How to flatten a 2-Dimensional vector in C++

Hello Guys, In this tutorial we are going to learn how to flatten a 2-Dimensional vector in C++. Before we get into this let’s discuss what is a 2-D vector, so it is a vector of vectors. Let’s say we have v as a 2-D then it can be defined as

vector<vector<int> >v;

For example,

v= {{2,3,4};{1,5,3};{6,7};{};{1}};

Few things are to be noted here,

• Vectors in ‘v’ can have different sizes.
• The size of a vector can be zero.

Algorithm for flattening a 2-D vector in C++

There is a standard approach to flatten a 2-D vector is using iterators (two-pointers) but with this, I will discuss one more way to do this.
Let’s see the standard one first;

• First, I would recommend you to read this post on Classes and objects, because we will be using this in the implementation of this algorithm.
• We define two pointers one for rows and another for columns.
• Then we set them to the first element of the first vector and update them iteratively.
• And finally terminate the program after printing all elements.

Code

#include<bits/stdc++.h>
using namespace std;

class Flatten2DVector{
    public:
    // a temporary copy of given vector, will use this furthur to avoid corruptions in original data
    vector<vector<int>> temp;
    int row, col;//two pointers 
    Flatten2DVector(vector<vector<int>>& v) {
        temp = v;
        row = 0;// initialising pointers to first index
        col = 0;
    }
    // returns the elements from temp vector   
    int next() {
        forward();
        return temp[row][col++];
    }    
    // checks whether next element is possible or not
    // also takes care of skipping the vectors of zero size
    bool hasNext() {
        forward();
        return (row < temp.size());
    }
    // helps to move forward in the vector
    void forward() {
        while ((row < temp.size()) && (col>=temp[row].size())) {
            row++;
            col = 0;
        }        
    }
};

int main(){
    vector<vector<int>>v;
    v={{1,3},{2,9,5,5},{},{4,5,6},{8}};//given 2Dvector
    Flatten2DVector it(v); // Class which has iterator(it) as his object
    // check if next element is posible or not, if possible print it
    while (it.hasNext()) {
        cout << it.next() << " "; 
    }
    cout<<endl;
    return 0; 
}

Now let’s discuss another approach;

This is a very simple approach where I will take all elements of the original 2D vector into a new 1D vector using nested for loops. And then print the new vector.

Code

#include<bits/stdc++.h>
using namespace std;

int main(){
  vector<vector<int>>v;
  vector<int>n;//new 1D vector
    v={{1,3},{2,9,5,5},{},{4,5,6},{8}};//given 2Dvector
  // using nested for loops to construct a new 1D vector
  for(int i=0; i<v.size(); i++){
    for(int j=0; j<v[i].size(); j++){
      n.push_back(v[i][j]);
    }
  }
  // Printing New vector
  for(int i=0; i<n.size(); i++){
    cout<<n[i]<<" ";
  }
  cout<<endl;
  return 0;
}

Now, look at the output for both implementations.

Output

1 3 2 9 5 5 4 5 6 8

This is it for this tutorial.
I hope it will help you.

You can also check out these tutorials related to vectors.

• How to Sort a Vector of Pairs in C++
• Implement Vector as a Stack in C++

Thank you.