collections

Common Collections

Vectors

Vectors allow you to store a variable amount of items of the same type on the heap, next to each other.

let v: Vec<i32> = Vec::new();  // Type is required because it's empty
let v = vec![1, 2, 3];  // Rust can infer the type with the vec! macro

Vectors have the usual operations push and get (which returns an Option).

Other than get, we can use indexing notation:

let v = vec![1, 2, 3, 4, 5];
let tenth_element = &v[100];

But this will panic because we tried to access an out-of-bounds index.

To modify the elements on an iteration, we can use a for loop on a mutable reference:

let mut v = vec![100, 32, 57];
for i in &mut v {
    *i += 50;
}

If we don't need modifying, we can just use &v instead.

UTF-8 encoded text

Both the String and the string slice &str types are UTF-8 encoded. In libraries, the owned and borrow variants of strings tend to end in String and Str, respectively.

Internally, a String is a wrapper over a Vec<u8>.

Types that implement the Display trait have the to_string method.

The push_str method appends a string slice to the end, while just push can add a single character.

The compiler can coerce a &String into a &str when passing to a function. This is called deref coercion.

For concatenating strings, we can add a slice, but note that this doesn't copy and the added-to string is invalidated. This is because of the signature:

fn add(self, s: &str) -> String {}

Alternatively, if we don't want ownership to be taken we can use the format! macro:

let s = format!("{}-{}-{}", s1, s2, s3);

There are three ways to think of an “element” of a String.

1. Bytes: Using the bytes() method to iterate.
2. Scalar values: Using the char() method to iterate.
3. Grapheme clusters: Not available to iterate on the standard library, complex.

You need to be careful with string slices because Rust will panic if the slice doesn't end on a char boundary.

Hash Maps

Available as std::collections::HashMap. We can create one via ::new(), and add elements via the insert method.

Alternatively, we can use the collect method with zip, for example.

To update a value based on an old one, the pattern is to use or_insert:

for word in text.split_whitespace() {
    let count = map.entry(word).or_insert(0);
    *count += 1;
}

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