Standard Library Containers

Reading the Documentation

In this section I’ll briefly introduce some common parts of the Rust standard library. The documentation is excellent but a little context and a few examples is always useful.

Initially, reading the Rust documentation can be challenging, so I’ll go through Vec as an example. A useful tip is to tick the ‘[-]’ box to collapse the docs. (If you download the standard library source using rustup component add rust-src a ‘[src]’ link will appear next to this.) This gives you a bird’s eye view of all the available methods.

The first point to notice is that not all possible methods are defined on Vec itself. They are (mostly) mutable methods that change the vector, e.g. push. Some methods are only implemented for vectors where the type matches some constraint. For example, you can only call dedup (remove duplicates) if the type is indeed something that can be compared for equality. There are multiple impl blocks that define Vec for different type constraints.

Then there’s the very special relationship between Vec<T> and &[T]. Any method that works on slices will also directly work on vectors, without explicitly having to use the as_slice method. This relationship is expressed by Deref<Target=[T]>. This also kicks in when you pass a vector by reference to something that expects a slice - this is one of the few places where a conversion between types happens automatically. So slice methods like first, which maybe-returns a reference to the first element, or last, work for vectors as well. Many of the methods are similar to the corresponding string methods, so there’s split_at for getting a pair of slices split at an index, starts_with to check whether a vector starts with sequence of values, and contains to check whether a vector contains a particular value.

There’s no search method for finding the index of a particular value, but here’s a rule of thumb: if you can’t find a method on the container, look for a method on the iterator:

    let v = vec![10,20,30,40,50];
    assert_eq!(v.iter().position(|&i| i == 30).unwrap(), 2);

(The & is because this is an iterator over references - alternatively you could say *i == 30 for the comparison.)

Likewise, there’s no map method on vectors, because iter().map(...).collect() will do the job just as well. Rust does not like to allocate unnecessarily - often you don’t need the result of that map as an actual allocated vector.

So I’d suggest you become familiar with all the iterator methods, because they are crucial to writing good Rust code without having to write loops out all the time. As always, write little programs to explore iterator methods, rather than wrestling with them in the context of a more complicated program.

The Vec<T> and &[T] methods are followed by the common traits: vectors know how to do a debug display of themselves (but only if the elements implement Debug). Likewise, they are clonable if their elements are clonable. They implement Drop, which happens when vectors get to finally die; memory is released, and all the elements are dropped as well.

The Extend trait means values from iterators can be added to a vector without a loop:

let mut strings = vec!["hello".to_string(), "dolly".to_string()];
strings.extend(["you","are","fine"].iter().map(|s| s.to_string()));

There’s also FromIterator, which lets vectors be constructed from iterators. (The iterator collect method leans on this.)

Any container needs to be iterable as well. Recall that there are three kinds of iterators

for x in v {...} // returns T, consumes v
for x in &v {...} // returns &T
for x in &mut v {...} // returns &mut T

The for statement relies on the IntoIterator trait, and there’s indeed three implementations.

Then there is indexing, controlled by Index (reading from a vector) and IndexMut (modifying a vector.) There are many possibilities, because there’s slice indexing as well, like v[0..2], returning these return slices, as well as plain v[0] which returns a reference to the first element.

There’s a few implementations of the From trait. For instance Vec::from("hello".to_string()) will give you a vector of the underlying bytes of the string Vec<u8>. Now, there’s already a method into_bytes on String, so why the redundancy? It seems confusing to have multiple ways of doing the same thing. But it’s needed because explicit traits make generic methods possible.

Sometimes limitations of the Rust type system make things clumsy. An example here is how PartialEq is separately defined for arrays up to size 32! (This will get better.) This allows the convenience of directly comparing vectors with arrays, but beware the size limit.

