Question

I want to create a transpose function for dynamic 2d arrays. I want the functions to have as parameters the 2d array and the rows and columns. I ve decided to use double pointer. However i m a bit confused about how i gonna call the function from main. So i ve got the above code

#include<iostream>
using namespace std;


void transposeMatrix(double **mat, int rows, int columns)
{


   mat = new double*[rows];

   for (int i = 0; i < rows; ++i)
   {
      mat[i] = new double[columns];
   }


   double temp;

   for (int i = 0; i<rows; i++)
   {

      for (int j = i+1; j<columns; j++)
      {

         temp=mat[i][j];
         mat[i][j]=mat[j][i];
         mat[j][i]=temp;
      }
   }


   cout<< "\n";

   for (int i = 0; i<rows; i++)
   {

      for (int j = 0; j<columns; j++)
      {

         cout << mat[i][j] << " \t";
      }

      cout << "\n";
   }
}


int main()
{
   int rows = 10;
   int columns = 10;
   double mat[rows][columns];

   for (int i = 0; i<rows; i++)
   {

      for (int j = 0; j<columns; j++)
      {

         mat[i][j] = j;
      }
   }

   for (int i = 0; i<rows; i++)
   {

      for (int j = 0; j<columns; j++)
      {

         cout << mat[i][j] << " \t";
      }

      cout << "\n";
   }

   //mat = new double[50][1];
   transposeMatrix(mat, 10, 10);



   system("pause");
   return 0;
}

Any idea?

Answer 1

You’re very close. You’re calling the function correctly and the function’s parameter list is correct. First, remove this section from the transpose function:

 mat = new double*[rows];

 for (int i = 0; i < rows; ++i)
     mat[i] = new double[columns];

 }

Now make sure all your brackets match up. (There was one missing.)
You can’t define a static array (one that looks like this: x[y][z]) with non-constant variables as the size arguments. (I.e. y and z must be constants.) But actually, you’re passing a dynamic array to the transpose function anyway, and rows and columns don’t have to be constants to do that. So, in main, define a dynamic array like this:

double** mat = new double*[rows];
for (int i = 0; i < rows; i++)
    mat[i] = new double[columns];

After that, your code should work. But you could also make it better by putting your matrix display code in a function. Then, instead of cutting and pasting it everywhere, all you have to do is call the function! It’s an important habit to get into. Have fun!