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Reverse map lookup

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囚心锁ツ
囚心锁ツ 2020-11-30 05:37

I have a 1 to 1 map. What\'s the best way to find keys from values,

i.e.

For examples if the map is this

KEY VALUE

a    1
b    2
c
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  • -上瘾入骨i
    -上瘾入骨i (楼主)
    2020-11-30 06:03

    Unless the map is huge, or you have some other way of knowing that linear search is too slow, I'd start with linear search:

    #include 
using std::cout;

#include 
using std::map;

#include 
using std::find_if;

#include 
using boost::assign::map_list_of;

typedef map Map;
typedef Map::key_type Key;
typedef Map::value_type Pair;
typedef Map::mapped_type Value;


struct finder {
    const Value v;
    finder(const Value& v) : v(v) {}
    bool operator()(const Pair& p) {
        return p.second == v;
    }
};

Map m = map_list_of('a', 1)('b', 2)('c', 3)('d', 4)('e', 5);

int main() {
    Pair v = *find_if(m.begin(), m.end(), finder(3));
    cout << v.second << "->" << v.first << "\n";
}

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