this would be much easier and less confusing, if we'd use "_" instead of " " characters.

NIcely done, one thing that's good to know:

Inclusive ranges can be created like this 0..=10

i.e.

for n in 0..=3 {
  println!("{}", n);
}

will print

0
1
2
3

In rust we have a handy little macro that manages these things ((x + 48) as char) for us.
format! accepts the same arguments as print!, but it yields a string rather than printing to stdout

I think you can use a simple array here, which saves a call to malloc