Array of Vectors in C++ STL

Prerequisite: Arrays in C++, Vector in C++ STL

An array is a collection of items stored at contiguous memory locations. It is to store multiple items of the same type together. This makes it easier to get access to the elements stored in it by the position of each element.

Vectors are known as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container automatically.

Therefore, array of vectors is two dimensional array with fixed number of rows where each row is vector of variable length. Each index of array stores a vector which can be traversed and accessed using iterators.

Syntax:

vector <data_type> V[size];

Example:

vector <int> A[5];
where A is the array of vectors of int of size 5

Insertion: Insertion in array of vectors is done using push_back() function.

For Example:

for i in [0, n) {
  A[i].push_back(35)
}

Above pseudo-code inserts element 35 at every index of vector <int> A[n].

Traversal: Traversal in an array of vectors is perform using iterators.

For Example:

for i in [0, n) {
   for(iterator it = A[i].begin(); 
       it!=A[i].end(); it++) {
      print(*it)
    }
}

Above pseudo-code traverses vector <int> A[n] at each index using starting iterators A[i].begin() and ending iterator A[i].end(). For accessing the element it uses (*it) as iterators are pointers pointing to elements in vector <int> A[n].

Below is the program to illustrate the insertion in the array of vectors.

// C++ program to illustrate
// array of vectors

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

// Declaring array of vectors
// globally
vector<int> v[5];

// Function for inserting elements
// in array of vectors
void insertionInArrayOfVectors()
{

    for (int i = 0; i < 5; i++) {

        // Inserting elements at every
        // row i using push_back()
        // function in vector
        for (int j = i + 1; j < 5; j++) {
            v[i].push_back(j);
        }
    }
}

// Function to print elements in array
// of vectors
void printElements()
{

    // Traversing of vectors v to print
    // elements stored in it
    for (int i = 0; i < 5; i++) {

        cout << "Elements at index "
             << i << ": ";

        // Displaying element at each column,
        // begin() is the starting iterator,
        // end() is the ending iterator
        for (auto it = v[i].begin();
             it != v[i].end(); it++) {

            // (*it) is used to get the
            // value at iterator is
            // pointing
            cout << *it << ' ';
        }
        cout << endl;
    }
}

// Function to illustrate array
// of vectors
void arrayOfVectors()
{
    // Inserting elements in array
    // of vectors
    insertionInArrayOfVectors();

    // Print elements stored in array
    // of vectors
    printElements();
}

// Driver code
int main()
{
    arrayOfVectors();
    return 0;
}

Output:

Elements at index 0: 1 2 3 4 
Elements at index 1: 2 3 4 
Elements at index 2: 3 4 
Elements at index 3: 4 
Elements at index 4: