Find Mapped Value of Map

Find mapped value of map

Because of how a map is designed, you'll need to do the equivalent of a search on unordered data.

for (auto it = someMap.begin(); it != someMap.end(); ++it)
    if (it->second == someValue)
        return it->first;

Finding an element in map by its value

You can use the member function find to search for key only. To search for a value, you can use a std::find_if with a lambda function (if you use C++11), or to traverse the map (ok in previous C++ version):

for (HandleMap::const_iterator it = map.begin(); it != map.end(); ++it) {
  if (it->second == name) return it->first;
} 
// or value not found

On the other hand, if searching for a value is a very common operation, you may want to have two maps: std::unordered_map<Handle, std::string> and std::unordered_map<std::string, Handle>. In that case, you have to make sure you perform insertions, deletions, etc. in both maps to keep then synchronized.

How to create map with mapped value?

You are correct that no lambda will work there. There are a few alternative options, but the one I'd use would be:

Map<String, String> result = inputStrings.stream()
    .map(str -> new AbstractMap.SimpleImmutableEntry<>(str, hashFunc(str)))
    .filter(entry -> entry.getValue().startsWith("00"))
    .collect(Collectors.toMap(Entry::getKey, Entry::getValue));

(If I weren't collecting to a Map, I would create my own custom tuple type appropriate for the use case rather than using Map.Entry, but here Map.Entry is a sufficient type.)

How to get values and keys from HashMap?

To get all the values from a map:

for (Tab tab : hash.values()) {
    // do something with tab
}

To get all the entries from a map:

for ( Map.Entry<String, Tab> entry : hash.entrySet()) {
    String key = entry.getKey();
    Tab tab = entry.getValue();
    // do something with key and/or tab
}

Java 8 update:

To process all values:

hash.values().forEach(tab -> /* do something with tab */);

To process all entries:

hash.forEach((key, tab) -> /* do something with key and tab */);

Finding Matching Keys and Values in a Map

You are putting all the values from first to output by passing it to the constructor.

Map<String, Integer> output = new HashMap<String, Integer>(first); // you are passing first to the constructor.

You don't need to create another Set, keySet() method returns set so the below lines not required.

Set<String> keyFirst = new HashSet<String>(); // stores all the keys for first
    for (String key: first.keySet()) { // goes through each key in input
        keyFirst.add(key); // adds all keys from first into keyFirst
}

Here's the correct implemetataion.

// we need to store the keys, then get the values in common, then put the key/map into map
public static Map<String, Integer> intersect(Map<String, Integer> first, Map<String, Integer> second) {
    Map<String, Integer> output = new HashMap<String, Integer>(); // combined output

    // goes through each key in common and checks to see if they reference to the same value    
    Iterator<String> keyFirstItr = first.keySet().iterator();
    while (keyFirstItr.hasNext()) {
        String keyTemp = keyFirstItr.next();
        if (first.get(keyTemp).equals(second.get(keyTemp))) { // If same key, same value mapped
            output.put(keyTemp, first.get(keyTemp)); // add key value to map
        }
    }
    return output;
}

how to use map::find on map of maps

To find a value in a map, you cannot use find member function, which finds a key. However, you can find a value, e.g., by using std::find_if with a custom comparator:

using value_t = std::map<std::string, std::string>;
using key_t = std::string;
using map_t = std::map<key_t, value_t>;

map_t m { { "key" , { { "hello", "world" } } } };

value_t v { { "hello", "world" } };  // value to be found
auto iter = std::find_if(m.begin(), m.end(),
               [&v](const auto& e){ return e.second == v; });
std::cout << (iter != m.end()) << std::endl;

Live demo: https://wandbox.org/permlink/1b0bjbnnPY8E0uiU

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Note that this will work since C++14. In C++11, you need to write:

auto iter = std::find_if(m.begin(), m.end(),
               [&v](const map_t::value_type& e){ return e.second == v; });

UPDATE

It's still kind-of unclear to me what you are trying to achieve. If you need a reference to the inner-most value only if it's both-level keys exist, then you can do (C++17 syntax):

if (auto i1 = m.find("key"); i1 != m.end())
   if (auto i2 = i1->second.find("hello"); i2 != i1->second.end())
      std::cout << i2->second << std::endl;  // same as m["key"]["hello"] here but faster

How to map values in a map in Java 8?

You need to stream the entries and collect them in a new map:

Map<String, String> result = map.entrySet()
    .stream()
    .collect(Collectors.toMap(Entry::getKey, e -> String.valueOf(e.getValue()));

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