Hash maps

Last reviewed May 28, 2026 Content v20260528

Track mode: server_compiled
Means: Compiled runner
Reading: ~1 min
Level: intermediate

This lesson

This lesson teaches Hash maps: the syntax, APIs, and habits you need before advancing in Rust.

Teams ship Hash maps on every Rust codebase—skipping it leaves gaps in debugging and code reviews.

You will apply Hash maps in contexts like: Infrastructure CLIs, proxies, game engines, blockchain nodes, and latency-sensitive backends.

Write Rust with fn main(), click Run on server—the dev runner compiles main.rs with rustc and runs the binary; fix borrow errors from stderr (requires Rust toolchain; LEARNING_RUNNER_ENABLED=true).

When you can explain the previous lesson's ideas without copying starter code.

HashMap<K, V> stores key-value pairs with average O(1) lookup. Keys need Hash and Eq; ownership rules apply when inserting owned keys.

Insert and get

use std::collections::HashMap;
let mut m = HashMap::new();
m.insert("a", 1);

Self-check

Which trait do keys need for hashing?
What does get return for missing keys?

Entry API

entry(key).or_insert(0) avoids double lookup when counting frequencies—common in interview “group by” questions.

Iterating a map yields references; order is randomized—do not rely on key order unless using BTreeMap.

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

Self-reflection (saved on this device)

Not saved yet.

Playground

Runs on the configured server runner (dev: npm run runner with LEARNING_RUNNER_ENABLED=true). Output appears below the editor.

Code runner not available

Server runner is disabled. Set LEARNING_RUNNER_ENABLED=true and LEARNING_RUNNER_URL in .env (see .env.example).

Check yourself

Multiple choice — immediate feedback.

Discussion

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Starter discussion topics

Entry API when?
Hash requirement?

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