Check if element exists in map c++

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Is there any function to find if an element exists in a map (not a key). For instance I have a map and want to find if a word is the second element of any pair from the map, and if so, what is its key.

Maps are organized by key, so short of iterating over each element there is no way to do this.

Assuming you are searching for look_for, you can use the standard library's std::find_if:

auto it = std::find_if(map.begin(),
                       map.end(),
                       [&look_for](const auto& kv_pair) {
                            return kv_pair.second == look_for;
                       });
if (it != map.end()) {
    auto key = it->first;
    // ...
} else {
    // Value not found
    // ...
}

It is also possible to use a manual loop (std::find_if is often considered nicer, but here that might be overkill)

for (const auto& kv_pair : map)
    if (kv_pair.second == look_for)
        return kv_pair.first;
// If you reach this, the value isn't in your map

How to find if a given key exists in a C++ std::map, To retrieve a value from the map via a key that you know to exist, use map::at: value = m.at(key) If you want to use other API, then find go for m.count(c)>0 In this article we will discuss how to find if a given key exists in map or not. Map internally store elements in Key-Value pair. It provides 2 member functions to check if a given key exists in map i.e. std::map provides a member function count () i.e. It finds & returns the count of number of elements in map with key K.

Use std::find_if with a lambda:

map<int,int> m;
m[1] = 10;
m[2] = 20;
m[3] = 30;
auto target = 20;
auto f20 = std::find_if(std::begin(m), std::end(m), [&](const auto& p) {
    return p.second == target;
});
if (f20 != std::end(m)) {
    // We have our value
}

Demo.

Note that, unlike a search of keys that is logarithmic, search on values is linear.

Check if a given key exists in a map or not in C++, To check for the existence of a particular key in the map, the standard solution unordered map container which returns an iterator to the key-value pair if the In this post, we will see how to check if a given key exists in a map or not in C++. To check for the existence of a particular key in the map, the standard solution is to use the public member function find() of the ordered or the unordered map container which returns an iterator to the key-value pair if the specified key is found, or iterator to the end of the container if the specified key is not found.

Scanning original key-value dictionary requires O(n) time and it might be slow.

To find specific value and its key fast, creating reverse dictionary is a way. To original std::map for key-value, reverse dictionary is std::unordered_multimap for value-key. The lookup time is O(1).

Check if a key is present in a C++ map or unordered_map , shuffle vs random_shuffle in C++ · Difference between set, multiset, unordered_set, Operator (%) in C/C++ with Examples · How to find the Entry with largest Value in a C++ Map Check if a key is present in a C++ map or unordered_map. in this case, if intensityValue[x][y] does not exist in map then after checking it will automatically insert intensityValue[x][y] in the map which I don't want. Please suggest an efficient way, so that I can check whether intensityValue[x][y] already exist in the map or not without inserting it in the map.

C++ map::find - Search Element, Related Posts: search · Search jobs · Binary Search Tree inorder traversal – C Program · Program to Check Even or Odd  Check if a key is present in a C++ map or unordered_map Check if a key is present in a C++ map or unordered_map A C++ map and unordered_map are initialized to some keys and their respective mapped values.

map::find - C++ Reference, Searches the container for an element with a key equivalent to k and returns an function, map::count, can be used to just check whether a particular key exists. std::map< char , int >::iterator it; mymap[ 'a' ]=50; mymap[ 'b' ]=100; mymap[ 'c' ]=  Searches the container for an element with k as key and returns an iterator to it if found, otherwise it returns an iterator to unordered_map::end (the element past the end of the container). Another member function, unordered_map::count, can be used to just check whether a particular key exists.

JSON for Modern C++: nlohmann::basic_json::contains, key value to check its existence. Returns: true if an element with specified key exists. If no such element with such key is found or the JSON value is not  The only way I have found to check for duplicates is by inserting and checking the std::pair.second for false, but the problem is that this still inserts something if the key is unused, whereas what I want is a map.contains(key); function.

Comments
  • std::find_if But remember that you have to think of cases where the value could be multiple times in the map.
  • Looks like you chose a wrong data structure for your problem. Look at Boost bimap or Boost muli-index
  • Might want to add a check for map.end() before dereferencing it.
  • Whoops, added that
  • Good recommendation. Just thought it should be mentioned that it requires you to be able to hash your value type.