cpp-flashcards/org/cpp/map.org

• Task: Create a std::map with initial values
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• Task: Access a map element safely with at()
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• Task: Insert into map with insert()
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• Task: Insert or assign with insert_or_assign
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• Task: Construct element in-place with emplace
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• Task: Insert only if key missing with try_emplace
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• Task: Iterate over all key-value pairs
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• Task: Check if key exists with contains (C++20)
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• Task: Find element and check existence
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• Task: Erase element by key
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• Task: Range query with lower_bound and upper_bound
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• std::map vs std::unordered_map
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• What is value_type for std::map?
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• insert() return type
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• Why avoid operator[] for lookup?
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• What does extract() do? (C++17)
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• What does merge() do? (C++17)
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Task: Create a std::map with initial values   cpp map retrieval production

Front

Write C++ code to create a std::map<std::string, int> initialized with {{"apple", 1}, {"banana", 2}}

Back

std::map<std::string, int> m = {{"apple", 1}, {"banana", 2}};

Task: Access a map element safely with at()   cpp map retrieval production

Front

Write code to safely access m.at("apple") from a std::map<std::string, int>

Back

std::map<std::string, int> m = {{"apple", 1}};
int val = m.at("apple");  // returns 1, throws if not found

Task: Insert into map with insert()   cpp map retrieval production

Front

Write code to insert {"cherry", 3} into a std::map<std::string, int> using insert()

Back

auto [it, success] = m.insert({"cherry", 3});
// Returns pair<iterator, bool> — success is true if inserted

Task: Insert or assign with insert_or_assign   cpp map retrieval production

Front

Write code to insert {"apple", 99} or update it if it already exists

Back

auto [it, inserted] = m.insert_or_assign("apple", 99);
// If "apple" exists, updates value to 99
// inserted is false if key existed, true if newly inserted

Task: Construct element in-place with emplace   cpp map retrieval production

Front

Write code to construct a std::map<std::string, std::vector<int>> element in-place without copying

Back

std::map<std::string, std::vector<int>> m;
m.emplace(std::piecewise_construct,
          std::forward_as_tuple("nums"),
          std::forward_as_tuple(3, 1));  // vector with {1,1,1}

Task: Insert only if key missing with try_emplace   cpp map retrieval production

Front

Write code to only insert "banana" if it's NOT already in the map

Back

m.try_emplace("banana", 99);
// Does nothing if "banana" exists, inserts if missing
// Unlike insert_or_assign, never overwrites existing value

Task: Iterate over all key-value pairs   cpp map retrieval production

Front

Write code to iterate over a std::map<std::string, int> and print all key-value pairs

Back

for (const auto& [key, val] : m) {
    std::cout << key << ": " << val << "\n";
}

Task: Check if key exists with contains (C++20)   cpp map retrieval production

Front

Write code to check if "apple" exists in a std::map<std::string, int> using C++20

Back

if (m.contains("apple")) {
    // key exists
}

Task: Find element and check existence   cpp map retrieval production

Front

Write code to find "apple" in a map and only process if found

Back

auto it = m.find("apple");
if (it != m.end()) {
    // it->first = key, it->second = value
    int val = it->second;
}

Task: Erase element by key   cpp map retrieval production

Front

Write code to remove the element with key "apple" from a std::map

Back

size_t erased = m.erase("apple");
// Returns number of elements erased (0 or 1)

Task: Range query with lower_bound and upper_bound   cpp map retrieval production

Front

Write code to find all elements with keys between "apple" and "cherry" (inclusive)

Back

auto [start, end] = m.equal_range("apple");
// lower_bound = first >= "apple"
// upper_bound = first > "apple"
for (auto it = start; it != end; ++it) {
    // process it->first, it->second
}

std::map vs std::unordered_map   cpp map retrieval comparison

Front

What are the key differences between std::map and std::unordered_map?

Back

• std::map: sorted by key, O(log n) operations, uses red-black tree
• std::unordered_map: unsorted, O(1) avg operations, uses hash table
• Use std::map when you need sorted iteration or range queries
• Use std::unordered_map when you need fastest lookups and order doesn't matter

What is value_type for std::map?   cpp map retrieval recognition

Front

What is the value_type of std::map<Key, T>?

Back

std::pair<const Key, T>

insert() return type   cpp map retrieval recognition

Front

What does insert() return on a std::map?

Back

std::pair<iterator, bool>

• iterator points to the inserted (or existing) element
• bool is true if insertion happened, false if key already existed

Why avoid operator[] for lookup?   cpp map retrieval recognition

Front

Why is it risky to use map["missing"] to check if a key exists?

Back

operator[] inserts a default-constructed value if the key doesn't exist, modifying the map as a side effect. Use find() or contains() (C++20) for read-only lookup.

What does extract() do? (C++17)   cpp map retrieval recognition

Front

What does m.extract(key) do on a std::map?

Back

Removes the element with that key and returns a node_type (a node handle). The node can be inserted into another container without copying. O(log n) but no allocator overhead.

What does merge() do? (C++17)   cpp map retrieval recognition

Front

What does m1.merge(m2) do?

Back

Transfers elements from m2 to m1. For each element transferred, if a key already exists in m1, that element stays in m2. No element copies are made.