从满足谓词的无序映射中删除元素

Posted 2023-02-17

【中文标题】从满足谓词的无序映射中删除元素

【英文标题】：Remove Elements from an Unordered Map Fulfilling a Predicate

【发布时间】：2012-02-09 11:25:11

【问题描述】：

我想从std::unordered_map（直方图）中删除满足作为 lambda 表达式给出的预测（计数为零的直方图箱）的元素（直方图箱），如下所示

std::remove_if(begin(m_map), end(m_map), [](const Bin & bin)  return bin.second == 0; );

但是 GCC-4.6.1 抱怨如下

/usr/include/c++/4.6/bits/stl_pair.h:156:2: error: assignment of read-only member ‘std::pair<const unsigned char, unsigned char>::first’
/usr/include/c++/4.6/bits/stl_pair.h: In member function ‘std::pair<_T1, _T2>& std::pair<_T1, _T2>::operator=(std::pair<_T1, _T2>&&) [with _T1 = const unsigned char, _T2 = long unsigned int, std::pair<_T1, _T2> = std::pair<const unsigned char, long unsigned int>]’:
/usr/include/c++/4.6/bits/stl_algo.h:1149:13:   instantiated from ‘_FIter std::remove_if(_FIter, _FIter, _Predicate) [with _FIter = std::__detail::_Hashtable_iterator<std::pair<const unsigned char, long unsigned int>, false, false>, _Predicate = pnw::histogram<V, C, H>::pack() [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]::<lambda(const Bin&)>]’
tests/../histogram.hpp:68:13:   instantiated from ‘void pnw::histogram<V, C, H>::pack() [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/../histogram.hpp:85:13:   instantiated from ‘void pnw::histogram<V, C, H>::normalize(uint) [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >, uint = unsigned int]’
tests/../histogram.hpp:121:51:   instantiated from ‘H& pnw::histogram<V, C, H>::add(It, It) [with It = __gnu_cxx::__normal_iterator<const unsigned char*, std::vector<unsigned char> >, V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/../histogram.hpp:129:55:   instantiated from ‘H& pnw::histogram<V, C, H>::add(const V&) [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/../histogram.hpp:57:60:   instantiated from ‘pnw::histogram<V, C, H>::histogram(const V&, pnw::histogram<V, C, H>::TYPE_t) [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/t_histogram.cpp:38:61:   instantiated from ‘void test_dense_histogram() [with T = unsigned char, C = long unsigned int]’
tests/t_histogram.cpp:64:5:   instantiated from ‘void test_histograms() [with C = long unsigned int]’
tests/t_histogram.cpp:200:29:   instantiated from here
/usr/include/c++/4.6/bits/stl_pair.h:156:2: error: assignment of read-only member ‘std::pair<const unsigned char, long unsigned int>::first’
make: *** [tests/t_histogram.o] Error 1

std::remove_if不适用于std::unordered_map吗？

【问题讨论】：

我也落入了这个陷阱，但现在想起来 std::remove（将元素移动到容器的末尾）无法与关联容器一起使用。 “容器的尽头”已经没有任何意义了。

【参考方案1】：

答案是否定的（您不能在关联容器上使用remove_if）。你需要做一个简单的循环； erase(iterator) 成员现在返回下一个有效的迭代器 - 所以你的循环变成：

for(auto it = begin(m_map); it != end(m_map);)

  if (it->second == 0)
  
    it = m_map.erase(it); // previously this was something like m_map.erase(it++);
  
  else
    ++it;

【参考方案2】：

Stephan T. Lavavej 有a proposal of Uniform Container Erasure：

template <class K, class T, class H, class P, class A, class Predicate>
void erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

即

std::erase_if(m_map, [](const Bin& bin)  return bin.second == 0; );

不幸的是，它没有成为 C++17。我想这与 Ranges TS 的采用有关。