How to allocate space for Vec <T> in Rust?

Question

I want to create Vec<T>and make some space for this, but I don’t know how to do it, and, to my surprise, there is almost nothing in the official documentation about this basic type.

let mut v: Vec<i32> = Vec<i32>(SIZE); // How do I do this ?

for i in 0..SIZE {
    v[i] = i;
}

I know that I can create an empty one Vec<T>and fill it with pushes, but I do not want to do this, because I do not always know when I write a value to index iif the value has already been inserted there. I do not want to write, for obvious performance reasons, something like:

if i >= len(v) {
    v.push(x);
} else {
    v[i] = x;
}

And of course, I cannot use the syntax vec!.

Answer 1

vec!, vec![elem; count]. :

vec![1; 10]

Vec<_>, 10 1 ( _ - i32). elem, , Clone. count .

Answer 2

vec![elem; count] , , .

Vec::with_capacity() , . , , , push() , push() :

fn main() {
    let mut v = Vec::with_capacity(10);
    for i in 0..10 {
        v.push(i);
    }
    println!("{:?}", v);
}

collect() . :

fn main() {
    let v: Vec<_> = (1..10).collect();
    println!("{:?}", v);
}

, , (, ). Vec::with_capacity() + set_len() :

fn main() {
    let mut v = Vec::with_capacity(10);
    unsafe { v.set_len(10); }
    for i in 0..10 {
        v[i] = i;
    }
    println!("{:?}", v);
}

, , - , . , ( ). , mem::uninitialized().

Answer 3

Vec::resize:

fn resize(&mut self, new_len: usize, value: T)

1024 , 7:

let mut vec: Vec<i32> = Vec::new();
vec.resize(1024, 7);