Segmentation fault caused by simple c++ code - c++

Question

Absolute beginner here. I'm trying to solve this but get a segmentation fault. I tried searching for a solution but couldn't find the reason why this doesn't work.
To reproduce the error simply copy the code below and paste it into the editor under the link above.
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <sstream>
#include <algorithm>
//using namespace std;
int main() {
int n, q;
std::cin >> n >> q;
std::vector<std::vector<int>> arr;
// handle n lines representing the arrays
for (int i = 0; i < n; i++) {
std::vector<int> numbers;
std::string line;
getline(std::cin, line);
std::istringstream iss(line);
int enterNumber;
while (iss >> enterNumber)
{
numbers.push_back(enterNumber);
}
arr.push_back(numbers);
}
// handle q lines representing i, j
int x, y;
for (int i = 0; i < q; i++) {
std::cin >> x >> y;
std::cout << arr[x][y];
}
return 0;
}
What am I missing? Why doesn't it work?
Input that causes the segmentation fault:
2 2
3 1 5 4
5 1 2 8 9 3
0 1
1 3
Expected output:
5
9

Answer 1

1) getline(std::cin, line); . Line contains space and numbers.
2) Handle spaces in lines
3) Handle first int in line for array length. (You tried to add array length in array itself)
Here is working code for reference. (Passed all test cases)
#include <vector>
#include <iostream>
using namespace std;
int main() {
int n, q;
cin >> n >> q;
vector< vector<int> > arr;
int temp, array_count;
for(int i = 0; i < n; i++) {
vector<int> numbers;
cin>>array_count;
for(int j = 0; j < array_count; j++) {
cin>>temp;
numbers.push_back(temp);
}
arr.push_back(numbers);
numbers.clear();
}
// handle q lines representing i, j
int x, y;
for (int i = 0; i < q; i++) {
cin >> x >> y;
cout << arr[x][y]<<"\n";
}
return 0;
}

Answer 2

To fix the segmentation fault simply add std::cin.get(); after the first std::cin(third line in your main function).
The segmentation fault happened because getline(std::cin, line);returned an empty string during the first iteration of the for loop. See this.
Note that even after fixing the segmentation fault issue your code is still wrong (doesn't solve the challenge) :p
Try this:
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <sstream>
#include <algorithm>
//using namespace std;
int main() {
int n, q;
std::cin >> n >> q;
std::cin.get();
std::vector<std::vector<int>> arr;
// handle n lines representing the arrays
for (int i = 0; i < n; i++) {
std::vector<int> numbers;
std::string line;
getline(std::cin, line);
std::istringstream iss(line);
int enterNumber;
while (iss >> enterNumber)
{
numbers.push_back(enterNumber);
}
numbers.erase(numbers.begin()); // because first value is k
arr.push_back(numbers);
}
// handle q lines representing i, j
int x, y;
for (int i = 0; i < q; i++) {
std::cin >> x >> y;
std::cout << arr[x][y] << std::endl;
}
return 0;
}