And there are Hidden Gems buried deep in the documentation. As Karol Kuczmarski says “Because let’s be honest: no one scrolls that far”. How does one handle errors in an iterator? Say you map over some operation that might fail and so returns Result, and then want to collect the results:

fn main() {
    let nums = ["5","52","65"];
    let iter = nums.iter().map(|s| s.parse::<i32>());
    let converted: Vec<_> = iter.collect();
//[Ok(5), Ok(52), Ok(65)]

Fair enough, but now you have to unwrap these errors - carefully!. But Rust already knows how to do the Right Thing, if you ask for the vector to be contained in a Result - that is, either is a vector or an error:

    let converted: Result<Vec<_>,_> = iter.collect();
//Ok([5, 52, 65])

And if there was a bad conversion? Then you would just get Err with the first error encountered. It’s a good example of how extremely flexible convert is. (The notation here can be intimidating - Vec<_> means “this is a vector, work out the actual type for meandResult,_>` is furthermore asking Rust to work out the error type as well.)

So there’s a lot of detail in the documentation. But it’s certainly clearer than what the C++ docs say about std::vector

The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable, but many member functions impose stricter requirements.

With C++, you’re on your own. The explicitness of Rust is daunting at first, but as you learn to read the constraints you will know exactly what any particular method of Vec requires.

I would suggest that you get the source using rustup component add rust-src, since the standard library source is very readable and the method implementations are usually less scary than the method declarations.


Maps (sometimes called associative arrays or dicts) let you look up values associated with a key. It’s not really a fancy concept, and can be done with an array of tuples:

    let entries = [("one","eins"),("two","zwei"),("three","drei")];

    if let Some(val) = entries.iter().find(|t| t.0 == "two") {

This is fine for small maps and just requires equality to be defined for the keys, but the search takes linear time - proportional to the size of the map.

A HashMap does much better when there are a lot of key/value pairs to be searched:

use std::collections::HashMap;

let mut map = HashMap::new();

assert_eq! (map.contains_key("two"), true);
assert_eq! (map.get("two"), Some(&"zwei"));

&"zwei"? This is because get returns a reference to the value, not the value itself. Here the value type is &str, so we get a &&str. In general it has to be a reference, because we can’t just move a value out of its owning type.

get_mut is like get but returns a possible mutable reference. Here we have a map from strings to integers, and wish to update the value for the key ‘two’:

let mut map = HashMap::new();

println!("before {}", map.get("two").unwrap());

    let mut mref = map.get_mut("two").unwrap();
    *mref = 20;

println!("after {}", map.get("two").unwrap());
// before 2
// after 20

Note that getting that writable reference takes place in its own block - otherwise, we would have a mutable borrow lasting until the end, and then Rust won’t allow you to borrow from map again with map.get("two"); it cannot allow any readable references while there’s already a writable reference in scope. (If it did, it could not guarantee that those readable references would remain valid.) So the solution is to make sure that mutable borrow doesn’t last very long.

It is not the most elegant API possible, but we can’t throw away any possible errors. Python would bail out with an exception, and C++ would just create a default value. (This is convenient but sneaky; easy to forget that the price of a_map["two"] always returning an integer is that we can’t tell the difference between zero and ‘not found’, plus an extra entry is created!)

And no-one just calls unwrap, except in examples. However, most Rust code you see consists of little standalone examples! Much more likely for a match to take place:

if let Some(v) = map.get("two") {
    let res = v + 1;
    assert_eq!(res, 3);
match map.get_mut("two") {
    Some(mref) => *mref = 20,
    None => panic!("_now_ we can panic!")

We can iterate over the key/value pairs, but not in any particular order.

for (k,v) in map.iter() {
    println!("key {} value {}", k,v);
// key one value eins
// key three value drei
// key two value zwei

There are also keys and values methods returning iterators over the keys and values respectively, which makes creating vectors of values easy.

Example: Counting Words

An entertaining thing to do with text is count word length frequency. It is straightforward to break text into words with split_whitespace, but really we must respect punctuation. So the words should be defined as consisting only of alphabetic characters. And the words need to be compared as lower-case as well.

Doing a mutable lookup on a map is straightforward, but also handling the case where the lookup fails is a little awkward. Fortunately there’s an elegant way to update the values of a map:

let mut map = HashMap::new();

for s in text.split(|c: char| ! c.is_alphabetic()) {
    let word = s.to_lowercase();
    let mut count = map.entry(word).or_insert(0);
    *count += 1;

If there’s no existing count corresponding to a word, then let’s create a new entry containing zero for that word and insert it into the map. Its exactly what a C++ map does, except it’s done explicitly and not sneakily.

