Module 4HashMap: Add, Get, RemoveLesson 7 of 2322 min

HashMap Insert, Get, Update

insert, get, get_mut, contains_key — and owned String keys.

Create & insert

rust

use std::collections::HashMap;

fn main() {
    let mut m: HashMap<String, i32> = HashMap::new();
    m.insert("alice".into(), 10);
    m.insert("bob".into(), 7);
    println!("alice={:?}", m.get("alice"));
    println!("carol={:?}", m.get("carol"));
    println!("has bob={}", m.contains_key("bob"));
}

Output

alice=Some(10)
carol=None
has bob=true

Get

rust

m.get("alice")      // Option<&i32>
m.get_mut("alice")  // Option<&mut i32>
m.contains_key("bob")

Update patterns

rust

*m.entry("alice".into()).or_insert(0) += 5;
// or
if let Some(v) = m.get_mut("bob") { *v += 1; }

Iteration

rust

use std::collections::HashMap;

fn main() {
    let mut m: HashMap<String, i32> = HashMap::new();
    m.insert("alice".into(), 10);
    m.insert("bob".into(), 7);
    let mut keys: Vec<_> = m.keys().cloned().collect();
    keys.sort(); // stable demo — HashMap itself is unordered
    for k in keys {
        println!("{k} {}", m[&k]);
    }
}

Output

alice 10
bob 7

Order is random — sort keys if you need stable output.

Complexity

Average O(1) insert/get/remove.

Challenge

Use entry API to set default 0 then += 1 for keys in ["a","b","a","c","a"].

Tips

get returns Option
entry avoids double lookup
HashMap order is unstable

Try it live

HashMap Insert, Get, UpdateRust 2021

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Output

Press Run to compile and execute.