C++ Map Insert Example

In this article we will discuss how to insert a key value pair in std::map.

Map internally store elements in a pair of key value i.e.

std::pair<key Type, Value Type>

So, to add an element in map we can use one of its member function insert() i.e.

pair<iterator,bool> insert (const value_type& element);

It accepts an object of key value pair and returns an pair of map iterator and bool.

In the returned pair i.e.

pair<iterator,bool>

bool represents the result of insertion and iterator represents the position of newly added element in map.

If insertion in map is successful then,
bool —> true
Iterator —-> Points to Position of newly added element.

If insertion in map failed then,
bool —> false
Iterator —-> Pointing to the passed pair

Let’s see an example,

Suppose we have a map of string & int as key value pair i.e.

std::map<std::string, int> mapOfWordCount;

To insert an element in it we will create a pair of string & int i.e.

std::pair<std::string, int>("first", 1)

insert() function will return a pair of map iterator & bool i.e.

std::pair<std::map<std::string, int>::iterator, bool > result;
result = mapOfWordCount.insert(std::pair<std::string, int>("first", 1));

Now, to check if insertion we will first check the bool in result pair i.e.

if(result.second == false)
{
   std::cout<<"Failed to add . duplicate key :: "<<result.first->first<<std::endl;
}
else
{
   std::cout<<"Successful in Adding , key :: "<<result.first->first<<std::endl;
}

Complete example is as follows,

#include <iostream>
#include <map>
#include <string>
#include <iterator>
#include <algorithm>

void testResult(
		std::pair<std::map<std::string, int>::iterator, bool> & result) {
	// Check if Insertion was successful
	if (result.second == false) {
		// Insertion Failed
		std::cout << "Failed to add . duplicate key :: " << result.first->first
				<< std::endl;
	} else {
		// Insertion was successful
		std::cout << "Successful in Adding , key :: " << result.first->first
				<< std::endl;
	}
}

int main() {

	// Map of string and int
	std::map<std::string, int> mapOfWordCount;

	// Pair of Map Iterator and bool
	std::pair<std::map<std::string, int>::iterator, bool> result;

	// Insert Element in map
	result = mapOfWordCount.insert(std::pair<std::string, int>("first", 1));
	// Test its result
	testResult(result);

	// Insert Element in map
	result = mapOfWordCount.insert(std::pair<std::string, int>("second", 2));
	// Test its result
	testResult(result);

	// Insert Element in map
	result = mapOfWordCount.insert(std::pair<std::string, int>("third", 3));
	// Test its result
	testResult(result);

	// Try to add duplicate element
	result = mapOfWordCount.insert(std::pair<std::string, int>("third", 4));
	// Test its result
	testResult(result);

	// Create a map iterator and point to beginning of map
	std::map<std::string, int>::iterator it = mapOfWordCount.begin();

	std::cout << "*****************************" << std::endl;
	// Iterate over a map using std::for_each and Lambda function
	std::for_each(mapOfWordCount.begin(), mapOfWordCount.end(),
			[](std::pair<std::string, int> element) {
				// Accessing KEY from element
				std::string word = element.first;
				// Accessing VALUE from element.
				int count = element.second;
				std::cout<<word<<" :: "<<count<<std::endl;
			});

	return 0;
}

Output:

Successful in Adding , key :: first
Successful in Adding , key :: second
Successful in Adding , key :: third
Failed to add . duplicate key :: third
*****************************
first :: 1
second :: 2
third :: 3

 

To compile the above example in linux use following command,

g++ -std=c++11 example.cpp