• Arrays can also be initialized using the shorthand syntax, e.g. [0; 1024]. This can be useful when you want to initialize all elements to the same value, or if you have a large array that would be hard to initialize manually.

• A value of the array type [T; N] holds N (a compile-time constant) elements of the same type T. Note that the length of the array is part of its type, which means that [u8; 3] and [u8; 4] are considered two different types. Slices, which have a size determined at runtime, are covered later.

• Try accessing an out-of-bounds array element. The compiler is able to determine that the index is unsafe, and will not compile the code:

fn main() {
    let mut a: [i8; 5] = [5, 4, 3, 2, 1];
    a[6] = 0;
    println!("a: {a:?}");
}

• Array accesses are checked at runtime. Rust can usually optimize these checks away; meaning if the compiler can prove the access is safe, it removes the runtime check for better performance. They can be avoided using unsafe Rust. The optimization is so good that it’s hard to give an example of runtime checks failing. The following code will compile but panic at runtime:

fn get_index() -> usize {
    6
}

fn main() {
    let mut a: [i8; 5] = [5, 4, 3, 2, 1];
    a[get_index()] = 0;
    println!("a: {a:?}");
}

• We can use literals to assign values to arrays.

• Arrays are not heap-allocated. They are regular values with a fixed size known at compile time, meaning they go on the stack. This can be different from what students expect if they come from a garbage-collected language, where arrays may be heap allocated by default.

• There is no way to remove elements from an array, nor add elements to an array. The length of an array is fixed at compile-time, and so its length cannot change at runtime.

Debug Printing

• The println! macro asks for the debug implementation with the ? format parameter: {} gives the default output, {:?} gives the debug output. Types such as integers and strings implement the default output, but arrays only implement the debug output. This means that we must use debug output here.

• Adding #, eg {a:#?}, invokes a “pretty printing” format, which can be easier to read.