How to make a two dimensional array with rows and collums selected by the user?

[neodrago1234]

So I have this problem with arrays. I have to make a program that makes the user input two numbers - one for the number of rows, and one for the columns in a 2d array and then the program has to fill the array with random numbers. The main problem is how do I make the program take variables for the number and rows and columns? Aren't they supposed to the a constant value?

[Thomas1965]

Aren't they supposed to the a constant value?

When you specify them at compiler time.

To do this at run time you have several options.
1. Use of variant length arrays if you use GCC, though it's not recommended.
2. Use of dynamic memory
3. Use of nested vectors
4. Use of a single vector - mapping row and col to a index

Options 2-4 could easily be inside a class.
I would recommend option 3

vector<vector<int>> array2d;
  int rows, cols;
  cin >> rows >> cols;
  for (int row = 0; row < rows; row++)
  {
    array2d.push_back(vector<int>(cols));
  }
  cout << "\nRows: " << array2d.size();
  cout << "\nCols: " << array2d[0].size();

[jlb]

My first suggestion would be to use std::vector instead, that's why they were designed.

#include <iostream>
#include <vector>

int main()
{
    int m = 10;
    int n = 12;
    std::vector<std::vector<int>> vec(m, std::vector<int> (n));

    std::cout << vec.size() << " " << vec[0].size() << std::endl;

}

But if you must use nasty arrays then you will need to manually allocate memory (new/delete) for the array if you want a runtime sized array.

[lastchance]

#include <iostream>
#include <vector>
#include <ctime>
#include <cstdlib>
using namespace std;

using matrix = vector< vector<int> >;            // Saves typing this long type declaration every time


//======================================================================


matrix genMatrix( int rows, int cols )
{
   matrix M( rows, vector<int>( cols, 0 ) );     // Declare container with specified number of rows and columns
   for ( int i = 0; i < rows; i++ )
   {
      for ( int j = 0; j < cols; j++ ) M[i][j] = rand() % 100;

   }
   return M;
}


//======================================================================


void print( const matrix &M )
{
   for ( auto row : M )
   {
      for ( auto e : row ) cout << e << '\t';
      cout << '\n';
   }
}


//======================================================================


int main()
{
   srand( time( 0 ) );

   int rows, cols;
   cout << "Enter the number of rows: "   ;   cin >> rows;
   cout << "Enter the number of columns: ";   cin >> cols;
   matrix M = genMatrix( rows, cols );
   print( M );
}


//====================================================================== 

Enter the number of rows: 5
Enter the number of columns: 10
39      84      37      95      34      71      7       59      42      40      
54      59      80      27      99      12      0       45      37      83      
61      59      15      43      30      16      48      63      22      14      
43      99      30      80      95      43      21      86      17      13      
61      17      31      29      8       96      27      81      37      59