Vectors in C++

Vectors are containers containing values of the same data type. I like to think of them like arrays. Vectors can resize automatically when an element is inserted or deleted . Since C++ is pretty close to hardware, vector elements are placed in adjacent storage so that they can travel across. Like arrays in JS, vector have methods such as insert( ), pop_back( ), erase( ) etc. An example of a 1 dimension vector is:

    vector<int> row1 = {3, 4, 5};

An example of a 2 dimensional vector is:

    vector<vector<int>> row2 {{1,2}, {3,4}};

To use vectors include them in your main program like so:

#include <vector>
using namespace std;

Declare and initialize a vector called “myvector”:

vector<int> myvector = {1,2,3};

Access and print the 2nd element in the vector called “myvector” above:

cout << myvector[1];

An example program:

#include <iostream>
#include <vector>
using namespace std;

int main() {
    // declaring and initializing vectors in 1 go
    vector<int> v2 = {3, 4, 5};
   vector<int> myvector = {1,2,3};
    cout << v2[1] << "\n";
    cout << myvector[1] << "\n";
}
/*
This prints out:
4
2
As remember arrays start at index 0 so if we want to access the 2nd element then we have to access the 1st index in the vector
*/

Now access and print out an element from a 2D vector:

int main() {
    vector<vector<int>> b = {{1, 1, 2, 13},
                             {2, 1, 2, 30},
                             {3, 1, 2, 300}};
    cout << b[0][3] <<"\n";
//prints out the last element in row 0 which is at index 3 and is the number 13
}

Vector sizes: Just like we can find out how many elements are in an array with the .length property. The same way we use the .size( ) method to find out how big an array is. Size of a 1D vector:

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> myvector = {0, 1, 2, 3,5,6,7};
    
    // Print the length of vector a to the console.
    cout << myvector.size() << "\n";
}

Size of a 2D vector: To do this access the row that you want and call the .size( ) method on it. For example:

#include <iostream>
#include <vector>
using namespace std;

int main() {

    vector<vector<int>> b = {{1, 2, 3,4, 5,6,7,8,9,10},
                             {2, 1, 2, 3},
                             {3}};
    // Print the length of an inner vector of b here.
    cout << b[0].size() << "\n";
// access row 0 (which is the 1st row) 
// prints  out 10 as there are 10 elements

   cout << b[2].size() << "\n";
// prints out the number of elements in the last row which is 1
}