Maps

Maps are associative containers that have elements with key-value pairs. The keys are used to sort and identify the elements, while the values store the content associated with the keys. Each mapped value must have a unique key value.

Syntax

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

std::map<type1, type2> mapName;

type1 and type2 are the data types of the key and value, respectively.

To set a list at declaration, the following syntax is used:

std::map<type1, type2> mapName { {key1, value1}, {key2, value2}, ...};

Each value must have a unique key assigned to it.

Example

The following example creates an empty map, emptyMap, and a map set with values, clothingStore:

#include <iostream>
#include <iterator>
#include <map>

int main() {
  // Initializing empty map
  std::map<std::string, int> emptyMap;

  // Initializing map with items
  std::map<std::string, int> clothingStore {{"tshirt", 10}, {"pants", 12}, {"sweaters", 18}};
}

Accessing Elements

Elements can be accessed within a map using the following square bracket syntax:

mapName[key]

This returns the mapped value associated with the key.

Example

The sweaters element is retrieved from the clothingStore map initialized in the previous example:

std::cout << clothingStore["sweaters"]; // Output: 18

Comparison Function

By default, elements are sorted by their key in ascending order.

#include <iostream>
#include <iterator>
#include <map>

int main() {
 // Initializing map with items
  std::map<int, std::string> reptiles {
    {10, "Komodo Dragon"}, {15, "Saltwater Crocodile"}, {8, "Leatherback Sea Turtle"}
  };


  // Initializing iterator
  std::map<int, std::string> :: iterator iter;

  for (iter = reptiles.begin(); iter != reptiles.end(); ++iter) {
    std::cout << '\t' << iter->first << '\t' << iter->second
    << '\n';
  }
}

The snippet above outputs the following:

8   Leatherback Sea Turtle
10  Komodo Dragon
15  Saltwater Crocodile

The default comparison function can be changed to std::greater<dataType> in order to sort the elements in descending order.

Syntax

To set the comparison function while initializing an empty map:

std::map<type1, type2, std::greater<dataType>> mapName;

To set the comparison function while initializing a map with items:

std::map<type1, type2, std::greater<dataType>> mapName { {key1, value1}, {key2, value2}, ...};

The dataType for the comparison function above must be the same as type1 which is the data type for the keys.

Codebyte Example

Setting the previous example’s comparison function to std::greater<int>:

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