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)
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    // 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;
          
    }

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    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.
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    // 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;
          
    }

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    Output:

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

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