There is exactly one explicit type in this snippet, and that’s the char needed because of a quirk of the string Pattern trait used by split. But we can deduce that the key type is String and the value type is i32.

Using The Adventures of Sherlock Holmes from Project Gutenberg, we can test this out more thoroughly. The total number of unique words (map.len()) is 8071.

How to find the twenty most common words? First, convert the map into a vector of (key,value) tuples. (This consumes the map, since we used into_iter.)

let mut entries: Vec<_> = map.into_iter().collect();

Next we can sort in descending order. sort_by expects the result of the cmp method that comes from the Ord trait, which is implemented by the integer value type:

    entries.sort_by(|a,b| b.1.cmp(&a.1));

And finally print out the first twenty entries:

    for e in entries.iter().take(20) {
        println!("{} {}", e.0, e.1);

(Well, you could just loop over 0..20 and index the vector here - it isn’t wrong, just a little un-idiomatic - and potentially more expensive for big iterations.)

the 5810
and 3088
i 3038
to 2823
of 2778
a 2701
in 1823
that 1767
it 1749
you 1572
he 1486
was 1411
his 1159
is 1150
my 1007
have 929
with 877
as 863
had 830

A little surprise - what’s that empty word? It is because split works on single-character delimiters, so any punctuation or extra spaces causes a new split.


Sets are maps where you care only about the keys, not any associated values. So insert only takes one value, and you use contains for testing whether a value is in a set.

Like all containers, you can create a HashSet from an iterator. And this is exactly what collect does, once you have given it the necessary type hint.

use std::collections::HashSet;

fn make_set(words: &str) -> HashSet<&str> {

fn main() {
    let fruit = make_set("apple orange pear orange");

    println!("{:?}", fruit);
// {"orange", "pear", "apple"}

Note (as expected) that repeated insertions of the same key have no effect, and the order of values in a set are not important.

They would not be sets without the usual operations:

let fruit = make_set("apple orange pear");
let colours = make_set("brown purple orange yellow");

for c in fruit.intersection(&colours) {
// "orange"

They all create iterators, and you can use collect to make these into sets.

Here’s a shortcut, just as we defined for vectors:

use std::hash::Hash;

trait ToSet<T> {
    fn to_set(self) -> HashSet<T>;

impl <T,I> ToSet<T> for I
where T: Eq + Hash, I: Iterator<Item=T> {

    fn to_set(self) -> HashSet<T> {


let intersect = fruit.intersection(&colours).to_set();

As with all Rust generics, you do need to constrain types - this can only be implemented for types that understand equality (Eq) and for which a ‘hash function’ exists (Hash). Remember that there is no type called Iterator, so I represents any type that implements Iterator.

This technique for implementing our own methods on standard library types may appear to be a little too powerful, but again, there are Rules. We can only do this for our own traits. If both the struct and the trait came from the same crate (particularly, the stdlib) then such implemention would not be allowed. In this way, you are saved from creating confusion.

Before congratulating ourselves on such a clever, convenient shortcut, you should be aware of the consequences. If make_set was written so, so that these are sets of owned strings, then the actual type of intersect could come as a surprise:

fn make_set(words: &str) -> HashSet<String> {
    words.split_whitespace().map(|s| s.to_string()).collect()
// intersect is HashSet<&String>!
let intersect = fruit.intersection(&colours).to_set();

And it cannot be otherwise, since Rust will not suddenly start making copies of owned strings. intersect contains a single &String borrowed from fruit. I can promise you that this will cause you trouble later, when you start patching up lifetimes! A better solution is to use the iterator’s cloned method to make owned string copies of the intersection.

// intersect is HashSet<String> - much better
let intersect = fruit.intersection(&colours).cloned().to_set();

A more robust definition of to_set might be self.cloned().collect(), which I invite you to try out.

Example: Interactive command processing

It’s often useful to have an interactive session with a program. Each line is read in and split into words; the command is looked up on the first word, and the rest of the words are passed as an argument to that command.

