variable sized arrays in c++ code example

Example 1: variable sized arrays hackerrank solution in c++

#include <iostream>
#include <vector>

using namespace std;

int main() {
	// get length of array 'a' and number of queries
	int n, q;
	cin >> n >> q;

	// create vector of vectors
	vector<vector<int>> a(n);

	// fill each 2D vector i with k_i values
	for (int i = 0; i < n; i++) {
		// get the length k of the vector at a[i]
		int k;
		cin >> k;

		// fill the vector with k values
		a[i].resize(k);
		for (int j = 0; j < k; j++) {
			cin >> a[i][j];
		}
	}

	// run queries on a
	for (int q_num = 0; q_num < q; q_num++) {
		// get i, j as the 'query' to get a value from a
		int i, j;
		cin >> i >> j;
		cout << a[i][j] << endl;
	}

	return 0;
}

Example 2: variable sized arrays hackerrank

int n,q;
cin >> n >> q;
vector< vector<int> > a(n);

// input each array
for (int i=0;i<n;i++) {
    int k;
    cin >> k;

    for (int j=0;j<k;j++) {
        int data;
        cin >> data;
        a[i].push_back(data);
    }
}

// do the queries
for (int i=0;i<q;i++) {
    int x,y;
    cin >> x >> y;
    cout << a[x][y] << endl;
}