Transpose a 2d dynamic matrix - c++

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!

Answer 2

There are a couple of major issues with your code.
The biggest one is that a double[10][10] is not convertible to a double** pointer.
You also have a memory leak (mat) in your transposeMatrix() implementation.
I recommend that you separate the concerns of printing a matrix and transposing a matrix. Perhaps a separate methods on a (templated) matrix class.
And now, having said that...
Why write one when a perfectly good implementation already exists?
Example:
#include <boost/numeric/ublas/matrix.hpp>
#include <boost/numeric/ublas/io.hpp>
int main ()
{
using namespace boost::numeric::ublas;
matrix<double> m(3, 3);
for (unsigned i = 0; i < m.size1(); ++i)
{
for (unsigned j = 0; j < m.size2(); ++j)
{
m(i, j) = 3 * i + j;
}
}
std::cout << m << std::endl;
std::cout << trans(m) << std::endl;
}
Output:
[3,3]((0,1,2),(3,4,5),(6,7,8))
[3,3]((0,3,6),(1,4,7),(2,5,8))

Answer 3

double ** transpose(double **matrix, int rows, int columns){
double ** trans;
trans=new double *[columns];
for(int i=0;i<columns;i++){
trans[i]=new double[rows];
for(int j=0;j<rows;j++)
trans[i][j]=matrix[j][i];
}
return trans;
for(int i=0;i<columns;i++)
delete[] trans[i];
delete[] trans;
}
Here is the code for transpose of the matrix.