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I'm working through the C++ Hackerrank modules, and I've come across a problem asking to output certain values in a two-dimensional array. Here is the problem:

Consider an n-element array, a, where each index i in the array contains a reference to an array of k integers (where the value of k varies from array to array)... Given a, you must answer q queries. Each query is in the format i j, where i denotes an index in array a and j denotes an index in the array located at a[i]. For each query, find and print the value of element j in the array at location a[i] on a new line.

I don't want to make this entire question a quote, so here is the link: Variable Sized Arrays. Basically, there is the first input line with the above-mentioned n and q. Then there are n lines of integers following, with the first integer in each line of input being k, and this is the number of integers following k itself in the input line. After these n lines is one line with two integers i and j, denoting the array index and element index respectively.

So, the solution is then to create a two-dimensional array that can be filled in with n sub-arrays. Here was my code originally:

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int n, q;
    cin >> n >> q;
    int a[n];
    for(int i=0; i<n; i++){
        int k;
        cin >> k;
        int subA[k];

        for(int j=0; j<k; j++)
            cin >> subA[j];

        arr[i] = subA[k];
    }

    for (int x=0; x<q; x++){
        int i, j;
        cin >> i>> j;
        cout << a[i][j] << endl;
    }
    return 0;
}

The error said 

invalid types 'int[int]' for array subscript
         cout << a[i][j] << endl;

and I realized that it was probably because an array isn't an integer type (right?). So then after googling, I came across this question, which just nonchalantly skips over the fact that one seems to need pointers to create two-dimensional arrays. So, when a two-dimensional array is created, does it require the use of pointers? If so, would I be right in stating that pointers are required because they aren't really a type like int or char, but just hold an address whose type doesn't have to be known?

P.S. I understand that the first link I provided provides in itself a link to something about vectors. It didn't give any examples and I have no idea what they are, and googling and Stackoverflow-ing hasn't really helped (especially the i/o part), so I just used something familiar to me, and it seems, through this process of googling and Stackoverflow-ing, that vectors are not needed for a solution such as the high-level one I posed above.

mpnm
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  • Please work through error messages top to bottom. There should be one before that about `arr` not being declared. [Do not use](https://stackoverflow.com/questions/39334435/variable-length-array-vla-in-c-compilers) [variable-length arrays](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) in C++. Don't use raw arrays at all. Use `std::vector` instead. And I also recommend to learn from a [good book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). C++ is too unforgiving to learn unstructured. – walnut Nov 25 '19 at 02:38
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    "*each index i in the array contains a reference to an array of k integers*": The way this assignment is phrased, it seems to assume that you are using raw arrays (which you shouldn't in modern C++) and it technically makes no sense since arrays of references are not possible in C++. You should not continue using the source of this assignment to learn C++. – walnut Nov 25 '19 at 02:47

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