Question

First I will give a specific case, and the I would like to see if it can be applied to a general problem.

Say I have map. And I want to get all the keys meeting a certain criteria. For example all keys that contain ‘COL’. My naive implementation will be

template<typename T> void  Filter (map<string, T> & m, std:set<string> & result, const std::string& condition) {     for(map<string,string> iter=m.begin();iter!m.end();iter++)     {            std::string key=iter->first;             size_t found=key.find(condition);            if (found!=string::npos)               result.insert(key);     }       } 

what is the good way to implement this?

Also, what is a good way to implement general problem when I want to filter map using algos?

Answer 1

I think your solution is rather good: it is clear, and except if you can ‘guess’ hash values based on the condition, I don’t think you could be much more performant. However, you could change your function to make it more generic:

template<typename TKey, typename TValue, typename Predicate> void filter (const map<TKey, TValue> & m,               set<TKey> & result,               Predicate & p) {     typename map<TKey,TValue>::const_iterator it = m.begin();     typename map<TKey,TValue>::const_iterator end = m.end();      for( ; it != end ; ++it)     {         TKey key = it->first;         if (p(key))             result.insert(key);     }      } 

Your example can then be writen using a functor as predicate:

struct Contains {     Contains(const string & substr) : substr_(substr) {}      bool operator()(const string & s)     {         return s.find(substr_) != string::npos;     }      string substr_; }; 

The call to filter will then look like this:

map<string, Obj> m; // Insert in m set<string> res; filter(m, res, Contains('stringToFind'));