Rust 中的 Generics - 迭代和索引通用 vec（频率计数函数）

Question

I feel this should be easy, but I'm stuck I can easily do a frequency count of a specific type (like u8), but how can I make it generic to allow counting u32s as well?

// Type specific works
fn freqs_u8(data: &[u8], size: usize) -> Vec<u32> {
    data.iter().fold(vec![0_u32; size], |mut freqs, &el| {
        freqs[el as usize] += 1;
        freqs
    })
}

// Not working generic because generic el cannot be typecast to size
fn freqs<T>(data: &[T], size: usize) -> Vec<u32> {
    data.iter().fold(vec![0_u32; size], |mut freqs, &el| {
        freqs[el as usize] += 1;
        freqs
    })
}

I've played with where restrictions on T, but to no avail. I've try creating an enum for T that is either a u8 or u32, but I'm not successful there either.

I wonder if I just don't know how to ask the question in the documentation - I've been looking in vain for days.

Answer 1

The problem is that there is no trait to specify for as .

There are two possible options:

1. Use num-traits , that provides the AsPrimitive trait:

fn freqs<T: num_traits::AsPrimitive<usize>>(data: &[T], size: usize) -> Vec<u32> {
    data.iter().fold(vec![0_u32; size], |mut freqs, &el| {
        freqs[el.as_()] += 1;
        freqs
    })
}

2. Or better, useTryInto , and panic if the value is too big for usize (this will be optimized away for types where usize is always big enough, eg u32 on 32bit and 64bit platforms):

fn freqs<T>(data: &[T], size: usize) -> Vec<u32>
where
    T: TryInto<usize> + Copy,
    T::Error: std::fmt::Debug,
{
    data.iter().fold(vec![0_u32; size], |mut freqs, &el| {
        freqs[el.try_into().unwrap()] += 1;
        freqs
    })
}