C++ map : Erase element by key or Iterator or Range

In this article we will discuss the different ways to delete a key-value pair from map.

std::map provides 3 overloaded version of erase() to remove elements from map i.e.

• Erase by key
• Erase by Iterator
• Erase a range

Lets discuss them one by one,

Erase Element from Map by Key

std::map provides a erase function that accepts the key and removes the elements (Key- Value pairs) whose key matches the passed key k.

size_type erase (const key_type& k);

It returns the number of elements deleted, but as there can be unique keys only in std::map. Therefore it will return 1 if element is deleted else it will return 0 if given key is not found in map.

Suppose we have a map of word and int i.e.

// Map of string & int i.e. words as key & there
// occurrence count as values
std::map<std::string, int> wordMap = { { "is", 6 }, { "the", 5 },
		{ "hat", 9 }, { "at", 6 }, { "of", 2 }, { "hello", 1 } };

Now, lets delete the element with key “is” i.e.

// Removes the element from map with given key.
int result = wordMap.erase("is");

Checkout Complete example as follows,

#include <iostream>
#include <map>
#include <string>

int main() {

	// Map of string & int i.e. words as key & there
	// occurrence count as values
	std::map<std::string, int> wordMap = { { "is", 6 }, { "the", 5 },
			{ "hat", 9 }, { "at", 6 }, { "of", 2 }, { "hello", 1 } };

	std::cout << "Map Entries Before Deletion" << std::endl;
	// Print the map elements
	for (auto elem : wordMap)
		std::cout << elem.first << " :: " << elem.second << std::endl;


	// Removes the element from map with given key.
	int result = wordMap.erase("is");

	// Check if element is actually deleted from map
	if(result == 1)
		std::cout<<"Element with key 'is' deleted"<<std::endl;
	else
		std::cout<<"Element with key 'is' Not Found"<<std::endl;


	std::cout << "Map Entries After Deletion" << std::endl;
	// Print the map elements
	for (auto elem : wordMap)
		std::cout << elem.first << " :: " << elem.second << std::endl;

	return 0;
}

Output:

Map Entries Before Deletion
at :: 6
hat :: 9
hello :: 1
is :: 6
of :: 2
the :: 5
Element with key 'is' deleted
Map Entries After Deletion
at :: 6
hat :: 9
hello :: 1
of :: 2
the :: 5

Erase Element from Map by Iterator

std::map provides a erase function that accepts the Iterator and removes the element pointed by the iterator.

iterator erase (const_iterator position);

It returns the iterator of the next element.

Suppose we have a map of word and int i.e.

// Map of string & int i.e. words as key & there
// occurrence count as values
std::map<std::string, int> wordMap = { { "is", 6 }, { "the", 5 },
		{ "hat", 9 }, { "at", 6 }, { "of", 2 }, { "hello", 1 } };

Now, lets delete the element with key “of” i.e.

Let’s first find the iterator pointing to it i.e.

// Get the iterator of element with key 'of'
std::map<std::string, int>::iterator it = wordMap.find("of");

Then check if iterator is valid or not. If its valid then only remove the element through it

if(it != wordMap.end())
{
	// Remove the element pointed by iterator
	wordMap.erase(it);
}

Checkout Complete example as follows,

#include <iostream>
#include <map>
#include <string>

int main() {

	// Map of string & int i.e. words as key & there
	// occurrence count as values
	std::map<std::string, int> wordMap = { { "is", 6 }, { "the", 5 },
			{ "hat", 9 }, { "at", 6 }, { "of", 2 }, { "hello", 1 } };

	std::cout << "Map Entries Before Deletion" << std::endl;
	// Print the map elements
	for (auto elem : wordMap)
		std::cout << elem.first << " :: " << elem.second << std::endl;


	// Get the iterator of element with key 'of'
	std::map<std::string, int>::iterator it = wordMap.find("of");

	// Check if iterator is valid.
	if(it != wordMap.end())
	{
		// Remove the element pointed by iterator
		wordMap.erase(it);

		std::cout<<"Element Removed"<<std::endl;
	}
	else
		std::cout<<"Key Not Found"<<std::endl;


	std::cout << "Map Entries After Deletion" << std::endl;
	// Print the map elements
	for (auto elem : wordMap)
		std::cout << elem.first << " :: " << elem.second << std::endl;



	return 0;
}

Output:

Map Entries Before Deletion
at :: 6
hat :: 9
hello :: 1
is :: 6
of :: 2
the :: 5
Element Removed
Map Entries After Deletion
at :: 6
hat :: 9
hello :: 1
is :: 6
the :: 5

Erase Element from Map by Iterator Range

std::map provides a erase function that accepts the Iterator start and end and removes all the elements in the given range i.e. from start to end -1 i.e.

void erase( iterator first, iterator last );

Suppose we have a map of word and int i.e.

// Map of string & int i.e. words as key & there
// occurrence count as values
std::map<std::string, int> wordMap = { { "is", 6 }, { "the", 5 },
		{ "hat", 9 }, { "at", 6 }, { "of", 2 }, { "hello", 1 } };

Now, lets delete the elements in range of two iterator it1 & it2 i.e.

// Remove the element pointed by iterator
 wordMap.erase(it1, it2);

It will delete the elements from it1 to it2 -1.

Checkout Complete example as follows,

#include <iostream>
#include <map>
#include <string>

int main() {

	// Map of string & int i.e. words as key & there
	// occurrence count as values
	std::map<std::string, int> wordMap = { { "is", 6 }, { "the", 5 },
			{ "hat", 9 }, { "at", 6 }, { "of", 2 }, { "hello", 1 } };


	std::cout << "Map Entries Before Deletion" << std::endl;
	// Print the map elements
	for (auto elem : wordMap)
		std::cout << elem.first << " :: " << elem.second << std::endl;

	// Create an iterator pointing to begin of map
	std::map<std::string, int>::iterator it1 = wordMap.begin();

	// Create an iterator pointing to begin of map
	std::map<std::string, int>::iterator it2 = wordMap.begin();
	// Increment Iterator
	it2++;
	// Increment Iterator
	it2++;
	// Itr2 is now pointing to 3rd element


	// Check if iterator is valid.
	if (it1 != wordMap.end() && it2 != wordMap.end())
	{
		// Remove the element pointed by iterator
		wordMap.erase(it1, it2);
		std::cout << "Elements Removed" << std::endl;
	}
	else
		std::cout << "Key Not Found" << std::endl;


	std::cout << "Map Entries After Deletion" << std::endl;
	// Print the map elements
	for (auto elem : wordMap)
		std::cout << elem.first << " :: " << elem.second << std::endl;

	return 0;
}

Output:

Map Entries Before Deletion
at :: 6
hat :: 9
hello :: 1
is :: 6
of :: 2
the :: 5
Elements Removed
Map Entries After Deletion
hello :: 1
is :: 6
of :: 2
the :: 5