John Doe John Doe - 3 months ago 27
C++ Question

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?

Answer

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;
}
Comments