## The Slice Type

Another data type that does not have ownership is the *slice*. Slices let you
reference a contiguous sequence of elements in a collection rather than the
whole collection.

Here’s a small programming problem: write a function that takes a string and
returns the first word it finds in that string. If the function doesn’t find a
space in the string, the whole string must be one word, so the entire string
should be returned.

Let’s think about the signature of this function:

```rust,ignore
fn first_word(s: &String) -> ?
```

This function, `first_word`, has a `&String` as a parameter. We don’t want
ownership, so this is fine. But what should we return? We don’t really have a
way to talk about *part* of a string. However, we could return the index of the
end of the word. Let’s try that, as shown in Listing 4-7.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-07/src/main.rs:here}}
```

<span class="caption">Listing 4-7: The `first_word` function that returns a
byte index value into the `String` parameter</span>

Because we need to go through the `String` element by element and check whether
a value is a space, we’ll convert our `String` to an array of bytes using the
`as_bytes` method:

```rust,ignore
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-07/src/main.rs:as_bytes}}
```

Next, we create an iterator over the array of bytes using the `iter` method:

```rust,ignore
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-07/src/main.rs:iter}}
```

We’ll discuss iterators in more detail in Chapter 13. For now, know that `iter`
is a method that returns each element in a collection and that `enumerate`
wraps the result of `iter` and returns each element as part of a tuple instead.
The first element of the tuple returned from `enumerate` is the index, and the
second element is a reference to the element. This is a bit more convenient
than calculating the index ourselves.

Because the `enumerate` method returns a tuple, we can use patterns to
destructure that tuple. We'll be discussing patterns more in Chapter 6. So in
the `for` loop, we specify a pattern that has `i` for the index in the tuple
and `&item` for the single byte in the tuple. Because we get a reference to the
element from `.iter().enumerate()`, we use `&` in the pattern.

Inside the `for` loop, we search for the byte that represents the space by
using the byte literal syntax. If we find a space, we return the position.
Otherwise, we return the length of the string by using `s.len()`:

```rust,ignore
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-07/src/main.rs:inside_for}}
```

We now have a way to find out the index of the end of the first word in the
string, but there’s a problem. We’re returning a `usize` on its own, but it’s
only a meaningful number in the context of the `&String`. In other words,
because it’s a separate value from the `String`, there’s no guarantee that it
will still be valid in the future. Consider the program in Listing 4-8 that
uses the `first_word` function from Listing 4-7.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-08/src/main.rs:here}}
```

<span class="caption">Listing 4-8: Storing the result from calling the
`first_word` function and then changing the `String` contents</span>

This program compiles without any errors and would also do so if we used `word`
after calling `s.clear()`. Because `word` isn’t connected to the state of `s`
at all, `word` still contains the value `5`. We could use that value `5` with
the variable `s` to try to extract the first word out, but this would be a bug
because the contents of `s` have changed since we saved `5` in `word`.

Having to worry about the index in `word` getting out of sync with the data in
`s` is tedious and error prone! Managing these indices is even more brittle if
we write a `second_word` function. Its signature would have to look like this:

```rust,ignore
fn second_word(s: &String) -> (usize, usize) {
```

Now we’re tracking a starting *and* an ending index, and we have even more
values that were calculated from data in a particular state but aren’t tied to
that state at all. We now have three unrelated variables floating around that
need to be kept in sync.

Luckily, Rust has a solution to this problem: string slices.

### String Slices

A *string slice* is a reference to part of a `String`, and it looks like this:

```rust
{{#rustdoc_include ../listings/ch04-understanding-ownership/no-listing-17-slice/src/main.rs:here}}
```

This is similar to taking a reference to the whole `String` but with the extra
`[0..5]` bit. Rather than a reference to the entire `String`, it’s a reference
to a portion of the `String`.

We can create slices using a range within brackets by specifying
`[starting_index..ending_index]`, where `starting_index` is the first position
in the slice and `ending_index` is one more than the last position in the
slice. Internally, the slice data structure stores the starting position and
the length of the slice, which corresponds to `ending_index` minus
`starting_index`. So in the case of `let world = &s[6..11];`, `world` would be
a slice that contains a pointer to the byte at index 6 of `s` with a length
value of 5.

Figure 4-6 shows this in a diagram.

<img alt="world containing a pointer to the byte at index 6 of String s and a length 5" src="img/trpl04-06.svg" class="center" style="width: 50%;" />

<span class="caption">Figure 4-6: String slice referring to part of a
`String`</span>

With Rust’s `..` range syntax, if you want to start at index zero, you can drop
the value before the two periods. In other words, these are equal:

```rust
let s = String::from("hello");

let slice = &s[0..2];
let slice = &s[..2];
```

By the same token, if your slice includes the last byte of the `String`, you
can drop the trailing number. That means these are equal:

```rust
let s = String::from("hello");

let len = s.len();

let slice = &s[3..len];
let slice = &s[3..];
```

You can also drop both values to take a slice of the entire string. So these
are equal:

```rust
let s = String::from("hello");

let len = s.len();

let slice = &s[0..len];
let slice = &s[..];
```

> Note: String slice range indices must occur at valid UTF-8 character
> boundaries. If you attempt to create a string slice in the middle of a
> multibyte character, your program will exit with an error. For the purposes
> of introducing string slices, we are assuming ASCII only in this section; a
> more thorough discussion of UTF-8 handling is in the [“Storing UTF-8 Encoded
> Text with Strings”][strings]<!-- ignore --> section of Chapter 8.

