Maps and sets intro

Last reviewed May 28, 2026 Content v20260528

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

This lesson

An orientation to the C++ track—how the compiled playground works, core vocabulary, and what you will practice next.

You need a clear map of the C++ track so classes, RAII, templates, and the STL do not feel like magic.

You will apply Maps and sets intro in contexts like: Game engines, trading systems, desktop apps, and performance-critical libraries.

Write C++ in main.cpp with int main(), click Run on server—the dev runner compiles with c++/g++ -std=c++17 -Wall and runs the binary; read template errors in stderr (LEARNING_RUNNER_ENABLED=true). Also read the interview prep blocks.

After the C track or equivalent—C++ builds on C memory ideas and adds OOP, templates, and the STL.

Associative containers store keys efficiently. std::map holds sorted key-value pairs; std::set holds unique sorted keys.

map example

std::map<std::string, int> scores;
scores["ada"] = 100;
scores["linus"] = 95;

For frequent lookups without order requirement, std::unordered_map offers average O(1) hashing (also STL).

Important interview questions and answers

Q: map vs unordered_map?
A: map keeps keys sorted (tree); unordered_map uses hash table—pick based on ordering and performance needs.
Q: operator[] on map?
A: Inserts default-constructed value if key missing—use .at() or .find() when insert is undesired.

Self-check

What does std::set guarantee about duplicates?
When prefer unordered_map?

Pitfall: map[key] inserts default values—use .find() or .at() when insertion side effects are unwanted.

Interview prep

map vs unordered_map?: map keeps sorted keys (tree); unordered_map uses hashing for average O(1) lookup.
operator[] side effect?: Inserts default value if key missing—use .at() or .find() when insert is undesired.

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

map vs unordered_map?
Key requirements?

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