比较 priority_queue/stl 中的 function

Question

 bool operator() (const pair<int,string>& a,const pair<int,string>& b){
        if(a.first!=b.first)
            return a.first>b.first;
        else
            return a.second < b.second;
    }

i have been doing heap problems, found out about this comparator function, here its comparing frequency of 2 words, but im not getting how exactly does it work and can't find any appropriate explanation of it. Could someone please explain it in a simple way? Thank you.

Answer 1

The comparison function sorts on the int s first in descending order. If two compared int s are the same, it'll do a lexicographical comparison of the string s and sort on those in ascending order.

Example, given a container with this content:

std::vector<std::pair<int, std::string>> vec{
    {1, "F"}, {2, "E"}, {3, "D"}, {4, "C"}, {5, "B"}, {6, "A"},
    {1, "A"}, {2, "B"}, {3, "C"}, {4, "D"}, {5, "E"}, {6, "F"},
};

Using a standard std::sort with your comparison function would produce this order:

6 A
6 F
5 B
5 E
4 C
4 D
3 C
3 D
2 B
2 E
1 A
1 F

For a std::priority_queue the extraction order will be the reverse since it puts the largest value on top and your comparison function says that a smaller number has higher priority than a larger number.

Demo with debug prints

Note that I used std::tie in the demo. It is often preferred when creating comparison functions that are required to do strict weak ordering because it makes it a lot easier to avoid making mistakes. It does the same as your original though:

bool operator()(const std::pair<int, std::string>& a,
                const std::pair<int, std::string>& b) 
{
    return std::tie(b.first, a.second) < std::tie(a.first, b.second);
}