使用以位集为键的映射时出现的问题

Problems using a map with a bitset as a key

本文关键字：映射 问题      更新时间：2023-10-16

我正试图用bitset作为键在C++中创建一个map。但是编译器会生成以下错误消息

In file included from /usr/include/c++/4.6/string:50:0,
                 from /usr/include/c++/4.6/bits/locale_classes.h:42,
                 from /usr/include/c++/4.6/bits/ios_base.h:43,
                 from /usr/include/c++/4.6/ios:43,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iostream:40,
                 from test2.cpp:1:
/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = std::bitset<8u>]’:
/usr/include/c++/4.6/bits/stl_map.h:452:2:   instantiated from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::bitset<8u>, _Tp = int, _Compare = std::less<std::bitset<8u> >, _Alloc = std::allocator<std::pair<const std::bitset<8u>, int> >, std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int, std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::bitset<8u>]’
test2.cpp:22:30:   instantiated from here
/usr/include/c++/4.6/bits/stl_function.h:236:22: error: no match for ‘operator<’ in ‘__x < __y’
/usr/include/c++/4.6/bits/stl_function.h:236:22: note: candidates are:
/usr/include/c++/4.6/bits/stl_pair.h:207:5: note: template<class _T1, class _T2> bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
/usr/include/c++/4.6/bits/stl_iterator.h:291:5: note: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.6/bits/stl_iterator.h:341:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.6/bits/basic_string.h:2510:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/bits/basic_string.h:2522:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT*)
/usr/include/c++/4.6/bits/basic_string.h:2534:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&)
/usr/include/c++/4.6/bits/stl_tree.h:856:5: note: template<class _Key, class _Val, class _KeyOfValue, class _Compare, class _Alloc> bool std::operator<(const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&, const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_set.h:713:5: note: template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::set<_Key, _Compare, _Alloc>&, const std::set<_Key, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_multiset.h:696:5: note: template<class _Key, class _Compare, class _Alloc> bool std::operator<(const std::multiset<_Key, _Compare, _Alloc>&, const std::multiset<_Key, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_map.h:894:5: note: template<class _Key, class _Tp, class _Compare, class _Alloc> bool std::operator<(const std::map<_Key, _Tp, _Compare, _Alloc>&, const std::map<_Key, _Tp, _Compare, _Alloc>&)
/usr/include/c++/4.6/bits/stl_multimap.h:812:5: note: template<class _Key, class _Tp, class _Compare, class _Alloc> bool std::operator<(const std::multimap<_Key, _Tp, _Compare, _Alloc>&, const std::multimap<_Key, _Tp, _Compare, _Alloc>&)

程序代码如下我正在尝试使用位集作为C++中映射的键。然而，每次运行下面的代码时，我都会遇到errros。

#include <iostream>
#include <algorithm>
#include <string>
#include <bitset>
#include <set>
#include <utility>
using namespace std;
int main(int argc, char *argv[])
{
    bitset<8> test;
    test = 9;
    cout<<"Set to 9"<<endl;
    map <bitset<8> , int> mymap;
    pair <biset<8> , int> p;
    p.first = test;
    p.second = 9;
    string teststring;
    teststring = test.to_string<char,char_traits<char>,allocator<char> >();
    cout<<teststring<<temymap[test]<<endl;
    return 0;
}

只需使用您自己的比较器类：

struct Comparer {
    bool operator() (const bitset<8> &b1, const bitset<8> &b2) const {
        return b1.to_ulong() < b2.to_ulong();
    }
};
/* ... */
map <bitset<8> , int, Comparer> mymap;

请注意，您可以扩展此解决方案以支持任意长度的位集，只要它们足够小，可以转换为无符号长：

template<size_t sz> struct bitset_comparer {
    bool operator() (const bitset<sz> &b1, const bitset<sz> &b2) const {
        return b1.to_ulong() < b2.to_ulong();
    }
};
map <bitset<8> , int, bitset_comparer<8> > mymap;
map <bitset<16> , int, bitset_comparer<16> > mymap16;

如果仍然满足您的要求，另一种解决方案是简单地使用unordered_map。

这可能是std::unordered_map<bitset<N>, T>或boost::unordered_map<bitset<N>, T>，具体取决于C++版本或性能方面的考虑。

这就避免了比较的需要，而且可能会更快，具体取决于需求。

您可以定义比较函数。如果假设bitset为无符号整数值建模，则以下函数将按递增顺序对bitset s进行排序（适用于任何N）。

template <size_t N>
class LessThan { 
public:
   bool operator() (const std::bitset<N> &lhs, const std::bitset<N> &rhs) const 
   { 
      size_t i = N;
      while ( i > 0 ) {
         if ( lhs[i-1] == rhs[i-1] ) {
            i--;
         } else if ( lhs[i-1] < rhs[i-1] ) {
            return true;
         } else {
            return false;
         }
      }
      return false;
   } 
};

如果运行以下代码段：

  const size_t mysz = 10;
  std::map< std::bitset<mysz>, size_t, Less<mysz> > mymap;
  for ( size_t i = 0; i < 10; i++ ) {
     mymap.insert( std::make_pair(std::bitset<mysz>(i),i) );
  }

你会有这个地图：

mymap[0]    is the pair ((0,0,0,0,0,0,0,0,0,0), 0)  
mymap[1]    is the pair ((1,0,0,0,0,0,0,0,0,0), 1)  
mymap[2]    is the pair ((0,1,0,0,0,0,0,0,0,0), 2)  
mymap[3]    is the pair ((1,1,0,0,0,0,0,0,0,0), 3)  
mymap[4]    is the pair ((0,0,1,0,0,0,0,0,0,0), 4)  
mymap[5]    is the pair ((1,0,1,0,0,0,0,0,0,0), 5)  
mymap[6]    is the pair ((0,1,1,0,0,0,0,0,0,0), 6)  
mymap[7]    is the pair ((1,1,1,0,0,0,0,0,0,0), 7)  
mymap[8]    is the pair ((0,0,0,1,0,0,0,0,0,0), 8)  
mymap[9]    is the pair ((1,0,0,1,0,0,0,0,0,0), 9)

这将允许映射<位集<N>；，int>直接填充：

namespace std{
    template<size_t N>
    struct less<bitset<N> > : binary_function <bitset<N>,bitset<N>,bool>{
        bool operator()(const bitset<N> &L, const bitset<N> &R) const{
            for(unsigned int i=0;i<L.size();i++)
                if(L.test(i)){
                    if(!R.test(i))return false;
                }else{
                    if(R.test(i))return true;
            }
            return false; //same
        }
    };
}

对于映射中的存储，如果大型位集不能转换为u_long，则可以将位集转换为字符串；对于更新，可以更改回位集并进行更改并存储回字符串。

map<string , int> mymap;
bitset<N> mybs("10100"); // converting string to bitset
map[mybs.to_string()] = 34; // bitset to string for map

只要你不在乎比较，这是有效的。

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