freeCodeCamp/guide/english/cplusplus/map/index.md

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Map

Introduction of map

map is an associative container that store elements in key-value pair. Just like in Java we have collection, associative array in PHP and so on.

Benefits of using map

• It stores only unique keys and that too in sorted order based on its assigned sorting criteria.
• As keys are in sorted order therefore searching element in map through key is very fast i.e. it takes logarithmic time.
• In map there will be only one value attached with the every key.
• map can be used as associative arrays.
• It might be implemented using balanced binary trees.

Here is an example:

#include <iostream>
#include <map>

using namespace std;

int main (){
  map<char,int> first;
  
  //initializing
  first['a']=10;
  first['b']=20;
  first['c']=30;
  first['d']=40;
  
   map<char, int>::iterator it;
   for(it=first.begin(); it!=first.end(); ++it){
      cout << it->first << " => " << it->second << '\n';
   }
   
  return 0;
}

Output:

a => 10
b => 20
c => 30
d => 40

Creating map object

map<string, int> myMap;

Get Size

Get size of map with size function

map<int, int > myMap;
myMap[100] = 3
count << "size of map is " << myMap.size() << '\n';

Output:

size of map is 1

Insertion

Inserting data with insert member function.

myMap.insert(make_pair("earth", 1));
myMap.insert(make_pair("moon", 2));

We can also insert data in std::map using operator [] i.e.

myMap["sun"] = 3;

If "sun" is already mapped before, this action will override the value mapped to key.

Erase

Erasing data with erase function

map<int, int > myMap;
myMap[10] = 1000;
cout << "before erase, size of map is " << myMap.size() << '\n';
myMap.erase(10);
cout << "after erase, size of map is " << myMap.size() << '\n';

Output:

before erase, size of map is 1
after erase, size of map is 0

Accessing map value

To access map values, simply call Map[key]. For example:

map<string, int > M;
M["abc"] = 1;
M["def"] = 2;
cout << "value of abc is " << M["abc"] << '\n';
cout << "value of def is " << M["def"] << '\n';

Output:

value of abc is 1
value of def is 2

Accessing map elements

To access map elements, you have to create iterator for it. Here is an example as stated before.

map<char, int>::iterator it;
for(it=first.begin(); it!=first.end(); ++it){
  cout << it->first << " => " << it->second << '\n';
}

N.B: All code in example are in C++11 version. You can learn more about C++ version Here

Unordered Map

There exists another associative container similar to map, named unordered_map, which is implemented using a Hash Table as opposed to a balanced binary tree as is the case with map. The key values of the unordered_map are hashed into indices of the hash table. Unlike map the elements of unordered_map are not stored in a sorted manner. All operations permissible on map are applicable to unordered_map as well.

Benefits of Unordered Map

• Average Cost of insert, delete, search is O(1) if there are no hash collisions.
• Implemented using Hash Table

Unordered Map vs Map

• Unordered Map is faster if you want single element access.
• Use map when you want to traverse through key values in a sorted fashion or if you want some kind of ordering in the map.
• Unordered Map has more memory overhead, although it is typically negligible.

References:

For more information about the differences between map and unordered_map: Map vs Unordered Map