Insert Element at Specific index in Vector in C++

In this article we will discuss how to insert one or multiple elements at specific index in vector.

Vector provides different overloaded version of member function insert() , to insert one or more elements in between existing elements.
Let’s discuss them in detail,

Inserting a single element at specific position in vector

We are going to use first overloaded version of Vector’s insert() function i.e.

iterator insert (const_iterator pos, const value_type& val);

It Inserts a copy of give element “val”, before the iterator position “pos” and also returns the iterator pointing to new inserted element.

Let’s understand by example,

Suppose we have a vector of int i.e.

std::vector<int> vecOfNums { 1, 4, 5, 22, 33, 2, 11, 89, 49 };

Now we want to insert an element at index position 4th (In vector position index start from 0),

// Create Iterator pointing to 4th Position
auto itPos = vecOfNums.begin() + 4;

// Insert element with value 9 at 4th Position in vector
auto newIt = vecOfNums.insert(itPos, 9);

Vector’s contents will be now,

1 , 4 , 5 , 22 , 9 , 33 , 2 , 11 , 89 , 49

Inserting an element in vector will increase the vector size by 1.
As in vector all elements are stored at continuous memory locations, so inserting an element in between will cause all the elements in right to shift or complete reallocation of all elements.

Inserting multiple elements or a range at specific position in vector

Some times we encounter a situation where we want to insert multiple elements in vector at specific position. These multiple elements can from another vector , array or any other container.

For this, vector provides an overloaded version of insert() function to insert multiple elements i.e.

iterator insert (const_iterator pos, InputIterator first, InputIterator last);

It inserts the elements in range from [first, end) before iterator position pos and returns the iterator pointing to position first newly added element.

Let’s understand by an example,

Suppose we have 2 vectors of strings i.e.

std::vector<std::string> vec1 { "at" , "hello", "hi", "there", "where", "now", "is", "that" };
std::vector<std::string> vec2 { "one" , "two", "two" };

Now insert all the elements in vec2 at position 3 in vec1 i.e.

// Insert all the elements in vec2 at 3rd position in vec1
auto iter = vec1.insert(vec1.begin() + 3, vec2.begin(), vec2.end());

Contents of vec1 will be now,

at , hello , hi , one , two , two , there , where , now , is , that ,

Inserting multiple elements using Initializer list

Another overloaded version of vector’s insert() function is as follows,

iterator insert (const_iterator position, initializer_list<value_type> list);

It copies all the elements in given initializer list before given iterator position pos and also returns the iterator of first of the newly added elements.

Suppose we have vector of int i.e.

std::vector<int> vecOfInts { 1, 4, 5, 22, 33, 2, 11, 89, 49 };

Let’s add all elements in initialisation list to the existing vector i.e.

// Insert all elements from initialization_list to vector at 3rd position
auto iter2 = vecOfInts.insert(vecOfInts.begin() + 3, {34,55,66,77});

Contents of vecOfInts will be now,

1 , 4 , 5 , 34 , 55 , 66 , 77 , 22 , 33 , 2 , 11 , 89 , 49

vector.insert() and Iterator invalidation

Inserting elements in vector will cause existing elements to shift places or sometimes complete reallocation, which will invalidates all the existing iterators.

Complete example is as follows,

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

template <typename T>
void print(T & vecOfElements, std::string delimeter = " , ")
{
	for(auto elem : vecOfElements)
		std::cout<<elem<<delimeter;
	std::cout << std::endl;
}
int main()
{

	std::vector<int> vecOfNums { 1, 4, 5, 22, 33, 2, 11, 89, 49 };

	/*
	 * Inserting an element at specific position in vector
	 */

	// Create Iterator pointing to 4th Position
	auto itPos = vecOfNums.begin() + 4;

	// Insert element with value 9 at 4th Position in vector
	auto newIt = vecOfNums.insert(itPos, 9);

	std::cout << "Element added in vector is : " << *newIt << std::endl;
	std::cout << "Modified vecOfNums = ";
	print(vecOfNums);

	/*
	 * Inserting multiple elements / range at specific position in vector
	 */

	std::vector<std::string> vec1 { "at" , "hello", "hi", "there", "where", "now", "is", "that" };
	std::vector<std::string> vec2 { "one" , "two", "two" };

	// Insert all the elements in vec2 at 3rd position in vec1
	auto iter = vec1.insert(vec1.begin() + 3, vec2.begin(), vec2.end());

	std::cout << "First of the newly added elements : " << *iter << std::endl;
	std::cout << "Modified vec1 = ";
	print(vec1);

	/*
	 * Inserting all elements in initialization_list in another vector
	 * at specific position.
	 */

	std::vector<int> vecOfInts { 1, 4, 5, 22, 33, 2, 11, 89, 49 };

	// Insert all elements from initialization_list to vector at 3rd position
	auto iter2 = vecOfInts.insert(vecOfInts.begin() + 3, {34,55,66,77});

	std::cout << "First of the newly added elements : " << *iter2 << std::endl;
	std::cout << "Modified vecOfInts = ";
	print(vecOfInts);

	return 0;

}

Output:

Element added in vector is : 9
Modified vecOfNums = 1 , 4 , 5 , 22 , 9 , 33 , 2 , 11 , 89 , 49 , 
First of the newly added elements : one
Modified vec1 = at , hello , hi , one , two , two , there , where , now , is , that , 
First of the newly added elements : 34
Modified vecOfInts = 1 , 4 , 5 , 34 , 55 , 66 , 77 , 22 , 33 , 2 , 11 , 89 , 49 ,

To Compile the above example in linux use following command,

g++ –std=c++11 example.cpp