How do I create a HashMap literal?

Question

How I can create a HashMap literal in Rust? In Python I can do it so:

hashmap = {
   \'element0\': {
       \'name\': \'My New Element\',
       \'childs\':          


        

Answer 1

I have seen a bunch of fancy solutions, but I just wanted something simple. To that end, here is a Trait:

use std::collections::HashMap;

trait Hash {
   fn to_map(&self) -> HashMap<&str, u16>;
}

impl Hash for [(&str, u16)] {
   fn to_map(&self) -> HashMap<&str, u16> {
      self.iter().cloned().collect()
   }
}

fn main() {
   let m = [("year", 2019), ("month", 12)].to_map();
   println!("{:?}", m)
}

I think it's a good option, as it's essentially what's already used by Ruby and Nim:

• https://nim-lang.org/docs/tables.html#toTable,openArray[]
• https://ruby-doc.org/core/Array.html#method-i-to_h

Answer 2

As noted by @Johannes in the comments, it's possible to use vec![] because:

• Vec<T> implements the IntoIterator<T> trait
• HashMap<K, V> implements FromIterator<Item = (K, V)>

which means you can do this:

let map: HashMap<String, String> = vec![("key".to_string(), "value".to_string())]
    .into_iter()
    .collect();

You can use &str but you might need to annotate lifetimes if it's not 'static:

let map: HashMap<&str, usize> = vec![("one", 1), ("two", 2)].into_iter().collect();

Answer 3

I recommend the maplit crate.

To quote from the documentation:

Macros for container literals with specific type.

use maplit::hashmap;

let map = hashmap!{
    "a" => 1,
    "b" => 2,
};

The maplit crate uses => syntax for the mapping macros. It is not possible to use : as separator due to syntactic the restrictions in regular macro_rules! macros.

Note that rust macros are flexible in which brackets you use for the invocation. You can use them as hashmap!{} or hashmap![] or hashmap!(). This crate suggests {} as the convention for the map & set macros, it matches their Debug output.

Macros

• btreemap Create a BTreeMap from a list of key-value pairs
• btreeset Create a BTreeSet from a list of elements.
• hashmap Create a HashMap from a list of key-value pairs
• hashset Create a HashSet from a list of elements.

Answer 4

There isn't a map literal syntax in Rust. I don't know the exact reason, but I expect that the fact that there are multiple data structures that act maplike (such as both BTreeMap and HashMap) would make it hard to pick one.

However, you can create a macro to do the job for you, as demonstrated in Why does this rust HashMap macro no longer work?. Here is that macro simplified a bit and with enough structure to make it runnable in the playground:

macro_rules! map(
    { $($key:expr => $value:expr),+ } => {
        {
            let mut m = ::std::collections::HashMap::new();
            $(
                m.insert($key, $value);
            )+
            m
        }
     };
);

fn main() {
    let names = map!{ 1 => "one", 2 => "two" };
    println!("{} -> {:?}", 1, names.get(&1));
    println!("{} -> {:?}", 10, names.get(&10));
}

This has no additional allocation and is maximally efficient.

---

In a nightly version of Rust, you can avoid both unneeded allocation and the need for a macro:

#![feature(array_value_iter)]

use std::array::IntoIter;
use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
use std::iter::FromIterator;

fn main() {
    let s = Vec::from_iter(IntoIter::new([1, 2, 3]));
    println!("{:?}", s);

    let s = BTreeSet::from_iter(IntoIter::new([1, 2, 3]));
    println!("{:?}", s);

    let s = HashSet::<_>::from_iter(IntoIter::new([1, 2, 3]));
    println!("{:?}", s);

    let s = BTreeMap::from_iter(IntoIter::new([(1, 2), (3, 4)]));
    println!("{:?}", s);

    let s = HashMap::<_, _>::from_iter(IntoIter::new([(1, 2), (3, 4)]));
    println!("{:?}", s);
}

This logic can then be wrapped back into a macro as well:

#![feature(array_value_iter)]

use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};

macro_rules! collection {
    // map-like
    ($($k:expr => $v:expr),* $(,)?) => {
        std::iter::Iterator::collect(std::array::IntoIter::new([$(($k, $v),)*]))
    };
    // set-like
    ($($v:expr),* $(,)?) => {
        std::iter::Iterator::collect(std::array::IntoIter::new([$($v,)*]))
    };
}

fn main() {
    let s: Vec<_> = collection![1, 2, 3];
    println!("{:?}", s);

    let s: BTreeSet<_> = collection! { 1, 2, 3 };
    println!("{:?}", s);

    let s: HashSet<_> = collection! { 1, 2, 3 };
    println!("{:?}", s);

    let s: BTreeMap<_, _> = collection! { 1 => 2, 3 => 4 };
    println!("{:?}", s);

    let s: HashMap<_, _> = collection! { 1 => 2, 3 => 4 };
    println!("{:?}", s);
}

See also:

• Add hashmap, hashset, treemap, and treeset macros

Answer 5

For one element

If you wish to initialize the map with only one element in one line (and without visible mutation in your code), you could do:

let map: HashMap<&'static str, u32> = Some(("answer", 42)).into_iter().collect();

This is thanks to the usefulness of Option being able to become an Iterator using into_iter().

In real code, you probably don't need to help the compiler with the type:

use std::collections::HashMap;

fn john_wick() -> HashMap<&'static str, u32> {
    Some(("answer", 42)).into_iter().collect()
}

fn main() {
    let result = john_wick();

    let mut expected = HashMap::new();
    expected.insert("answer", 42);

    assert_eq!(result, expected);
}

There is also a way to chain this to have more than one element doing something like Some(a).into_iter().chain(Some(b).into_iter()).collect(), but this is longer, less readable and probably has some optimization issues, so I advise against it.

Answer 6

There is an example of how to achieve this in the documentation for HashMap:

let timber_resources: HashMap<&str, i32> = [("Norway", 100), ("Denmark", 50), ("Iceland", 10)]
    .iter()
    .cloned()
    .collect();