Checking value exist in a std::map - C++

Question

I know find method finds the supplied key in std::map and return an iterator to the element. Is there anyway to find the value and get an iterator to the element? What I need to do is to check specified value exist in std::map. I have done this by looping all items in the map and comparing. But I wanted to know is there any better approach for this.

Here is what I have wrote

bool ContainsValue(Type_ value)
{
    bool found = false;
    Map_::iterator it = internalMap.begin(); // internalMap is std::map
    while(it != internalMap.end())
    {
        found = (it->second == value);
        if(found)
            break;
        ++it;
    }
    return found;
}

Edit

How about using another map internally which stores value,key combination. So I can call find on it? Is find() in std::map doing sequential search?

Thanks

Answer 1

Possible that I don't fully understand what you're trying to accomplish. But to simply test whether or not a map contains a value, I believe you can use the std::map's built in find.

bool ContainsValue(Type_ value)
{
    return (internalMap.find(value) != internalMap.end());
}

Answer 2

How about using another map internally which stores value,key combination. So I can call find on it?

Yes: maintain two maps, with one map using one type of key and the other using the other.

Is find() in std::map doing sequential search?

No it's a binary search of a sorted tree: its speed is O(log(n)).

Answer 3

I am adding this answer, if someone come here and looks for c++11 and above..

    //DECLARE A MAP
    std::map<int, int> testmap;

    //SAMPLE DATA
    testmap.insert(std::make_pair(1, 10));
    testmap.insert(std::make_pair(2, 20));
    testmap.insert(std::make_pair(3, 30));
    testmap.insert(std::make_pair(4, 20));

    //ELEMENTS WITH VALUE TO BE FOUND
    int value = 20;

    //RESULTS
    std::map<int, int> valuesMatching;

    //ONE STEP TO FIND ALL MATCHING MAP ELEMENTS
    std::copy_if(testmap.begin(), testmap.end(), std::inserter(valuesMatching, valuesMatching.end()), [value](const auto& v) {return v.second == value; });

Answer 4

try this function:

template <class Map, class Val> typename Map::const_iterator MapSearchByValue(const Map & SearchMap, const Val & SearchVal)
{
    Map::const_iterator iRet = SearchMap.end();
    for (Map::const_iterator iTer = SearchMap.begin(); iTer != SearchMap.end(); iTer ++)
    {
        if (iTer->second == SearchVal)
        {
            iRet = iTer;
            break;
        }
    }
    return iRet;
}

i think it's useful