With all this information in mind, let’s rewrite `first_word` to return a
slice. The type that signifies “string slice” is written as `&str`:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch04-understanding-ownership/no-listing-18-first-word-slice/src/main.rs:here}}
```

We get the index for the end of the word in the same way as we did in Listing
4-7, by looking for the first occurrence of a space. When we find a space, we
return a string slice using the start of the string and the index of the space
as the starting and ending indices.

Now when we call `first_word`, we get back a single value that is tied to the
underlying data. The value is made up of a reference to the starting point of
the slice and the number of elements in the slice.

Returning a slice would also work for a `second_word` function:

```rust,ignore
fn second_word(s: &String) -> &str {
```

We now have a straightforward API that’s much harder to mess up, because the
compiler will ensure the references into the `String` remain valid. Remember
the bug in the program in Listing 4-8, when we got the index to the end of the
first word but then cleared the string so our index was invalid? That code was
logically incorrect but didn’t show any immediate errors. The problems would
show up later if we kept trying to use the first word index with an emptied
string. Slices make this bug impossible and let us know we have a problem with
our code much sooner. Using the slice version of `first_word` will throw a
compile-time error:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch04-understanding-ownership/no-listing-19-slice-error/src/main.rs:here}}
```

Here’s the compiler error:

```console
{{#include ../listings/ch04-understanding-ownership/no-listing-19-slice-error/output.txt}}
```

Recall from the borrowing rules that if we have an immutable reference to
something, we cannot also take a mutable reference. Because `clear` needs to
truncate the `String`, it needs to get a mutable reference. The `println!`
after the call to `clear` uses the reference in `word`, so the immutable
reference must still be active at that point. Rust disallows the mutable
reference in `clear` and the immutable reference in `word` from existing at the
same time, and compilation fails. Not only has Rust made our API easier to use,
but it has also eliminated an entire class of errors at compile time!

#### String Literals Are Slices

Recall that we talked about string literals being stored inside the binary. Now
that we know about slices, we can properly understand string literals:

```rust
let s = "Hello, world!";
```

The type of `s` here is `&str`: it’s a slice pointing to that specific point of
the binary. This is also why string literals are immutable; `&str` is an
immutable reference.

#### String Slices as Parameters

Knowing that you can take slices of literals and `String` values leads us to
one more improvement on `first_word`, and that’s its signature:

```rust,ignore
fn first_word(s: &String) -> &str {
```

A more experienced Rustacean would write the signature shown in Listing 4-9
instead because it allows us to use the same function on both `&String` values
and `&str` values.

```rust,ignore
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-09/src/main.rs:here}}
```

<span class="caption">Listing 4-9: Improving the `first_word` function by using
a string slice for the type of the `s` parameter</span>

If we have a string slice, we can pass that directly. If we have a `String`, we
can pass a slice of the `String` or a reference to the `String`. This
flexibility takes advantage of *deref coercions*, a feature we will cover in
the [“Implicit Deref Coercions with Functions and
Methods”][deref-coercions]<!--ignore--> section of Chapter 15. Defining a
function to take a string slice instead of a reference to a `String` makes our
API more general and useful without losing any functionality:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch04-understanding-ownership/listing-04-09/src/main.rs:usage}}
```

### Other Slices

String slices, as you might imagine, are specific to strings. But there’s a
more general slice type, too. Consider this array:

```rust
let a = [1, 2, 3, 4, 5];
```

Just as we might want to refer to a part of a string, we might want to refer
to part of an array. We’d do so like this:

```rust
let a = [1, 2, 3, 4, 5];

let slice = &a[1..3];

assert_eq!(slice, &[2, 3]);
```

This slice has the type `&[i32]`. It works the same way as string slices do, by
storing a reference to the first element and a length. You’ll use this kind of
slice for all sorts of other collections. We’ll discuss these collections in
detail when we talk about vectors in Chapter 8.

## Summary

The concepts of ownership, borrowing, and slices ensure memory safety in Rust
programs at compile time. The Rust language gives you control over your memory
usage in the same way as other systems programming languages, but having the
owner of data automatically clean up that data when the owner goes out of scope
means you don’t have to write and debug extra code to get this control.

Ownership affects how lots of other parts of Rust work, so we’ll talk about
these concepts further throughout the rest of the book. Let’s move on to
Chapter 5 and look at grouping pieces of data together in a `struct`.

[strings]: ch08-02-strings.html#storing-utf-8-encoded-text-with-strings
[deref-coercions]: ch15-02-deref.html#implicit-deref-coercions-with-functions-and-methods
