C++: Transpose of Matrix, using SINGLE DIMENSION dynamic array to save elements - c++

Question

Dear Friends I am having problem to transpose a matrix. The transposed matrix has elements that are undefined. Not sure what is wrong. Thank you for your time!
entries[i] is the dynamic array storing the elements in the matrix. Elements are stored row by row, from left to right. i.e. in a 3X3 matrix, entries[2] is 3rd element on the 1st row, entries[3] is 1st element on the 2nd row
n is the number of rows of matrix
m is the number of columns of matrix
Matrix Matrix::Transpose() const {
double* temp;
temp = new double[n * m];
for (int i = 1; i <= n; i++)
{
for (int j = 1; j <= m; j++)
temp[(j - 1) * m + i - 1] = entries[(i - 1) * m + j - 1];
}
Matrix Result(m, n, temp);
delete temp;
return Result;
}
When the original matrix is a square, all elements of the transposed matrix are defined. When the original matrix is 1x3, then the resulting transposed 3x1 matrix has undefined elements for the 2nd and 3rd elements. I.e. (1 1 3) after transposed returns (1 -3452346326236 -12351251515)
The Matrix Print out function is below. The error likely comes from here too.
void Matrix::Print() const
{
for (int i = 1; i <= n; i++)
{
for (int j = 1; j <= m; j++)
cout << setw(13) << entries[(i - 1) * m + j - 1];
cout << endl;
}
}

Answer 1

When the matrix is not square, the line
temp[(j - 1) * m + i - 1] = entries[(i - 1) * m + j - 1];
is not right. It needs to be:
temp[(j - 1) * n + i - 1] = entries[(i - 1) * m + j - 1];
// ^^ needs to be n, not m.
Think of the 2D analogue. You want to use:
temp[j][i] = entries[i][j];
entries is a n x m matrix. For it, the 2D indices [i][j] are translated as [i*m + j] for the 1D index.
temp is a m x n matrix. For it, the 2D indices [j][i] are translated as [j*n + i] for the 1D index.
Suggestion for improved readability
Instead of n and m, use num_rows, and num_columns. You will find your code a lot more readable.