rust/src/doc/trpl/arrays-vectors-and-slices.md

% Arrays, Vectors, and Slices

Like many programming languages, Rust has list types to represent a sequence of
things. The most basic is the *array*, a fixed-size list of elements of the
same type. By default, arrays are immutable.

```{rust}
let a = [1, 2, 3]; // a: [i32; 3]
let mut m = [1, 2, 3]; // mut m: [i32; 3]
```

There's a shorthand for initializing each element of an array to the same
value. In this example, each element of `a` will be initialized to `0`:

```{rust}
let a = [0; 20]; // a: [i32; 20]
```

Arrays have type `[T; N]`. We'll talk about this `T` notation later, when we
cover generics.

You can get the number of elements in an array `a` with `a.len()`, and use
`a.iter()` to iterate over them with a for loop. This code will print each
number in order:

```{rust}
let a = [1, 2, 3];

println!("a has {} elements", a.len());
for e in a.iter() {
    println!("{}", e);
}
```

You can access a particular element of an array with *subscript notation*:

```{rust}
let names = ["Graydon", "Brian", "Niko"]; // names: [&str; 3]

println!("The second name is: {}", names[1]);
```

Subscripts start at zero, like in most programming languages, so the first name
is `names[0]` and the second name is `names[1]`. The above example prints
`The second name is: Brian`. If you try to use a subscript that is not in the
array, you will get an error: array access is bounds-checked at run-time. Such
errant access is the source of many bugs in other systems programming
languages.

A *vector* is a dynamic or "growable" array, implemented as the standard
library type [`Vec<T>`](../std/vec/) (we'll talk about what the `<T>` means
later). Vectors always allocate their data on the heap. Vectors are to slices
what `String` is to `&str`. You can create them with the `vec!` macro:

```{rust}
let v = vec![1, 2, 3]; // v: Vec<i32>
```

(Notice that unlike the `println!` macro we've used in the past, we use square
brackets `[]` with `vec!`. Rust allows you to use either in either situation,
this is just convention.)

There's an alternate form of `vec!` for repeating an initial value:

```
let v = vec![0; 10]; // ten zeroes
```

You can get the length of, iterate over, and subscript vectors just like
arrays. In addition, (mutable) vectors can grow automatically:

```{rust}
let mut nums = vec![1, 2, 3]; // mut nums: Vec<i32>

nums.push(4);

println!("The length of nums is now {}", nums.len()); // Prints 4
```

Vectors have many more useful methods.

A *slice* is a reference to (or "view" into) an array. They are useful for
allowing safe, efficient access to a portion of an array without copying. For
example, you might want to reference just one line of a file read into memory.
By nature, a slice is not created directly, but from an existing variable.
Slices have a length, can be mutable or not, and in many ways behave like
arrays:

```{rust}
let a = [0, 1, 2, 3, 4];
let middle = &a[1..4]; // A slice of a: just the elements 1, 2, and 3

for e in middle.iter() {
    println!("{}", e); // Prints 1, 2, 3
}
```

You can also take a slice of a vector, `String`, or `&str`, because they are
backed by arrays. Slices have type `&[T]`, which we'll talk about when we cover
generics.

We have now learned all of the most basic Rust concepts. We're ready to start
building ourselves a guessing game, we just need to know one last thing: how to
get input from the keyboard. You can't have a guessing game without the ability
to guess!