CODEWITHSUNDEEP
c++arraysc++11
John Doe
John Doe

Reputation: 136

Passing 2d std::array to function cpp

I am trying to write a function in c++ that will take 2 input std::arrays, and return an array of the products via matrix multiplication. However, the function cannot take an array with different dimensions (ex. 4x4 works, 3x4 does not)

here is the code:

#include <iostream> 
#include <array>


template <std::size_t SIZE>
void dot(std::array<std::array<double, SIZE>, SIZE>& array1, 
     std::array<std::array<double, SIZE>, SIZE>& array2)
{
  int x1 = array1.size();
  int y1 = array1[0].size();


  int x2 = array2.size();
  int y2 = array2[0].size();
}

int main()
{
  using std::array;
  array<array<double, 4>, 4> syn0 = {{ {1,2,4},
                       {2,3,4},
                       {6,8,6},
                       {1,2,4} }};
  dot(syn0, syn0);
  return 0;
}

Using the template example as posed in this question, it will accept arrays such as 4x4 like in the code.

Changing the matrix to unequal numbers yields the following errors:

newer.cpp: In function ‘int main()’:
newer.cpp:23:21: error: too many initializers for ‘std::__array_traits<std::array<double, 4ul>, 3ul>::_Type {aka std::array<double, 4ul> [3]}’
            {1,2,4} }};
                     ^
newer.cpp:24:17: error: no matching function for call to ‘dot(std::array<std::array<double, 4ul>, 3ul>&, std::array<std::array<double, 4ul>, 3ul>&)’
   dot(syn0, syn0);
                 ^
newer.cpp:24:17: note: candidate is:
newer.cpp:6:6: note: template<long unsigned int SIZE> void dot(std::array<std::array<double, SIZE>, SIZE>&, std::array<std::array<double, SIZE>, SIZE>&)
 void dot(std::array<std::array<double, SIZE>, SIZE>& array1, 
      ^
newer.cpp:6:6: note:   template argument deduction/substitution failed:
newer.cpp:24:17: note:   deduced conflicting values for non-type parameter ‘SIZE’ (‘4ul’ and ‘3ul’)
   dot(syn0, syn0);
                 ^
newer.cpp:24:17: note:   ‘std::array<std::array<double, 4ul>, 3ul>’ is not derived from ‘std::array<std::array<double, SIZE>, SIZE>

I assume the reason for this is that the template only assigns one variable, so if i assign 2 to the same one, it throws the error. I tested to see if i could stack templates for two different variables, but that is not possible.

How can I allow the function to take a 2d array of any size and not cause that error?

Upvotes: 2

Views: 1738

Answers (1)

Jean-Fran&#231;ois Fabre
Jean-Fran&#231;ois Fabre

Reputation: 140287

Seems that you were really close.

You just need to add SIZE2 as a parameter to your template:

template <std::size_t SIZE,std::size_t SIZE2>
void dot(std::array<std::array<double, SIZE>, SIZE2>& array1, 
     std::array<std::array<double, SIZE>, SIZE2>& array2)

and it compiles all right BTW your syn0 size was wrong

int main()
{
  using std::array;
  array<array<double, 3>, 4> syn0 = {{ {1,2,4},
                       {2,3,4},
                       {6,8,6},
                       {1,2,4} }};
  dot(syn0, syn0);
  return 0;
}

Upvotes: 4

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