Different ways to delete elements in std::map (erase() and clear())

This article deals with the deletion part of Maps.

1. Using erase() : erase() is used to erase the pair in map mentioned in argument, either its position, its value or a range of number.
   • erase(key) : Erases the key-value pair using key mentioned in its argument. reorders the map after deletion. It returns the number of entries deleted. If non-existing keys is deleted, 0 is returned.
     Time complexity : log(n) (n is size of map)
   • erase(iter) : Erases the pair at the position pointed by the iterator mentioned in its argument.
     Time complexity : log(n) (n is size of map)
   • erase(strt_iter, end_iter) : Erases the range of pairs starting from “strt_iter” to the “end_iter”.
     Time complexity : O(k) (k is size of map)

   // C++ code to demonstrate the working of erase()
   
   #include<iostream>
   #include<map> // for map operations
   using namespace std;
   
   int main()
   {
       // declaring map
       // of char and int
       map< char, int > mp;
   
       // declaring iterators
       map<char, int>::iterator it ;
       map<char, int>::iterator it1;
       map<char, int>::iterator it2;
   
       // inserting values
       mp['a']=5;
       mp['b']=10;
       mp['c']=15;
       mp['d']=20;
       mp['e']=30;
   
       // printing initial map elements
       cout << "The initial map elements are : \n";
   
       for (it1 = mp.begin(); it1!=mp.end(); ++it1)
           cout << it1->first << "->" << it1->second << endl;
   
       it = mp.begin();
   
       cout << endl;
   
       // erasing element using iterator
       // erases 2nd element
       // 'b'
       ++it;
       mp.erase(it);
   
       // printing map elements after deletion
       cout << "The map elements after 1st deletion are : \n";
   
       for (it1 = mp.begin(); it1!=mp.end(); ++it1)
           cout << it1->first << "->" << it1->second << endl;
   
       cout << endl;
   
       // erasing element using value
       int c = mp.erase('c');
   
       // printing map elements after deletion
       cout << "The map elements after 2nd deletion are : \n";
   
       for (it1 = mp.begin(); it1!=mp.end(); ++it1)
           cout << it1->first << "->" << it1->second << endl;
   
       cout << "The number of elements deleted in 2nd deletion are : ";
       cout << c << endl;
   
       cout << endl;
   
       // erasing element using value
       // key not present
       int d = mp.erase('w');
   
       // printing map elements after deletion
       cout << "The map elements after 3rd deletion are : \n";
   
       for (it1 = mp.begin(); it1!=mp.end(); ++it1)
           cout << it1->first << "->" << it1->second << endl;
   
       cout << "The number of elements deleted in 3rd deletion are : ";
       cout << d << endl;
   
       cout << endl;
   
   
       ++it;
       ++it;
   
       // erasing element using range iterator
       // deletes "d" and "e" keys
       mp.erase(it, mp.end());
   
       // printing map elements 4th deletion
       cout << "The map elements after 4th deletion are : \n";
   
       for (it1 = mp.begin(); it1!=mp.end(); ++it1)
           cout << it1->first << "->" << it1->second << endl;
   
       cout << endl;
   
   }

   Output:

   The initial map elements are : 
   a->5
   b->10
   c->15
   d->20
   e->30
   
   The map elements after 1st deletion are : 
   a->5
   c->15
   d->20
   e->30
   
   The map elements after 2nd deletion are : 
   a->5
   d->20
   e->30
   The number of elements deleted in 2nd deletion are : 1
   
   The map elements after 3rd deletion are : 
   a->5
   d->20
   e->30
   The number of elements deleted in 3rd deletion are : 0
   
   The map elements after 4th deletion are : 
   a->5
   

2. Using clear() : This function clears all the elements present in the map. After this function is called, the size of map becomes 0.

   // C++ code to demonstrate the working of clear()
   
   #include<iostream>
   #include<map> // for map operations
   using namespace std;
   
   int main()
   {
       // declaring map
       // of char and int
       map< char, int > mp;
   
       // declaring iterator
       map<char, int>::iterator it ;
   
       // inserting values
        mp['a']=5;
        mp['b']=10;
        mp['c']=15;
        mp['d']=20;
        mp['e']=30;
   
       // printing initial map elements
       cout << "The initial map elements are : \n";
       for (it1 = mp.begin(); it1!=mp.end();  ++it1)
       cout << it1->first << "->" << it1->second << endl;
   
       // using clear() to erase all elements in map
       mp.clear();
   
       // printing map elements after deletion
       cout << "The map elements after clearing all elements are : \n";
       for (it1 = mp.begin(); it1!=mp.end();  ++it1)
       cout << it1->first << "->" << it1->second << endl;
   
   }

   Output:

   The initial map elements are : 
   a->5
   b->10
   c->15
   d->20
   e->30
   The map elements after clearing all elements are : 
   

This article is contributed by Manjeet Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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