A natural implementation is a map from command names to closures. But how do we store closures, given that they will all have different sizes? Boxing them will copy them onto the heap:

Here’s a first try:

    let mut v = Vec::new();
    v.push(Box::new(|x| x * x));
    v.push(Box::new(|x| x / 2.0));

    for f in v.iter() {
        let res = f(1.0);
        println!("res {}", res);

We get a very definite error on the second push:

  = note: expected type `[ 4:28]`
  = note:    found type `[ 5:28]`
note: no two closures, even if identical, have the same type

rustc has deduced a type which is too specific, so it’s necessary to force that vector to have the boxed trait type before things just work:

    let mut v: Vec<Box<Fn(f64)->f64>> = Vec::new();

We can now use the same trick and keep these boxed closures in a HashMap. We still have to watch out for lifetimes, since closures can borrow from their environment.

It’s tempting as first to make them FnMut - that is, they can modify any captured variables. But we will have more than one command, each with its own closure, and you cannot then mutably borrow the same variables.

So the closures are passed a mutable reference as an argument, plus a slice of string slices (&[&str]) representing the command arguments. They will return some Result - We’ll use String errors at first.

D is the data type, which can be anything with a size.

type CliResult = Result<String,String>;

struct Cli<'a,D> {
    data: D,
    callbacks: HashMap<String, Box<Fn(&mut D,&[&str])->CliResult + 'a>>

impl <'a,D: Sized> Cli<'a,D> {
    fn new(data: D) -> Cli<'a,D> {
        Cli{data: data, callbacks: HashMap::new()}

    fn cmd<F>(&mut self, name: &str, callback: F)
    where F: Fn(&mut D, &[&str])->CliResult + 'a {

cmd is passed a name and any closure that matches our signature, which is boxed and entered into the map. Fn means that our closures borrow their environment but can’t modify it. It’s one of those generic methods where the declaration is scarier than the actual implementation! Forgetting the explicit lifetime is a common error here - Rust won’t let us forget that these closures have a lifetime limited to their environment!

Now for reading and running commands:

    fn process(&mut self,line: &str) -> CliResult {
        let parts: Vec<_> = line.split_whitespace().collect();
        if parts.len() == 0 {
            return Ok("".to_string());
        match self.callbacks.get(parts[0]) {
            Some(callback) => callback(&mut,&parts[1..]),
            None => Err("no such command".to_string())

    fn go(&mut self) {
        let mut buff = String::new();
        while io::stdin().read_line(&mut buff).expect("error") > 0 {
                let line = buff.trim_left();
                let res = self.process(line);
                println!("{:?}", res);


This is all reasonably straightforward - split the line into words as a vector, look up the first word in the map and call the closure with our stored mutable data and the rest of the words. An empty line is ignored and not considered an error.

Next, let’s define some helper functions to make it easier for our closures to return correct and incorrect results. There’s a little bit of cleverness going on; they are generic functions that work for any type that can be converted to a String.

fn ok<T: ToString>(s: T) -> CliResult {

fn err<T: ToString>(s: T) -> CliResult {

So finally, the Main Program. Look at how ok(answer) works - because integers know how to convert themselves to strings!

use std::error::Error;

fn main() {
    println!("Welcome to the Interactive Prompt! ");

    struct Data {
        answer: i32

    let mut cli = Cli::new(Data{answer: 42});

    cli.cmd("go",|data,args| {
        if args.len() == 0 { return err("need 1 argument"); }
        data.answer = match args[0].parse::<i32>() {
            Ok(n) => n,
            Err(e) => return err(e.description())
        println!("got {:?}", args);

    cli.cmd("show",|data,_| {


The error handling is a bit clunky here, and we’ll later see how to use the question mark operator in cases like this. Basically, the particular error std::num::ParseIntError implements the trait std::error::Error, which we must bring into scope to use the description method - Rust doesn’t let traits operate unless they’re visible.

And in action:

Welcome to the Interactive Prompt!
go 32
got ["32"]
goop one two three
Err("no such command")
go 42 one two three
got ["42", "one", "two", "three"]
go boo!
Err("invalid digit found in string")

Here are some obvious improvements for you to try. First, if we give cmd three arguments with the second being a help line, then we can store these help lines and automatically implement a ‘help’ command. Second, having some command editing and history is very convenient, so use the rustyline crate from Cargo.