std::set 与自定义比较器

Question

I am trying to order wordItems in descending wordCount order in a set.

I am in data structures, and our professor gives us challenges to use STL containers on previous assignments. The reason an std::set is being used here is because my Prof. wants to show us how to use an STL BST in practice.

I cannot seem to figure out the std::set and all of its functionality...

Here is what I believe to be the relevant code pertaining to my wordItem struct:

// header file (HashTable.hpp)
struct wordItem
{
    std::string word;
    int count;
    wordItem* next;
};

struct comp 
{
    // I have also tried bool operator<()(wordItem const &lhs, wordItem const &rhs)     
    bool operator()(wordItem const &lhs, wordItem const &rhs) const {
        if (lhs.count == rhs.count) {
            return lhs.word > rhs.word;
        }
        return lhs.count > rhs.count;
    }
};

And then here is my function that would actually use this set:

// implementation file (HashTable.cpp)
#include "HashTable.hpp"
void HashTable::printTopN(int n) {
    set<wordItem,comp> s;

    for (int i = 0; i < hashTableSize; i++) {
        if (hashTable[i] != nullptr) {
            wordItem *temp = hashTable[i];
            s.insert(*temp);
            while (temp->next != nullptr) {
                temp = temp->next;
                s.insert(*temp);
            }
        }
    }
}

However, I am getting an error message that says I am using undeclared identifiers. Both comp and s are causing this... This is the exact error message:

HashTable2.cpp:129:16: error: use of undeclared identifier 'comp' 
set<wordItem,comp> s;
             ^
HashTable2.cpp:134:7: error: use of undeclared identifier 's'
s.insert(*temp);
^
HashTable2.cpp:137:9: error: use of undeclared identifier 's'
s.insert(*temp); 
^

The set is supposed to contain wordItem 's with the largest wordCount first, and if two words share the same count the tiebreaker should be lexicographical order.

I feel like the issue is probably coming from the fact that I am probably not comparing these wordItem 's correctly

Lastly, I apologize because I assume its very messy to mix my own custom data structures with the STL structures, but any help would be greatly appreciated

Answer 1

When you use a custom comparator std::map uses it as

comp_object(a, b)

In

struct comp {
    bool operator<(wordItem const &lhs, wordItem const &rhs) const {
        if (lhs.count == rhs.count) {
            return lhs.word > rhs.word;
        }
        return lhs.count > rhs.count;
    }
};

you define an operator < , not an operator () , so it wont work. Change it to

struct comp {
    bool operator()(wordItem const &lhs, wordItem const &rhs) const {
        if (lhs.count == rhs.count) {
            return lhs.word > rhs.word;
        }
        return lhs.count > rhs.count;
    }
};

And it will work. This is called a functor and you can read more about there here: What are C++ functors and their uses?