Pointer refer different values - c++

Question

I got a problem while solving a beginner c++ challenge in hackerrank.
https://www.hackerrank.com/challenges/variable-sized-arrays/problem
The challenge require me to create an array of int array which simply is a 2d array, but the order of input constraints is so bad that I cannot make it. Suddenly, it pops up to me an idea of creating an array of pointers *arrA that each element refers &arrB[0] of its sub-array arrB.
Note: It is similar to vector<vector<int>> but I would not use vector here.
int main() {
int n, q, k, i, j;
cin >> n >> q;
int *arr[n];
for (int l = 0; l < n; l++) {
cin >> k;
int arr_i[k];
for (int m = 0; m < k; m++) {
cin >> arr_i[m];
}
arr[l] = &arr_i[0];
cout << *arr[l] << " " << arr[l] << endl;
}
// after 2 loop, it prints
// 1 0x7fff17940030
// 2 0x7fff17940020
for (int l = 0; l < q; l++) {
cin >> i >> j;
cout << *arr[l] << " " << arr[l] << endl;
}
// after 2 loop, it prints
// random integer(eg: 395575472) 0x7fff17940030
// random integer(eg: 922493088 ) 0x7fff17940020
return 0;
}
The constraints are:
2 2 // n q
3 1 5 4 // k k[0] k[1] k[2]
5 2 2 8 9 3 // k k[0] k[1] k[2] k[3] k[4]
0 1 // i j
1 3 // i j
Back to the problem, the first loop prints exactly what i need. But in the second loop, the values are missing though the address are the same. I have search many stackoverflow question but none of them meets my needs.
Can someone explain this to me. Many thanks !!!

Answer 1

The scope for int arr_i[k]; is the first loop, and while it's fine to take it's address with arr[l] = &arr_i[0]; those addresses are out of scope when you dereference them in the 2nd loop.

Answer 2

The C++ way to have variable sized arrays is std::vector. And you can have a vector-of-vectors.
Example:
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
int main() {
int nrOfVectors, q /* what is q?*/;
std::cin >> nrOfVectors >> q;
std::vector<std::vector<int>> vecvec(nrOfVectors);
for(auto& vec : vecvec) {
int nrOfElements;
std::cin >> nrOfElements;
vec.reserve(nrOfElements);
std::copy_n(std::istream_iterator<int>(std::cin), nrOfElements, back_inserter(vec));
}
}
edit:
An array solution has no added value (actually makes thing unnecessarily complex and error-prone), but whatever
#include <iostream>
#include <memory>
int main() {
int nrOfVectors, q /* what is q?*/;
std::cin >> nrOfVectors >> q;
std::unique_ptr<std::unique_ptr<int[]>[]> vecvec(new std::unique_ptr<int[]>[nrOfVectors]);
for (int i = 0; i < nrOfVectors; ++i) {
int nrOfElements;
std::cin >> nrOfElements;
auto vec = new int[nrOfElements];
for (int j = 0; j < nrOfElements; ++j) {
std::cin >> vec[j];
}
vecvec[i].reset(vec);
}
}