C++ creating an array pointing to different arrays - c++

Question

The inputs to this program are as follows:
2 2
3 1 5 4
5 1 2 8 9 3
0 1
1 3
I would like n to be an array that points to other integer arrays. So, n should essentially be {{1, 5, 4}, {1, 2, 8, 9, 3}}. If I wanted to access the 0th array and the 1st index, the value should return 5, and if I were to access the 1st array and the 3rd index, the value should be 9.
However, the values that this code returns are 32764 and 32764.
#include <cstdio>
#include <iostream>
using namespace std;
int main() {
int n_l; // integer variable that will hold the length of array n
int q_l; // integer variable that will hold the length of the number of queries
cin >> n_l >> q_l; // assigns values to the variables n_l and q_l
int *n[n_l]; // creates an array that will contain pointers
for (int i = 0; i < n_l; i++){ // loops through the length of array n
int amount; // declares the variable amount
cin >> amount; // assigns a value to the variable amount
int k[amount]; // creates one of the arrays that will be added to n
for (int x= 0; x < amount; x++){ // loops through the length of k and assigns a value to each index
cin >> k[x];
}
n[i] = k; // adds the array k to the position in array n
}
for (int i = 0; i < q_l; i++){
int arraynum;
int index;
cin >> arraynum >> index;
cout << n[arraynum][index] << endl;
}
}

Answer 1

cin >> n_l >> q_l; // assigns values to the variables n_l and q_l
int *n[n_l];
This isn't allowed in C++. The size of an array variable must be compile time constant. You can create dynamic arrays. Most convenient way is to use std::vector class template from the standard library.
The issue with your pointers is that the automatic arrays that you create in the loop are automatically destroyed at the end of the loop statement and the pointers in the array are all dangling (i.e. invalid) pointers to destroyed arrays. When you later indirect through the invalid pointers, the behaviour of the program is undefined.
You want multiple arrays. What's a good way to create multiple objects? Array is a good way to create multiple objects. So, in order to create multiple arrays, you can create an array of arrays. Or, since you want dynamic sizes, you can create a vector of vectors. Here is an example of how to create a vector of vectors:
std::vector<std::vector<int>> n(n_l);
for(auto& vec : n) {
int amount;
std::cin >> amount;
vec.resize(amount);
for(auto& i : vec) {
cin >> i;
}
}
You could create a vector of pointers to the arrays within the vectors in the vector of vectors, but that would be pointless. You don't need the pointers.

Answer 2

You are defining your array inside for loop which its scope will be limited in that loop, try allocate area with new for array and save the address of newly allocated area to your pointer array.
for (int i = 0; i < n_l; i++){ // loops through the length of array n
int amount; // declares the variable amount
cin >> amount; // assigns a value to the variable amount
int *k = new int[amount];
for (int x= 0; x < amount; x++){ // loops through the length of k and assigns a value to each index
cin >> k[x];
}
n[i] = k; // adds the array k to the position in array n
}
After you’re done with it, don’t forget to delete allocated area:
for(int i = 0; i < n_l; i++)
delete[] n[i];

Answer 3

For starters variable length arrays are not a standard C++ feature
int *n[n_l];
Instead you could use std::vector<int *>. Or you could use std::vector<std::pair<int, int *>> where the first element of the pair stores the number of elements in the dynamically allocated array and the second element of the pair stores the pointer to the dynamically allocated array.
Moreover the array will contain invalid pointers because the arrays declared in the for loop
int k[amount];
will not be alive after exiting the for loop.
So at least you have to allocate dynamically arrays in the for loop.
And you need to check whether an entered index is a valid for a given array before accessing an element of the dynamically allocated array.