Re: How to pass a third argument to compare function?
Lambda wrote:
I defined a class:
class inverted_index
{
private:
std::map<std::string, std::vector<size_t> > index;
public:
std::vector<size_t> intersect(const std::vector<std::string>&);
};
bool compare(const std::string&, const std::string&);
vector<size_t> inverted_index::intersect(const vector<string>& query)
{
sort(query.begin(), query.end(), compare);
vector<size_t> result = index[query.front()];
return result;
}
bool compare(const string& s1, const string& s2)
{
return index[s1].size() < index[s2].size();
}
In the intersect function, I want to sort the query by the
string matching vector size. So I define a compare function fort
'sort'.
I think I can't define compare as member function of inverted_index,
there is a current object as a implicit parameter for member
functions.
And the compare will have three parameters. I think it's wrong!
Right.
But if I define compare as a nonmember function with two parameters,
how can it access the index object. Make it 'friend' is useless here.
That can't work either as you noticed. What you can do is have compare a
functor (a class type with operator()) that does work in syntax such
as "compare(string1, string2)" and that has state (has a reference to the
index it uses for compation). For example
struct compare
{
explicit compare(std::map<std::string, std::vector<size_t> > const& index)
:index_(index) {}
bool operator()(std::string const& s1, std::string const& s2) const {
return index_[s1].size() < index_[s2].size();
}
private:
std::map<std::string, std::vector<size_t> > const& index_;
};
Note: to have the code more clear I recommend some (nested?) typedef for
the "std::map<std::string, std::vector<size_t> >" type and using that.
In order to use this compare functor you have to construct it giving it a
reference to the index to use for ordering, like:
std::sort(begin, end, compare(some-index));
--
Dizzy