Gavin Wiggins

Arrays and Vectors in Rust

Written on November 14, 2022

In Rust, arrays and vectors are both collections that store multiple elements of the same type. Arrays are fixed-size collections allocated on the stack. Vectors are dynamic arrays allocated on the heap that can grow and shrink at runtime.

Arrays

An array is a collection of values where every element must have the same type. The length of an array is fixed and the first item is at index 0.

// Create an array of integers and access the first item
let a = [3, 4, 5, 6, 7, 8];
let first_item = a[0];

println!("First item in a is {first_item}");
// First item in a is 3

// Create an array with 3 elements of i64 and get the second item
let b: [i64; 3] = [12, 8, 5];
let second_item = b[1];

println!("Second item in b is {second_item}");
// Second item in b is 8

// Create an array of 4 elements where each element is 1
let c = [1; 4];

println!("The c array is {:?}", c);
// The c array is [1, 1, 1, 1]

// Get the length (number of elements) of an array
let d = [3, 4, 5, 6, 7];
let length_of_d = d.len();

println!("Length of d is {length_of_d}");
// Length of d is 5

Vectors

A vector is a resizable collection that stores items of the same type. Its values are stored next to each other in memory. The vec! macro can be used to initialize a vector.

// Create a vector of numbers
let v1 = vec![3, 5, 8, 10, 12, 9];

// Create a vector of strings
let v2 = vec!["one", "two", "three"];

Below is an example of creating an empty vector and adding values to it.

// Add elements to an empty vector
let mut v3 = Vec::new();
v3.push(1.5);
v3.push(3.0);
v3.push(8.2);

println!("v3 is {:?}", v3);
// This prints:
// v3 is [1.5, 3.0, 8.2]

Use indexing to get values from a vector. Notice the first element is at index zero.

// Read elements from a vector
let v4 = vec![22, 38, 42, 50];
let first = &v4[0];
let third = v4[2];
let last = v4.last().unwrap();

println!("v4 is {:?}", v4);
println!("first is {}", first);
println!("third is {}", third);
println!("last is {}", last);
// This prints:
// v4 is [22, 38, 42, 50]
// first is 22
// third is 42
// last is 50

Use of for loop to iterate over the values in a vector.

// Iterate over elements in a vector
let v5 = vec![3, 4, 5, 6, 8, 12];

for v in v5 {
    println!("v in v5 is {}", v);
}
// This prints:
// v in v5 is 3
// v in v5 is 4
// v in v5 is 5
// v in v5 is 6
// v in v5 is 8
// v in v5 is 12

// Iterate over elements in a vector by reference
let v5 = vec![3, 4, 5, 6, 8, 12];

for v in &v5 {
    println!("v in v5 is {}", v);
}
// This prints the same as above.