[Rust] Slice and Vec metadata

struct VecMetadata {
    first_elem_index: usize,
    length: usize,
    capacity: usize
}

struct SliceMetadata {
    first_elem_index: usize,
    length: usize
}

Slice is similar to Vec, but it is a slice of Vec. As notice in the defination, Slice doesn't have `capacity` property, because Slice always refer to existing reference in the memory.

let nums: Vec<u8> = vec![0,1,2,3];
let slice: &[u8] = &nums[0..2];

 

The important thing to notice is "Slice doesn't own the elements, just references them".

 

Full code of the Vec or String:

nums.as_slice()
strings.as_str()

 

Example:

fn main() {
    let numbers = vec![2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24];

    let sum_of_nums = sum(numbers.as_slice());
    let product_of_nums = product(numbers.as_slice()); 
    let average_of_nums = average(numbers.as_slice()); 

    println!("Sum of these numbers: {}", sum_of_nums);
    println!("Product of these numbers: {}", product_of_nums);
    println!("Average of these numbers: {}", average_of_nums);
}

fn sum(numbers: &[i64]) -> i64 {
    let mut total = 0;

    for num in numbers.iter() {
        total += num;
    }

    total
}

fn product(numbers: &[i64]) -> i64 {
    let mut total = 1;

    for num in numbers.iter() {
        total *= num;
    }

    total
}

fn average(numbers: &[i64]) -> i64 {
    let length = numbers.len() as i64;

    sum(numbers) / length
}

 

In the example we do: sum(numbers.as_slice()); calling `as_slice()` but actually it is not necessary, since we already define in function it takes slice, sum(numbers: &[i64]), what we can do to simpify it:

fn main() {
    let numbers = vec![2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24];

    let sum_of_nums = sum(&numbers);
    let product_of_nums = product(&numbers); 
    let average_of_nums = average(&numbers); 

    println!("Sum of these numbers: {}", sum_of_nums);
    println!("Product of these numbers: {}", product_of_nums);
    println!("Average of these numbers: {}", average_of_nums);
}

Rust can convert Slice and Vec easily for us.