unordered_multimap - 迭代find()的结果产生具有不同值的元素 [英] unordered_multimap - iterating the result of find() yields elements with different value
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问题描述
C ++中的多重映射似乎非常奇怪,我想知道为什么
The multimap in C++ seems to work really odd, i would like to know why
#include <iostream>
#include <unordered_map>
using namespace std;
typedef unordered_multimap<char,int> MyMap;
int main(int argc, char **argv)
{
MyMap map;
map.insert(MyMap::value_type('a', 1));
map.insert(MyMap::value_type('b', 2));
map.insert(MyMap::value_type('c', 3));
map.insert(MyMap::value_type('d', 4));
map.insert(MyMap::value_type('a', 7));
map.insert(MyMap::value_type('b', 18));
for(auto it = map.begin(); it != map.end(); it++) {
cout << it->first << '\t';
cout << it->second << endl;
}
cout << "all values to a" << endl;
for(auto it = map.find('a'); it != map.end(); it++) {
cout << it->first << '\t' << it->second << endl;
}
}
p>
this is the output:
c 3
d 4
a 1
a 7
b 2
b 18
all values to a
a 1
a 7
b 2
b 18
b $ b
为什么当我明确要求'a'时,输出仍然包含b作为键的任何东西?这是一个编译器还是stl错误?
why does the output still contain anything with b as the key when I am explicitly asking for 'a'? Is this a compiler or stl bug?
推荐答案
find
,返回与多图中的键匹配的第一个元素的迭代器(与任何其他映射一样)。您可能正在查找 equal_range
:
find
, as implemented, returns an iterator for the first element which matches the key in the multimap (as with any other map). You're likely looking for equal_range
:
// Finds a range containing all elements whose key is k.
// pair<iterator, iterator> equal_range(const key_type& k)
auto its = map.equal_range('a');
for (auto it = its.first; it != its.second; ++it) {
cout << it->first << '\t' << it->second << endl;
}
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