C ++在地图中按值查找 [英] c++ find by value in map
问题描述
我想在地图中按值找到对应的键.
我可以线性地迭代并找到密钥,但是如果我的所有值也都是唯一的,我们还有另一个更好的解决方案吗?
I wanted to find in map by value corresponding key.
I can iterate linearly and find the key, but do we have another better solution if my all values are also unique.
提前谢谢.
推荐答案
您可以使用 std :: find_if
#include <algorithm>
template<typename M, typename V>
auto get_map_iterator_from_value(const M& m, const V& v) {
return std::find_if(std::begin(m), std::end(m),
[&v](const auto& pair) { return pair.second == v; });
}
...
auto it = get_map_iterator_from_value(your_map, your_value);
,但是它比在 key 上查询时要慢,因为它也必须遍历地图中的每个条目,直到找到匹配项.
but it'll be slower than when querying on key since it too will have to go through every entry in the map until it finds a match.
与顺序搜索相比,在搜索包含多个条目的映射时的一项改进可能是利用 std :: find_if
的并行执行支持:
An improvement, compared to sequenced searching, when searching a map with many entries could be to make use of std::find_if
s parallel execution support:
#include <algorithm>
#include <execution>
template<typename M, typename V>
auto get_map_iterator_from_value(const M& m, const V& v) {
return std::find_if(std::execution::par, std::begin(m), std::end(m),
[&v](const auto& pair) { return pair.second == v; });
}
如果这真的使它更快或更不需要使用实际地图进行测试.
If that really makes it faster or not has to be tested with your actual map.
注意:如果您经常需要这样做,则可以使用双向地图,例如 Boost.Bimap 更好.
Note: If you need to do this often a bidirectional map like Boost.Bimap is better.
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