C++ unordered_map - erase() Function

The C++ unordered_map::erase function is used to delete either a single element or a range of elements from the unordered_map. It reduces the size of the unordered_map by number of elements deleted from the container.

Syntax

C++11

//Version 1 - Delete element at position
iterator  erase (const_iterator position);

//Version 2 - Delete specified key from the unordered_map
size_type erase (const key_type& k);

//Version 3 - Delete a range of elements in [first, last)
iterator  erase (const_iterator first, const_iterator last);

Parameters

`position`	Iterator specifying position of the element in the unordered_map to be removed.
`k`	Key of the element to be removed from the unordered_map.
`first`	Iterator specifying position of the first element of the range in the unordered_map. Elements in [first, last) position range will be deleted.
`last`	Iterator specifying position of the last element of the range in the unordered_map. Elements in [first, last) position range will be deleted.

Return Value

For version 2: the function returns number of elements erased. For other versions, the function returns an iterator pointed to the element that follows the last element removed. If the last element is removed, then the iterator will point to the unordered_map::end.

Time Complexity

Average case: Linear in the number of elements removed for version 1 and version 2.
Worst case: Linear in the container size..

Example:

In the example below, the unordered_map::erase function is used to delete a single element from uMap.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap;
  unordered_map<int, string>::iterator it;

  //populating unordered_map
  uMap[101] = "John";
  uMap[102] = "Marry";
  uMap[103] = "Kim";
  uMap[104] = "Jo";
  uMap[105] = "Ramesh";

  cout<<"uMap contains: \n ";
  for(it = uMap.begin(); it != uMap.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  //version 1: deletes element at position = 2
  it = uMap.begin();
  it++;
  uMap.erase(it);

  //version 2: deletes key=103 from the unordered_map
  uMap.erase(103);

  cout<<"\nuMap contains: \n ";
  for(it = uMap.begin(); it != uMap.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  return 0;
}

The output of the above code will be:

uMap contains: 
 105  Ramesh
 104  Jo
 103  Kim
 102  Marry
 101  John
 
uMap contains: 
 105  Ramesh
 102  Marry
 101  John

Example:

A range of elements can also be deleted from the unordered_map. Consider the example below:

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap;
  unordered_map<int, string>::iterator start_it;
  unordered_map<int, string>::iterator stop_it;
  unordered_map<int, string>::iterator it;

  //populating map
  uMap[101] = "John";
  uMap[102] = "Marry";
  uMap[103] = "Kim";
  uMap[104] = "Jo";
  uMap[105] = "Ramesh";

  cout<<"uMap contains: \n ";
  for(it = uMap.begin(); it != uMap.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";


  //setting the start position at 
  //third element of the map
  start_it = uMap.begin();
  start_it++;
  start_it++;

  //setting the stop position at end of the map
  stop_it = uMap.end();

  //version 3: erase a range of elements
  uMap.erase(start_it, stop_it);

  cout<<"\nuMap contains: \n ";
  for(it = uMap.begin(); it != uMap.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  return 0;
}

The output of the above code will be:

uMap contains: 
 105  Ramesh
 104  Jo
 103  Kim
 102  Marry
 101  John
 
uMap contains: 
 105  Ramesh
 104  Jo

❮ C++ <unordered_map> Library