C++ unordered_map - insert() Function

The C++ unordered_map::insert function is used to insert new elements in the container. This results into increasing the unordered_map size by the number of elements inserted. As the element's key in a unordered_map are unique, therefore the insertion operation first checks if the inserted element's key is unique to the map then the element is inserted.

Syntax

C++11

//single element version 
pair<iterator,bool> insert (const value_type& val);
template <class P> pair<iterator,bool> insert (P&& val);

//single element with hint version   
iterator insert (const_iterator position, const value_type& val);
template <class P> iterator insert (const_iterator position, P&& val);

//range version 
template <class InputIterator>
  void insert (InputIterator first, InputIterator last);

//initializer list version 
void insert (initializer_list<value_type> il);

Parameters

`position`	Specify the hint for the position where the element can be inserted.
`val`	Specify the value to be copied (or moved) to the inserted elements.
`first`	Specify the starting position of InputIterator. Copies of the elements in the range [first,last) are inserted in the container.
`last`	Specify the last position of InputIterator. Copies of the elements in the range [first,last) are inserted in the container.
`il`	Specify the initializer_list object.

Return Value

The single element version returns a pair, with pair::first unordered_map to an iterator pointing to either newly inserted element or to element with the equivalent key in the unordered_map. The pair::second is unordered_map to true if new element is inserted in the unordered_map, false otherwise.

The single element with hint version returns an iterator pointing to either newly inserted element or to the equivalent key present in the unordered_map.

Time Complexity

Single element insertion: Average case - constant i.e, Θ(1). Worst case - linear i.e, Θ(n).
Multiple elements insertion: Average case - linear in number of elements inserted. Worst case - quadratic: number of elements inserted multiplied by (container size + 1).

Example:

In the example below, the unordered_map::insert function is used to insert elements in the given unordered_map.

#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";

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

  //insert single element in the map 
  uMap.insert(pair<int, string>(104, "Jo"));

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

  //insert single element with hint in the map
  it = uMap.begin();
  uMap.insert(it, pair<int, string>(105, "Ramesh"));

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

Example:

A range of elements can also be inserted into a unordered_map. Consider the example below.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap1;
  unordered_map<int, string> uMap2;
  unordered_map<int, string>::iterator it;

  //populating uMap1
  uMap1[101] = "John";
  uMap1[102] = "Marry";
  uMap1[103] = "Kim";

  //populating uMap2
  uMap2[104] = "Jo";
  uMap2[105] = "Ramesh";

  cout<<"uMap1 contains: \n ";
  for(it = uMap1.begin(); it != uMap1.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  //inserts a range of elements from uMap2 to uMap1 
  uMap1.insert(uMap2.begin(), uMap2.end());

  cout<<"\nuMap1 contains: \n ";
  for(it = uMap1.begin(); it != uMap1.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  return 0;
}

The output of the above code will be:

uMap1 contains: 
 103  Kim
 102  Marry
 101  John
 
uMap1 contains: 
 104  Jo
 105  Ramesh
 103  Kim
 102  Marry
 101  John

Example:

Similarly, the initializer list version can be used to insert elements in the given unordered_map.

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

  //populating map
  uMap[101] = "John";
  uMap[102] = "Marry";
  uMap[103] = "Kim";

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

  //insert elements of initializer list into map 
  initializer_list<pair<const int, string>> ilist = 
    {{104,"Jo"}, {105, "Ramesh"}};
  uMap.insert(ilist); 

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

❮ C++ <unordered_map> Library

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