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  1. #include <array>
  2. #include <utility>
  3.  
  4. namespace detail {
  5.  
  6. template<typename IntType, IntType(*Step)(IntType), IntType Start, IntType ...Is> 
  7. constexpr auto make_integer_array(std::integer_sequence<IntType,Is...>)
  8. {
  9.     return std::array<IntType,sizeof...(Is)>{{Step(Start + Is)...}};
  10. }
  11.  
  12. template<typename IntType, IntType(*Step)(IntType), IntType Start, std::size_t Length> 
  13. constexpr auto make_integer_array()
  14. {
  15.     return make_integer_array<IntType,Step,Start>(
  16.         std::make_integer_sequence<IntType,Length>());
  17. }
  18.  
  19.  
  20. template<
  21.     typename IntType, std::size_t Cols, 
  22.     IntType(*Step)(IntType),IntType Start, std::size_t ...Rs
  23. > 
  24. constexpr auto make_integer_matrix(std::index_sequence<Rs...>)
  25. {
  26.     return std::array<std::array<IntType,Cols>,sizeof...(Rs)> 
  27.         {{make_integer_array<IntType,Step,Start + (Rs * Cols),Cols>()...}};
  28. }
  29.  
  30. } // namespace detail
  31.  
  32. /*
  33.     Return a compiletime initialized matrix (`std::array` of std::array`)
  34.     of `Cols` columns by `Rows` rows. Ascending elements from [0,0] 
  35.     in row-first order are populated with successive values of the
  36.     constexpr function `IntType Step(IntType i)` for `i` in
  37.     `[Start + 0,Start + (Rows * Cols))` 
  38. */
  39. template<
  40.     typename IntType, std::size_t Cols, std::size_t Rows, 
  41.     IntType(*Step)(IntType), IntType Start
  42. > 
  43. constexpr auto make_integer_matrix()
  44. {
  45.     return detail::make_integer_matrix<IntType,Cols,Step,Start>(
  46.         std::make_index_sequence<Rows>());
  47. }
  48.  
  49. /*
  50.     Return a compiletime initialized matrix (`std::array` of std::array`)
  51.     of `Cols` columns by `sizeof...(Starts)` rows. Successive rows are populated
  52.     with successive values of the constexpr function `IntType Step(IntType i)` 
  53.     for `i` in `[start + 0,start + Cols)`, for `start` successively in `...Starts`.  
  54. */
  55. template<typename IntType, std::size_t Cols, IntType(*Step)(IntType), IntType ...Starts> 
  56. constexpr auto make_integer_matrix()
  57. {
  58.     return std::array<std::array<IntType,Cols>,sizeof...(Starts)> 
  59.         {{detail::make_integer_array<IntType,Step,Starts,Cols>()...}};
  60. }
  61.  
  62. #include <iostream>
  63.  
  64. using namespace std;
  65.  
  66. template<typename IntType>
  67. constexpr auto times_3(IntType i)
  68. {
  69.     return i * 3;
  70. }
  71.  
  72. static constexpr auto m4x6 = make_integer_matrix<int,4,6,&times_3<int>,4>();
  73. static constexpr auto m5x1 = make_integer_matrix<int,5,&times_3<int>,7>();
  74. static constexpr auto m6x5 = make_integer_matrix<int,6,&times_3<int>,11,13,17,19,23>();
  75. static_assert(m4x6[0][0] == 12,"");
  76.  
  77. int main()
  78. {
  79.     cout << "A 4 x 6 matrix that wraps around in steps of `3i` from `i` = 4" << endl; 
  80.     for (auto const & ar  : m4x6) {
  81.         for (auto const & i : ar) {
  82.             cout << i << ' ';
  83.         }
  84.         cout << endl;
  85.     }
  86.     cout << endl;
  87.     cout << "A 6 x 5 matrix with rows of `3i` for initial `i` in <11,13,17,19,23>" 
  88.         << endl;
  89.     for (auto const & ar  : m6x5) {
  90.         for (auto const & i : ar) {
  91.             cout << i << ' ';
  92.         }
  93.         cout << endl;
  94.     }
  95.     cout << endl;
  96.     cout << "A 5 x 1 matrix with rows of of ` 3i` for initial `i` in <7>" 
  97.         << endl;
  98.     for (auto const & ar  : m5x1) {
  99.         for (auto const & i : ar) {
  100.             cout << i << ' ';
  101.         }
  102.         cout << endl;
  103.     }
  104.  
  105. 	return 0;
  106. }
  107.  
Success #stdin #stdout 0s 3460KB
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stdin
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Standard input is empty
stdout
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A 4 x 6 matrix that wraps around in steps of `3i` from `i` = 4
12 15 18 21 
24 27 30 33 
36 39 42 45 
48 51 54 57 
60 63 66 69 
72 75 78 81 

A 6 x 5 matrix with rows of `3i` for initial `i` in <11,13,17,19,23>
33 36 39 42 45 48 
39 42 45 48 51 54 
51 54 57 60 63 66 
57 60 63 66 69 72 
69 72 75 78 81 84 

A 5 x 1 matrix with rows of of ` 3i` for initial `i` in <7>
21 24 27 30 33
https://ideone.com/cZTplL
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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