Unordered Maps

Unordered Maps are associative containers that have elements with key-value pairs. Unlike Maps, the pairs in Unordered Maps are not sorted by their keys. Each mapped value must have a unique key value.

Syntax

An empty unordered map can be created by using the unordered_map keyword, declaring the data types of the key and value, and setting a mapName:

std::unordered_map<type1, type2> mapName;

• type1: Date type of the key in the unordered_map.
• type2: Date type of the value in the unordered_map.

Note: To use unordered_map, including the unordered_map library is necessary.

Examples

The below example shows how to use unordered_map.

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, int> myMap;

    // Inserting elements
    myMap["apple"] = 2;
    myMap["banana"] = 5;
    myMap["orange"] = 3;

    // Iterating over elements
    for (const std::pair<const std::string, int>& x : myMap) {
        std::cout << x.first << " " << x.second << std::endl;
    }
    return 0;
}

The output will be:

orange 3
banana 5
apple 2

Overriding Functions

By default, keys are treated as case-sensitive, but this can be avoided, so they can be treated as case-insensitive. In the example below, the keys are treated as case-insensitive.

#include <iostream>
#include <unordered_map>
#include <string>

struct CustomHash {
  std::size_t operator()(const std::string & key) const {
    // Custom hash: hash based on the length of the string
    return key.length();
  }
};

struct CustomEqual {
  bool operator()(const std::string & lhs,
    const std::string & rhs) const {
    // Custom equality: compare strings ignoring case
    return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end(),
      [](char a, char b) {
        return tolower(a) == tolower(b);
      });
  }
};

int main() {
  // Unordered map with custom hash and equality functions
  std::unordered_map < std::string, int, CustomHash, CustomEqual > myMap;

  myMap["apple"] = 1;

  // Will be treated as the same key as "apple"
  myMap["APPLE"] = 2;

  for (const auto & pair: myMap) {
    std::cout << pair.first << ": " << pair.second << std::endl;
  }
  return 0;
}

The above code snippet will return the following output:

apple: 2

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