std::piecewise_constant_distribution和std::vector的问题

Question

给定一堆字符串，我正在尝试创建一个程序，它可以模仿伪随机行为，并根据我的输入进行加权分布。

到目前为止，我想出了这个

#include <iostream>
#include <random>
#include <type_traits>
#include <map>
#include <vector>
#include <string>
#include <initializer_list>

#define N 100

int main()
{
    std::vector<std::string> interval{"Bread", "Castle", "Sun"};
    std::vector<float> weights { 0.40f, 0.50f, 0.10f };
    std::piecewise_constant_distribution<> dist(interval.begin(),
                                                interval.end(),
                                                weights.begin());

    std::random_device rd;
    std::mt19937 gen(rd()) ;

    for(int i = 0; i<N;i++)
    {
        std::cout << dist(gen) << "\n";
    }

    return(0);

}

但是这个东西不能工作，我也不知道为什么，根据在线例子，std::piecewise_constant_distribution的通常用法是用std::arrays，但我正在尝试用std::vector实现它，这是我发现的主要区别。

使用Clang++时，错误的输出为

/usr/bin/../lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/random.tcc:2409:10: error: no matching member function for
      call to 'push_back'
                _M_int.push_back(*__bbegin);
                ~~~~~~~^~~~~~~~~

但是我不能理解它，因为在我的代码中没有显式的.push_back，我也不能从它得到什么，因为调试一个模板化的类是一场噩梦，我只是从这个开始。

有人知道为什么代码不能工作吗？

Answer 1

std::piecewise_constant_distribution的默认结果类型是RealType (这里是double)。看起来你正在尝试从3个不同权重的string选项中进行选择，但这不是std::piecewise_constant_distribution的用途。它的设计目的是从具有给定权重的数值间隔生成均匀的随机值。如果您修改示例并将interval更改为：

std::vector<double> interval {1, 3, 5, 10, 15, 20};

一切都会顺利编译的。从外观上看，您想要std::discrete_distribution

...

std::vector<std::string> interval{"Bread", "Castle", "Sun"};
std::vector<float> weights { 0.40f, 0.50f, 0.10f };
std::discrete_distribution<> dist(weights.begin(), weights.end());

std::mt19937 gen(rd());

for(int i = 0; i<N;i++)
{
    std::cout << interval[dist(gen)] << "\n";
}

...

Answer 2

template< class RealType = double >
class piecewise_constant_distribution;

对RealType进行操作，不能对字符串进行操作。see here

产生随机浮点数，它们均匀分布在几个子区间[bi，bi+1]中的每个子区间内，每个子区间都有自己的权重wi。

将其更改为：

std::vector<float> weights { 0.40f, 0.50f, 0.10f };
std::discrete_distribution<> dist(weights.begin(), weights.end());

std::mt19937 gen(rd());

for(int i = 0; i<N;i++)
{
    std::cout << interval[dist(gen)] << "\n";
}

你就完事了。

建议:阅读由你的朋友编译器产生的消息

In file included from /usr/include/c++/4.7/random:51:0,
                 from prog.cpp:2:
/usr/include/c++/4.7/bits/random.tcc: In instantiation of ‘std::piecewise_constant_distribution<_RealType>::param_type::param_type(_InputIteratorB, _InputIteratorB, _InputIteratorW) [with _InputIteratorB = __gnu_cxx::__normal_iterator<std::basic_string<char>*, std::vector<std::basic_string<char> > >; _InputIteratorW = __gnu_cxx::__normal_iterator<float*, std::vector<float> >; _RealType = double]’:
/usr/include/c++/4.7/bits/random.h:4940:39:   required from ‘std::piecewise_constant_distribution<_RealType>::piecewise_constant_distribution(_InputIteratorB, _InputIteratorB, _InputIteratorW) [with _InputIteratorB = __gnu_cxx::__normal_iterator<std::basic_string<char>*, std::vector<std::basic_string<char> > >; _InputIteratorW = __gnu_cxx::__normal_iterator<float*, std::vector<float> >; _RealType = double]’
prog.cpp:17:64:   required from here
/usr/include/c++/4.7/bits/random.tcc:2407:3: error: no matching function for call to ‘std::vector<double>::push_back(std::basic_string<char>&)’
/usr/include/c++/4.7/bits/random.tcc:2407:3: note: candidates are:
In file included from /usr/include/c++/4.7/vector:65:0,
                 from /usr/include/c++/4.7/bits/random.h:34,
                 from /usr/include/c++/4.7/random:50,
                 from prog.cpp:2:
/usr/include/c++/4.7/bits/stl_vector.h:881:7: note: void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = double; _Alloc = std::allocator<double>; std::vector<_Tp, _Alloc>::value_type = double]
/usr/include/c++/4.7/bits/stl_vector.h:881:7: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘const value_type& {aka const double&}’
/usr/include/c++/4.7/bits/stl_vector.h:899:7: note: void std::vector<_Tp, _Alloc>::push_back(std::vector<_Tp, _Alloc>::value_type&&) [with _Tp = double; _Alloc = std::allocator<double>; std::vector<_Tp, _Alloc>::value_type = double]
/usr/include/c++/4.7/bits/stl_vector.h:899:7: note:   no known conversion for argument 1 from ‘std::basic_string<char>’ to ‘std::vector<double>::value_type&& {aka double&&}’

看见?

未知参数1从‘std：：basic_string’到‘std：：vector：：value_type&& {aka double&&}’的转换

什么都说出来了。