Hash Map in Rust

Hash map is one of common collections in Rust, with key value as a pair. HashMap<K, V>

It’s useful when we want to store and retrieve data by a certain name/key not by the index

Create a Hashmap

We can use insert method. Insert take 2 params. First the string as a key, and second is the value.

use std::collections::HashMap;

let mut scores = HashMap::new();

scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Yellow"), 50);

Warning: Hash maps are homogeneous: all of the keys must have the same type, and all of the values must have the same type.

Retrieve value from Hashmap

We can retrieve the values using get method

  use std::collections::HashMap;

  let mut scores = HashMap::new();

  scores.insert(String::from("Blue"), 10);
  scores.insert(String::from("Yellow"), 50);

  let team_name = String::from("Blue");
  let score = scores.get(&team_name);

How to iterate key value pair of Hashmap

We can iterate all the key value pair with

for (key, value) in &scores {
      println!("{}: {}", key, value);
  }

Update value of key

Each value in hashmap can only have one key. If we re-assign a value with the same key name, it will overwrite the previous one.

If we want to be safe, we can use or_insert to only insert data if the key don’t have any value yet

 scores.entry(String::from("Blue")).or_insert(50);

Storing two vectors in a Hashmap

Assume we have two different vector we get from some type of database. We want to store them as key -value pair in hashmap. Here is how:

  use std::collections::HashMap;

let teams = vec![String::from("Blue"), String::from("Yellow")];
let initial_scores = vec![10, 50];

let mut scores: HashMap<_, _> =
    teams.into_iter().zip(initial_scores.into_iter()).collect();

collect()return hash map from iterated tuples
zip() create an iterator of tuples.