这段代码可以工作，但是我想显式声明索引范围max的类型。但是uX或iX，其中X = 8，16或32会产生编译错误。正确的类型是什么？

fn main() {
    let mut arr2: [[f64; 3]; 3] = [[0.0; 3]; 3];
    let pi: f64 = 3.1415926535;

    let max = 3; // let max: i16 e.g. is wrong

    for ii in 0..max {
        for jj in 0..3 {
            let i = ii as f64;
            let j = jj as f64;
            arr2[ii][jj] = ((i + j) * pi * 41.0).sqrt().sin();
            println!("arr2[{}][{}] is {}", ii, jj, arr2[ii][jj]);
        }
    }
}