / 13000000;

It's not perfect, but it'll get us a pseudorandom number that will generally be between 1 and 100. One last thing: right now, we are printing our secret number! Where's the fun in that? Let's remove that line and now our guessing game is complete:

main.rs
use std::io;
use std::cmp::Ordering;
use std::time::{SystemTime, UNIX_EPOCH};
fn main() {
println!("Guess the number!");
let nanos = SystemTime::now()
.duration_since(UNIX_EPOCH)
.unwrap()
.subsec_nanos();
let secret_number = nanos / 13000000;
loop {
println!("Please input your guess.");
let mut guess = String::new();
io::stdin().read_line(&mut guess)
.expect("Failed to read line");
let guess: u32 = match guess.trim().parse() {
Ok(num) => num,
Err(msg) => {
println!("{}", msg);
continue;
},
};
println!("You guessed: {}", guess);
match guess.cmp(&secret_number) {
Ordering::Less => println!("Too small!"),
Ordering::Greater => println!("Too big!"),
Ordering::Equal => {
println!("You win!");
break;
},
}
}
}