function that reads a 2d array in c++ [closed] - c++

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I'm trying to write a function that reads a 2D array of integers where the user is the one who enters the size of the array (it shouldn't be defined before ) . I tried this but it's not working.
I appreciate your help!
line 6 is the declaration of the function
this is my code:
#include <iostream>
using namespace std;
void Fill_table(int mat[][], int s) {
for (int i = 0; i<s; i++) {
for (int j = 0; j<s; j++) {
cout << "mat[" << i << "][" << j << "]:" << endl;
cin >> mat[i][j];
}
}
}
int main()
{
int n;
cout << "Enter the size: ";
cin >> n;
int a[n][n];
Fill_table(a, n);
return 0;
}

Answer 1

Just to give you a simple alternative. Instead of declaring a 2D array with [][], you could use [n*n] and then subscript with i*s + j. The code would be
#include <iostream>
using namespace std;
void Fill_table(int* mat, int s) {
for (int i = 0; i<s; i++) {
for (int j = 0; j<s; j++) {
cout << "mat[" << i << "][" << j << "]:" << endl;
cin >> mat[i*s+j];
}
}
}
int main()
{
int n;
cout << "Enter the size: ";
cin >> n;
int a[n * n];
Fill_table(a, n);
return 0;
}
1D arrays can be passed as a pointer to a function. You also need to pass the size (as you did). For 2D arrays, you need to know the size at compile time.

Answer 2

You cannot set the size of a multidimensional array at runtime.
The error
error: declaration of ‘mat’ as multidimensional array must have bounds for all dimensions except the first
void Fill_table(int mat[][], int s)
tells you exactly that.
Consider using C++ data structure instead, in this case vector<vector<int>>, you have a somewhat simple how-to guide in Creating a Matrix using 2D vector in C++ – Vector of Vectors.