Performance Patterns

Hash Map Lookup

Cache Values by Key

A hash map stores values by key so repeated lookups do not need a linear scan.

Program

Play the program to choose a user id and see the cached credit value returned.

hash_map_lookup.rs 
use std::collections::HashMap;

fn main() {
    let user_id = ;
    let mut credits = HashMap::new();
    credits.insert(1, 10);
    credits.insert(2, 25);
    let total = lookup_credit(&credits, user_id);
    println!("user {user_id}: {total}");
}

fn lookup_credit(credits: &HashMap<i32, i32>, user_id: i32) -> i32 {
    credits.get(&user_id).copied().unwrap_or(0)
}

use std::collections::HashMap;

fn main() {
    let user_id = ;
    let mut credits = HashMap::new();
    credits.insert(1, 10);
    credits.insert(2, 25);
    let total = lookup_credit(&credits, user_id);
    println!("user {user_id}: {total}");
}

fn lookup_credit(credits: &HashMap<i32, i32>, user_id: i32) -> i32 {
    credits.get(&user_id).copied().unwrap_or(0)
}

use std::collections::HashMap;

fn main() {
    let user_id = ;
    let mut credits = HashMap::new();
    credits.insert(1, 10);
    credits.insert(2, 25);
    let total = lookup_credit(&credits, user_id);
    println!("user {user_id}: {total}");
}

fn lookup_credit(credits: &HashMap<i32, i32>, user_id: i32) -> i32 {
    credits.get(&user_id).copied().unwrap_or(0)
}
hash map `HashMap` stores values under keys for direct lookup.
copied `copied()` turns an `Option<&i32>` into an `Option<i32>` for this small value type.
fallback `unwrap_or(0)` supplies a default when a key is not cached.