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

25 May, 2018
This article deals with the deletion part of Maps.
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 operationsusing 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
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 operationsusing 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 : 

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 operationsusing 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

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 operationsusing 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

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 operationsusing 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 : 

// C++ code to demonstrate the working of clear()  #include<iostream>#include<map> // for map operationsusing 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 : 

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