How to find whether an element exists in std::map?

Question

My use case:

map cars;
bool exists(const string& name) {
  // somehow I should find whether my MAP has a car
  // with the name provid         


        

Answer 1

std::map::find(const key_type& x );

It returns map::end if the item doesn't exist.

Answer 2

bool exists(const std::map<std::string, Car>& cars, const std::string& name) {
  return cars.end() != cars.find(name);
}

Answer 3

What about:

template <typename KeyType, typename Collection>
bool exists_in(Collection const& haystack, KeyType const& needle) {
    return std::find(haystack.begin(), haystack.end(), needle) != haystack.end();
}

template <typename K, typename V>
bool exists_in(std::map<K,V> const& haystack, K const& needle) {
    return haystack.find(needle) != haystack.end();
}

This makes exists_in work with any standard container via std::find and use a special version for std::map since it offers a more efficient searching alternative. You could add additional specializations as necessary (e.g., for std::set and others).

Answer 4

bool exists(const string& name)
{
    return cars.find(name) != cars.end();
}

Answer 5

Apart from the answers with iterator-Value from find() and comparison to .end(), there is another way: map::count.

You can call map::count(key) with a specific key; it will return how many entries exist for the given key. For maps with unique keys, the result will be either 0 or 1. Since multimap exists as well with the same interface, better compare with != 0 for existence to be on the safe side.

for your example, that's

return (cars.count(name)>0);

The advantages I see are 1. shorter code, 2. benefit from whatever optimisations the library may apply internally, using its representation details.

Answer 6

#define itertype(v) typeof((v).begin())
itertype(cars) it = cars.find(name);
return it != cars.end();