Declaring and allocating a 2d array in C++

Question

I am a Fortran user and do not know C++ well enough. I need to make some additions into an existing C++ code. I need to create a 2d matrix (say A) of type double whose size (say

Answer 1

To allocate dynamically a construction similar to 2D array use the following template.

#include 

int main()
{
   int m, n;

   std::cout << "Enter the number of rows: ";
   std::cin >> m;

   std::cout << "Enter the number of columns: ";
   std::cin >> n;

   double **a = new double * [m];

   for ( int i = 0; i < m; i++ ) a[i] = new double[n]();

   //...

   for ( int i = 0; i < m; i++ ) delete []a[i];
   delete []a;
}

Also you can use class std::vector instead of the manually allocated pointers.

#include 
#include 

int main()
{
   int m, n;

   std::cout << "Enter the number of rows: ";
   std::cin >> m;

   std::cout << "Enter the number of columns: ";
   std::cin >> n;

   std::vector> v( m, std::vector( n ) );

   //...

}

As for this code snippet

int * x;
x = new int [10];

then x has type int * and x[0] has type int. So if the size of the pointer is equal to 4 and the size of an object of type int is equal also to 4 then sizeof( x ) / sizeof( x[0] ) will yields 1. Pointers do not keep the information whether they point to only a single object or the first object pf some sequence of objects.