C++ 查找地图的映射值
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Find mapped value of map
提问by wrongusername
Is there a way in C++ to search for the mapped value (instead of the key) of a map, and then return the key? Usually, I do someMap.find(someKey)->secondto get the value, but here I want to do the opposite and obtain the key (the values and keys are all unique).
C++ 中有没有办法搜索映射的映射值（而不是键），然后返回键？通常，我这样做是someMap.find(someKey)->second为了获取值，但在这里我想做相反的事情并获取键（值和键都是唯一的）。
采纳答案by Bill Lynch
Because of how a mapis designed, you'll need to do the equivalent of a search on unordered data.
由于 amap的设计方式，您需要对无序数据进行等效的搜索。
for (auto it = someMap.begin(); it != someMap.end(); ++it)
if (it->second == someValue)
return it->first;
回答by Pavan Chandaka
Using lambdas (C++11 and newer)
使用 lambdas（C++11 和更新版本）
//A MAP OBEJCT
std::map<int, int> mapObject;
//INSERT VALUES
mapObject.insert(make_pair(1, 10));
mapObject.insert(make_pair(2, 20));
mapObject.insert(make_pair(3, 30));
mapObject.insert(make_pair(4, 40));
//FIND KEY FOR BELOW VALUE
int val = 20;
auto result = std::find_if(
mapObject.begin(),
mapObject.end(),
[val](const auto& mo) {return mo.second == val; });
//RETURN VARIABLE IF FOUND
if(result != mapObject.end())
int foundkey = result->first;
回答by Paul
What you're looking for is a Bimap, and there is an implementation of it available in Boost: http://www.boost.org/doc/libs/1_36_0/libs/bimap/doc/html/index.html
您正在寻找的是 Bimap，并且在 Boost 中有它的实现：http: //www.boost.org/doc/libs/1_36_0/libs/bimap/doc/html/index.html
回答by lubgr
As this has not been mentioned yet: structured bindings (available since C++17) enable a convenient way of writing the same loop as depicted in Bill Lynch's answer, i.e.
由于尚未提及这一点：结构化绑定（自 C++17 起可用）启用了一种方便的方式来编写与Bill Lynch 的回答中描述的相同的循环，即
for (const auto& [key, value] : someMap)
if (value == someValue)
return key;
回答by Karan Saxena
We can create a reverseMap which maps values to keys.
我们可以创建一个将值映射到键的 reverseMap。
Like,
喜欢，
map<key, value>::iterator it;
map<value, key> reverseMap;
for(it = originalMap.begin(); it != originalMap.end(); it++)
reverseMap[it->second] = it->first;
This also is basically like a linear search but will be useful if you have a number of queries.
这也基本上类似于线性搜索，但如果您有多个查询，它将很有用。
回答by ???
struct test_type
{
CString str;
int n;
};
bool Pred( std::pair< int, test_type > tt )
{
if( tt.second.n == 10 )
return true;
return false;
}
std::map< int, test_type > temp_map;
for( int i = 0; i < 25; i++ )
{
test_type tt;
tt.str.Format( _T( "no : %d" ), i );
tt.n = i;
temp_map[ i ] = tt;
}
auto iter = std::find_if( temp_map.begin(), temp_map.end(), Pred );
