Notes (TRPL 08/21): Common Collections

8 Common Collections

8.1 Vectors

see src/


let mut v = Vec::new();


looks like it causes the compiler to infer that v : Vec<{integer}>. I think this is the case because if I force a type error with

let mut v = Vec::new();


The compiler says:

9 |   v3.push("a"); // type inference!
  |           ^^^ expected integral variable, found reference
  = note: expected type `{integer}`
             found type `&'static str`

I wonder how the compiler decides to pick a concrete type like i32 from {integer} though.

Reading Elements of Vectors

If you try to access an element out of bounds:

let v = vec![1, 2, 3, 4, 5];
let sixth: &i32 = &v[6];
println!("The sixth element is {}", sixth);

The thread will panic at runtime. Using .get we can get elements safely.

I really like that having an immutable reference to an element of a vector prevents creating mutable references and vice-versa:

This has two immutable references to v[0] so no problems:

let mut v = vec![1, 2, 3, 4, 5];
println!("v is {:?}", v);

let first = &v[0];

println!("first is {}", first);
//v[0] = 3;
//println!("first is {}", first);
println!("v is {:?}", v);

Hmmm, looks like we can create an immutable reference to v[0] and then mutate under it, as long as our immutable reference is never actually used.

let first = &v[0];

//println!("first is {}", first);
v[0] = 3;
//println!("first is {}", first);
/println!("v is {:?}", v);

I guess that makes sense because the scope we’re in already mutably owns v because we declared it here. But this still seems like something that could cause us some confusion later on if we forget exactly how the borrow checker works.

We can actually make the above code work by just pushing another value onto the stack with first’s value:

let first = &v[0]; // ref A
let first_ = first;
println!("first_ is {}", first_);

v[0] = 3; // allowed because first is never borrowed by anything
          // we could have done
          // let first = v[0].clone()
          // or simply
          // let first = v[0];

let first = &v[0]; // this &v[0] is potentially a different reference than ref A
println!("first is {}", first);
/println!("v is {:?}", v);

Okay, so that explains why we’re allowed to even have an immutable reference that never gets borrowed in the same scope as a mutable reference. We can use the value, but only to copy it’s value into new memory, whether onto the stack with a let or onto the heap with a .clone().

Iterating over the Values in a Vector

* is the dereference operator. *ref means the value at memory location ref. It’s important to distinguish let *x = y from let y = *x though. The former is a write operation that sets the value at x to equal the value of the variable y on the stack. The latter is a read operation that copies the value at x and allocates a new variable y on the stack with that value.

It’s a little confusing that = does different things to the left-hand and right-hand side. Keep in mind that in C family languages (of which Rust is one), the = assignment operator should properly be :=, which corresponds to the definition notation from mathematics. The only reason it isn’t is that in 1969 Dennis Ritchie and Ken Thompson were trying to fit their new B language efficiently into the 9.2 KB memory of the PDP-7, so they cut the : out of := to save space.

8.2 String

see src/

Storing UTF-8 Encoded Text with Strings

Wondering how + is overloaded to be concatenation on Strings and addition on numbers. Oh, I see, there’s just an Add trait:

Strings are UTF-8 encoded so since UTF-8 is variable width, slices and indexes of Strings don’t always map cleanly to characters. The Rust compiler flat out prevents you from accessing a single index of a string. String slices are allowed but can cause a thread to panic if the slice bounds don’t correspond to UTF-8 char boundaries.

tldr; Strings are complicated, use methods and libraries.

8.3 Hash Maps

see src/

A HashMap<K, V> is a map from keys of type K to values of type V.

The line:


feels kind-of functional, but I want to get a beter understanding of what .iter() does concretely.

Hash Maps and Ownership

Once they get passed into .insert(), the field_name and field_value references have been moved into the map.

How exactly does the for ... in in

for (key, value) in &scores

iterate over all the pairs in the HashMap? Is there a trait? I bet there is, there’s always a trait when I ask these kinds of questions…

Indeed there is! It’s the Iterator trait.


Mean, Median, Mode

fn median(ns: &Vec<i32>) -> f32 {
  let mut ms = ns.clone();
  let l = ns.len();
  if l % 2 == 1 {
    ms[l / 2] as f32
  } else {
    (ms[l / 2] as f32 + ms[l / 2 - 1] as f32) / 2.0

fn mean(ns: &Vec<i32>) -> f32 {
  let total: i32 = ns.iter().sum();
  let size = ns.len() as f32;
  total as f32 / size

fn mode(ns: &Vec<i32>) -> Vec<i32> {
  let mut map: HashMap<i32, i32> = HashMap::new();
  for n in ns {
    let count = map.entry(*n).or_insert(0);
    *count += 1;
  let max: i32 = *map.iter().max_by_key(|(_, &v)| v).unwrap().1;
    .filter(|(_, &v)| v == max)
    .map(|(&k, _)| k)

The mode function was particularly fun. One question I have is how Rust distinguishes between a reference to a struct vs a struct of references. In some of the tuples above they’re interchangeable, but not in all of them.

Pig Latin

For pig-latin, we’ll use the unicode-segmentation external crate

I don’t know what the best way to test if graphemes are consonants or vowel though… Properly this would require an is_vowel function that was total over every UTF-8 codepoint, but I don’t know if there’s a Rust library for that…

Text Interface for a department records system

see src/