unordered_multimap学习之查找操作和观察器操作count，find，contains(C++20)，equal_range,hash_function,key_eq

本篇学习unordered_multimap的查找操作和观察器操作&＃xff0c;具体函数如下&＃xff1a;

代码示例&＃xff1a;

#include
#include
#include using namespace std;void findOpertion()
{//1.count返回匹配特定键的元素数量std::unordered_multimap map1;map1.emplace(1, "Hero");map1.emplace(2, "Archer");map1.emplace(3, "Barry");int nkey &＃61; 0;for(nkey &＃61; 1; nkey <6; &＃43;&＃43;nkey){if(map1.count(nkey) > 0)std::cout < map2;map2.emplace(4, "camel");map2.emplace(5, "iran");std::unordered_multimap::iterator it;it &＃61; map2.find(4);if(it !&＃61; map2.end())std::cout <<"find element is " <second < map3;map3.emplace(1, "scott");map3.emplace(3, "camel");map3.emplace(5, "Sky");map3.emplace(7, "beer");pair::iterator, unordered_multimap::iterator> it2;pair::iterator, unordered_multimap::iterator> it3;std::unordered_multimap map &＃61; {{1,&＃39;a&＃39;},{1,&＃39;b&＃39;},{3,&＃39;d&＃39;},{5,&＃39;b&＃39;}};auto range &＃61; map.equal_range(1);for (auto it &＃61; range.first; it !&＃61; range.second; &＃43;&＃43;it) {std::cout <first <<&＃39; &＃39; <second <<&＃39;\n&＃39;;}std::cout <first <<" &＃61;> " <second <first <<" &＃61;> " <second <first <<" &＃61;> " <second <first <<" &＃61;> " <second <first <<" &＃61;> " <second <first <<" &＃61;> " <second <::iterator lowerIt;//lowerIt &＃61; map3.lower_bound(2);//这个key一定要存在unordered_map中&＃xff0c;否则返回end()迭代器//std::cout <<"lowerIt->first &＃61; " <first <<" lowerIt->second &＃61; " <second <first &＃61; " <first <<" lowerIt->second &＃61; " <second <::iterator upperIt;//upperIt &＃61; map3.upper_bound(4);//这个key一定要存在unordered_map中&＃xff0c;否则返回end()迭代器//std::cout <<"upperIt->first &＃61; " <first <<" upperIt->second &＃61; " <second <first &＃61; " <first <<" upperIt->second &＃61; " <second < map4;map4.emplace(1, "scott");map4.emplace(3, "beer");map4.emplace(5, "Sky");map4.emplace(7, "camel");std::unordered_multimap::key_compare funCompare &＃61; map4.key_comp();//返回一个比较键的函数bool b1 &＃61; funCompare(3, 5);bool b2 &＃61; funCompare(5, 3);std::cout <<"b1 &＃61; " < map5;map5.emplace(&＃39;1&＃39;, 12);map5.emplace(&＃39;3&＃39;, 8);map5.emplace(&＃39;5&＃39;, 54);map5.emplace(&＃39;7&＃39;, 6);std::unordered_multimap::iterator endIt &＃61; map5.end();auto lastElement &＃61; --endIt;//最后一个元素std::cout <<"lastElement.key &＃61; " <first <<" lastElement.value &＃61; " <second <::iterator firstIt &＃61; map5.begin();//第一个元素迭代器bool b3 &＃61; map5.value_comp()(*firstIt, *lastElement);&＃43;&＃43;firstIt;bool b4 &＃61; map5.value_comp()(*firstIt, *lastElement);std::cout <<"b3 &＃61; " < map6;unordered_multimap::hasher fn &＃61; map6.hash_function();std::cout <<"fn1 &＃61; " < map7;bool case_insensitive &＃61; map7.key_eq()("camle","CAMLE");std::cout <<"map7.key_eq() is ";std::cout <<( case_insensitive ? "case insensitive" : "case sensitive" );std::cout <}int main()
{findOpertion();cout <<"hello world" <}


运行结果&＃xff1a;

参考&＃xff1a;

https://zh.cppreference.com/w/cpp/container/unordered_multimap