Vectors

A Vector is an array whose size can be changed during the execution of the program.

The following code creates an empty vector and adds three elements to it:

#![allow(unused)]
fn main() {
let v = vec![1,2,3];
println!("{v:?}");
}

After this, the elements can be accessed like in an ordinary array:

#![allow(unused)]
fn main() {
let v = vec![1,2,3];
println!("{}", v[0]);
println!("{}", v[1]);
println!("{}", v[2]);
}

Access to an array element with direct access is not really idiomatic in Rust, it should be better to use std functions like .get(&index) that return a Result that could be managed. However, in competitive programming use direct access could be faster.

The function .len() returns the number of elements in the vector. The following code iterates through the vector and prints all elements in it:

#![allow(unused)]
fn main() {
let v = vec![1,2,3];
for i in 0..v.len(){
    println!("{}", v[i])
}
}

A shorter way to iterate through a vector is as follows:

#![allow(unused)]
fn main() {
let v = vec![1,2,3];
for x in v {
    println!("{x}");
}
}

The function .push() add an alement to the back of an existing Vec. The function .last() returns the last element in the Vec, and the function .pop() removes the last element:

#![allow(unused)]
fn main() {
let v = vec![5,2];
v.push(6);
println!("{v:?}");
println!("{:?}", v.last());
v.pop();
println!("{v:?}");
}

Another way to create a Vec is to give the number of elements and the initial value for each element:

#![allow(unused)]
fn main() {
// size 10, initial value 0
let v = vec![0;10]
println!("{v:?}");
}

#![allow(unused)]
fn main() {
// size 10, initial value 5
let v = vec![5;10]
println!("{v:?}");
}

The internal implementation of a vector uses a struct with three elements:

• Pointer to the first data on the heap
• length
• capacity

The capacity indicates how much memory is reserved for the Vec. Vectors have O(1) indexing, amortized O(1) push (to the end) and O(1) pop (from the end).