How can I insert element into beginning of vector?

Question

I need to insert values into the beginning of a std::vector and I need other values in this vector to be pushed to further positions for example: something adde

Answer 1

Use the std::vector::insert function accepting an iterator to the first element as a target position (iterator before which to insert the element):

#include <vector>

int main() {
    std::vector<int> v{ 1, 2, 3, 4, 5 };
    v.insert(v.begin(), 6);
}

Alternatively, append the element and perform the rotation to the right:

#include <vector>
#include <algorithm>

int main() {
    std::vector<int> v{ 1, 2, 3, 4, 5 };
    v.push_back(6);
    std::rotate(v.rbegin(), v.rbegin() + 1, v.rend());
}

Answer 2

You should consider using std::deque. It works alot like a std::vector but you can add and remove items from both the front and the end.

It does this by dividing the internal storage up into smaller blocks. You still have random-access iterators with good lookup speed.

If your container is small it should be fine to use the std::vector approach but if you are storing large amounts of data the std::deques performance for inserting/deleting at the front will be far superior.

Answer 3

You can insert values to std::vector from back and then use std::reverse:

Example:

#include <vector>
#include <algorhitm>
#include <iostream>

void printVector( std::vector< int > const & _vector )
{
    for( auto value : _vector )
    {
         std::cout << value << " ";
    }

    std::cout << std::endl;
}

int main()
{
    std::vector< int > someVec;
    
    someVec.push_back( 5 );
    someVec.push_back( 4 );
    someVec.push_back( 3 );
    someVec.push_back( 2 );
    someVec.push_back( 1 );

    // (1)
    printVector( someVec );

    std::reverse( someVec.begin(), someVec.end() );
    
    // (2)
    printVector( someVec );

    return 0;
}

Output (1):

5 4 3 2 1 

Output (2):

1 2 3 4 5 

Answer 4

You may try this

    vector<int> v={1,2,3,4,5};
    for(int i=0;i<5;i++){
        v.insert(v.begin(),i+1);
    }

Output is {5,4,3,2,1,1,2,3,4,5}

Every element is shifted to the right after insertion