What kind of value will map::find() return if "key" does'nt exist?

Question

Let's assume I have a map in my code:

map <string, set<string> > myMap;

... and I want to get an element from map:

myMap.find("key");

My question is: what kind of value will myMap return if "key" doesn't exist? `

///EDIT Can anyone point the reason of error? Compiler doesn't see any mistake but server which tests whole algorithm, doesn't accept it because of this function.

    map< string, set<string> >::iterator mapIterator = container.find(key);

    if(mapIterator != container.end()){
        set<string>::iterator setIterator = mapIterator->second.begin();
        if(!mapIterator->second.empty()){
            while(setIterator != mapIterator->second.end()){
                cout << *setIterator << endl;
                ++setIterator;
            }
        }else{
            .......
        }
    }else{
        ..........
    }

possible duplicate of [What happens if I read a map's value where the key does not exist?](http://stackoverflow.com/questions/10124679/what-happens-if-i-read-a-maps-value-where-the-key-does-not-exist) — honk, Apr 17 '14 at 18:44

score 2 · Answer 1 · answered Apr 17 '14 at 18:43

2

It returns an iterator equal to myMap.end(). You can easily test for that:

auto it = myMap.find("key");
if (it == myMap.end())
{
  std::cout << "key not found\n";
}

answered Apr 17 '14 at 18:43

juanchopanza

223,364
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qeadz · Answer 2 · 2014-04-17T18:51:20.910

2

http://en.cppreference.com/w/cpp/container/map/find

find returns an iterator, so if the key does not exist it returns an end iterator.

eg:

std::map< int, int > some_map;
if ( some_map.find( 10 ) != some_map.end() )
{
  ... key exists ...
}
else
{
  ... key does not exist ...
}

edited Apr 17 '14 at 18:51

answered Apr 17 '14 at 18:45

qeadz

1,476
1
9
17

Please don't link to cplusplus.com. It is known for inaccuracies and shortcomings. Please use en.cppreference.com for C++ reference – Tony The Lion Apr 17 '14 at 18:46
Sure. Changed, although I do like how CPP is organized. Beyond the scope of this question on SO, but I haven't had any issues with the site. I often look up constructors, items in functional or algorithm and so far they've all done what CPP says they do with the parameters that CPP says they have. I guess if it had missed any features, I probably wouldn't realize they're missing. – qeadz Apr 17 '14 at 18:57

4pie0 · Answer 3 · 2014-04-17T18:48:54.710

std::map::find returns iterator (or const_iterator) equal to map.end() if key is not present.

Return value

Iterator to an element with key equivalent to key. If no such element is found, past-the-end (see end()) iterator is returned.

http://en.cppreference.com/w/cpp/container/map/find

Example:

std::map<std::string, int> m;
if( m.find( searchedString) != m.end()) {
   //... present
} else {
   //... not found
}

score 0 · Answer 4 · answered Apr 17 '14 at 18:44

An iterator pointing past the end of the collection. This is true for all stl containers. Check for the value with

map <string, set<string> > myMap;
map <string, set<string> > :: iterator iter = myMap.find( "x" );
if( iter != myMap.end())
{
    // ... do something
}

What kind of value will map::find() return if "key" does'nt exist?

4 Answers4