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c++ - 编译时生成的表

转载 作者:搜寻专家 更新时间:2023-10-31 01:36:38 25 4
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由于一些技巧,我能够在编译时生成一个表,尽管表中的值不是很有用。例如一张 5x5 的 table 看起来像这样:

1,2,3,4,5,
1,2,3,4,5,
1,2,3,4,5,
1,2,3,4,5,
1,2,3,4,5,

逗号是为了清楚起见。创建此表的代码如下:

#include <iostream>

using ll = long long;

template<typename type,type...data>
struct sequence
{
static type seq_data[sizeof...(data)];
static const ll size;
type operator[](ll index){
return seq_data[size-index-1];
}
};

template<typename type,type...data>
type sequence<type,data...>::seq_data[sizeof...(data)] = { data... };
template<typename type,type...data>
const ll sequence<type,data...>::size{sizeof...(data)};

template<ll n,ll l,ll...data> struct create_row
{
typedef typename create_row<n-1,l+1,l,data...>::value value;
};
template<ll l,ll...data>
struct create_row<0,l,data...>
{
typedef sequence<ll,data...> value;
};

template<ll cols,ll rows>
struct table
{
typename create_row<cols,1>::value row_data[rows];
static const ll size;
};

template<ll cols,ll rows>
const ll table<cols,rows>::size{cols*rows};

using namespace std;

int main()
{
table<5,5> my_table;
for(int i{0};i<5;i++)
{
for(int j{0};j<5;j++)
{
cout<<my_table.row_data[i][j]<<",";
}
cout<<endl;
}
}

如您所见,为了创建单行,我不得不在 struct table 中使用硬编码值“1”,因此 create_table 将始终返回相同的序列,一系列从 1 到 n 的数字。因此,表中的每一行都具有相同的值。

我想做的是在编译时为每一行编码一个不同的起始值,以便有一个看起来像这样的表:

1,2,3,4,5,
6,7,8,9,10,
11 <CUT>

我找不到任何方法来创建这种类型的表格。

你知道怎么做吗?

最佳答案

在你的帖子最后我不确定你是否有兴趣在:-

  • 生成一个矩阵,该矩阵在编译时填充了连续的某些函数的值 f(i) 以行优先顺序环绕矩阵,例如
    Cols = 3; Rows = 3; f(i) = 2i; Vals = (1,2,3,4,5,6,7,8,9) ->    |02|04|06|    ----------    |08|10|12|    ----------    |14|16|18|

or:-

  • Generating a matrix that in which successive rows are compiletime populatedwith successive values of some function f(i) for some specified initial iper row, e.g.
    Cols = 3; f(i) = 3i; First_Vals = (4,7,10) ->     |12|15|18|    ----------    |21|24|27|    ----------    |30|33|36|

Anyhow there are ways to do both, and here is one you can use with aC++14 conforming compiler. (As @AndyG has commented, the appropriateimplementation for a compiletime matrix - leveraging the Standard Library- is an std::array of std::array.)

#include <array>
#include <utility>

namespace detail {

template<typename IntType, IntType(*Step)(IntType), IntType Start, IntType ...Is>
constexpr auto make_integer_array(std::integer_sequence<IntType,Is...>)
{
return std::array<IntType,sizeof...(Is)>{{Step(Start + Is)...}};
}

template<typename IntType, IntType(*Step)(IntType), IntType Start, std::size_t Length>
constexpr auto make_integer_array()
{
return make_integer_array<IntType,Step,Start>(
std::make_integer_sequence<IntType,Length>());
}


template<
typename IntType, std::size_t Cols,
IntType(*Step)(IntType),IntType Start, std::size_t ...Rs
>
constexpr auto make_integer_matrix(std::index_sequence<Rs...>)
{
return std::array<std::array<IntType,Cols>,sizeof...(Rs)>
{{make_integer_array<IntType,Step,Start + (Rs * Cols),Cols>()...}};
}

} // namespace detail

/*
Return a compiletime initialized matrix (`std::array` of std::array`)
of `Cols` columns by `Rows` rows. Ascending elements from [0,0]
in row-first order are populated with successive values of the
constexpr function `IntType Step(IntType i)` for `i` in
`[Start + 0,Start + (Rows * Cols))`
*/
template<
typename IntType, std::size_t Cols, std::size_t Rows,
IntType(*Step)(IntType), IntType Start
>
constexpr auto make_integer_matrix()
{
return detail::make_integer_matrix<IntType,Cols,Step,Start>(
std::make_index_sequence<Rows>());
}

/*
Return a compiletime initialized matrix (`std::array` of std::array`)
of `Cols` columns by `sizeof...(Starts)` rows. Successive rows are populated
with successive values of the constexpr function `IntType Step(IntType i)`
for `i` in `[start + 0,start + Cols)`, for `start` successively in `...Starts`.
*/
template<typename IntType, std::size_t Cols, IntType(*Step)(IntType), IntType ...Starts>
constexpr auto make_integer_matrix()
{
return std::array<std::array<IntType,Cols>,sizeof...(Starts)>
{{detail::make_integer_array<IntType,Step,Starts,Cols>()...}};
}

您可以通过附加以下内容来制作演示程序:

#include <iostream>

using namespace std;

template<typename IntType>
constexpr auto times_3(IntType i)
{
return i * 3;
}

static constexpr auto m4x6 = make_integer_matrix<int,4,6,&times_3<int>,4>();
static constexpr auto m5x1 = make_integer_matrix<int,5,&times_3<int>,7>();
static constexpr auto m6x5 = make_integer_matrix<int,6,&times_3<int>,11,13,17,19,23>();
static_assert(m4x6[0][0] == 12,"");

int main()
{
cout << "A 4 x 6 matrix that wraps around in steps of `3i` from `i` = 4" << endl;
for (auto const & ar : m4x6) {
for (auto const & i : ar) {
cout << i << ' ';
}
cout << endl;
}
cout << endl;
cout << "A 6 x 5 matrix with rows of `3i` for initial `i` in <11,13,17,19,23>"
<< endl;
for (auto const & ar : m6x5) {
for (auto const & i : ar) {
cout << i << ' ';
}
cout << endl;
}
cout << endl;
cout << "A 5 x 1 matrix with rows of of ` 3i` for initial `i` in <7>"
<< endl;
for (auto const & ar : m5x1) {
for (auto const & i : ar) {
cout << i << ' ';
}
cout << endl;
}

return 0;
}

应该输出:

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

查看at ideone

您可能还对 std::experimental::make_array 感兴趣

关于c++ - 编译时生成的表,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35334752/

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