There is an assumption that it is possible to turn with sort(v.begin(), v.end(), cmp) , but I don’t understand how the third parameter is involved / what the third parameter means (Google also did not clarify)

    3 answers 3

    Assuming that you are interested in vector<pair<int,pair<int,int>>> , cmp might look something like this, for example:

     bool cmp(const pair<int,pair<int,int>>& p1, const pair<int,pair<int,int>>& p2) { return p1.second.second < p2.second.second; }

      This is a link to a function of the form bool foo(const T& a, const T& b) , where T is the type of the elements of the sorted vector.

      The task of this function is to check whether a and b ordered in ascending order. In other words, does the value of a exceed that of b (in mathematical terms, whether the condition a < b is met).

      • Not necessarily a function, it is quite possible a functor (for example, lambda). And the signature should be bool (const T& a, const T& b) like. - VladD
      • I corrected the types of arguments (thanks for the remark), but is it worth it to load a novice with such concepts as “functor” and “lambda-function” (the latter, besides, is absent in the standards of the 98th and 2003th years)? - ߊߚߤߘ
      • one
        Well, at least show the opportunity to write a lambda as an advanced use? I would generally write “any garbage that can be called like this: const T& l; const T& r; bool v = fignya(l, r); const T& l; const T& r; bool v = fignya(l, r); ". Although this is probably too much. (But in the spirit of C ++.) - VladD
      • And who would explain to me how such a type is possible - vector<int,pair<int,int>> ? - Harry
      • @Harry, there is some typo. Well, the vector cannot accept the second template argument pair, only the memory management class for the elements. Probably meant by all the same vector<pair<int, pair<int, int> > > . - ߊߚߤߘ

      cmp is any functional object. It can be a lambda, a pointer to a function or an object of a class that has operator() defined. This function object must accept two objects from the list to be sorted, and return bool . The object must implement a strict weak ordering operation. This is how it will look like in your example:

       #include <iostream> #include <vector> #include <algorithm> typedef std::pair<int, std::pair<int, int>> IntIntInt; struct Compare{ inline bool operator()(const IntIntInt &x, const IntIntInt &y) const{ return x.second.second < y.second.second; } }; int main(){ std::vector<IntIntInt> v; v.push_back(std::make_pair(5, std::make_pair(3, 4))); v.push_back(std::make_pair(1, std::make_pair(9, 0))); v.push_back(std::make_pair(8, std::make_pair(6, 7))); v.push_back(std::make_pair(4, std::make_pair(2, 3))); v.push_back(std::make_pair(3, std::make_pair(1, 2))); std::sort(v.begin(), v.end(), Compare()); }

      PS:

      Google also did not clarify

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