How to determine if an element exists in std :: map?

Question

My use case:

map<string, Car> cars; bool exists(const string& name) { // somehow I should find whether my MAP has a car // with the name provided return false; } 

Could you suggest the best and most elegant way to do this in C ++? Thank.

Answer 1

Of course use an iterator

 map<string,Car>::const_iterator it = cars.find(name); return it!=cars.end(); 

Answer 2

 return cars.find(name) != cars.end(); 

Answer 3

You can also use

 bool exists(const string& name) { return cars.count(name) != 0; } 

Answer 4

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

You can call map :: count (key) with a specific key; it will return the number of records for this key. For cards with unique keys, the result will be 0 or 1. Since the multimap exists with the same interface, it is better to compare it with! = 0 for existence to be safe.

for your example, this

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

The advantages that I see are: 1. shorter code, 2. benefit from any optimizations that the library can apply internally using its presentation data.

Answer 5

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 the special version for std::map , as it offers a more efficient search alternative. You can add additional specializations as needed (for example, for std::set and others).

Answer 6

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

Answer 7

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

Returns map::end if the item does not exist.

Answer 8

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

Answer 9

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