Important Rust Topics

Rust HashMap

Store values under keys and look them up fast: insert, get, entry and counting.

Advanced
3 min read
1 runnable example

What you'll learn

insert and get
entry().or_insert()
Sorting the keys

Run code →

stock

"apple"5

"pear"5

"plum"1

Values found by key

A HashMap<K, V> stores values under keys, so you can find a value quickly by its key. It is Rust's version of a dictionary or associative array.

insert adds or replaces a value. entry(key).or_insert(0) returns the existing value or inserts a default, which makes counting easy. get returns an Option because the key may be missing. A HashMap has no fixed order, so the keys are sorted before printing.

Key points

get returns an Option: the key may be missing.
entry(key).or_insert(0) is the counting pattern.
A HashMap has no fixed order.

Runnable Rust example

Rust

Run code →

main.rs

use std::collections::HashMap;

fn main() {
    let mut stock: HashMap<String, u32> = HashMap::new();
    stock.insert(String::from("apple"), 3);
    stock.insert(String::from("pear"), 5);

    *stock.entry(String::from("apple")).or_insert(0) += 2;
    stock.entry(String::from("plum")).or_insert(1);

    println!("apples: {}", stock["apple"]);
    println!("has kiwi: {}", stock.contains_key("kiwi"));

    if let Some(count) = stock.get("pear") {
        println!("pears: {count}");
    }

    let mut names: Vec<&String> = stock.keys().collect();
    names.sort();
    println!("{:?}", names);
}

Get a Run button for this example Result of running the code

apples: 5
has kiwi: false
pears: 5
["apple", "pear", "plum"]

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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