How to use the custom comp function of std::lower_bound()?

Question

I want to specify to the lower_bound function that it should compare only the .first property of a pair and not both of them, as I assume it is default behavior.

In my situation, it's guaranteed the input is ordered, and I need a O(nlogn) solution. I'm not using std::binary_search because I need to access the position so I can get the .second value, as the code below shows.

I've read the documentation already, but I didn't find it clear, there were no specific examples there and a description not longer than a paragraph.

Here's my code, which is not producing the expected behavior:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

bool comp(pair<string, int> i, string value) {
    return (i.first == value);
}

int main() {
    vector<pair<string, int>> books;

    int n;
    cin >> n;
    for(int i = 0; i < n; i++) {
        string name; int code;
        pair<string, int> livro;
        cin >> name >> code;
        books.push_back(make_pair(name, code));
    }

    int m;
    cin >> m;
    for(int j = 0; j < m; j++) {
        string consult;
        cin >> consult;
        auto i = lower_bound(books.begin(), books.end(), consult, comp);
        if (i->first == consult) {
            if (i->second == 1) {
                cout << "Avaliable" << '\n';
            }
            else {
                cout << "Borrowed" << '\n';
            }
        }

        else {
            cout << "Not found" << '\n';
        }

    }
}

Answer 1

You should write comp func like below because you have to let lower_bound function know which value is bigger(or lesser) with two pair correctly in its algorithm.

bool comp(pair<string, int> i, string value) {
    return (i.first < value);
}

output is ok

3
aaa
Borrowed
bbb
Avaliable
ccc
Borrowed