[FIXED] How to find the maximum in std::set< pair<int, int> > in a given range?

Issue

I have a set of pairs, and I want to find the maximum number in the second entry of the pair between l and r inclusive.

This is what the set looks like: myset = [(0,2),(1,1),(2,4),(3,0),(4,3)]

Here is what I have tried:

#include <iostream>
#include <set>
using namespace std;

#define INPUT1(x)  scanf("%d", &x)
#define INPUT2(x, y)  scanf("%d%d", &x, &y)
#define OUTPUT1(x) printf("%d\n", x);

bool cmp(pair<int, int> A, pair<int, int> B) {
    return A.second < B.second;
}

int main(int argc, char const *argv[]) {
    int n;
    INPUT1(n);

    set< pair<int,int> > myset;
    set< pair<int,int> >::iterator it;

    for (int i = 0; i < n; i++) {
        int val;
        INPUT(val);
        myset.insert(make_pair(i, val));
    }

    int l, r;
    INPUT2(l, r);
    int max = std::max_element(myset.begin()+l, myset.begin()+r+1, cmp)->second;
    OUTPUT1(max);
}

This doesn't work but for l = 1 and r = 3 I want is for max to equal 4.

I get the following error:

invalid operands to binary expression

('iterator' (aka '__tree_const_iterator<std::__1::pair<int, int>, std::__1::__tree_node<std::__1::pair<int, int>, void *> *, long>') and 'int')

Solution

You cannot use std::max_element in such manner. The reason is that std::set provides bidirectional iterators, not random access iterators, so the things like myset.begin()+l are forbidden.

You should use something like this:

auto mx = std::numeric_limits<int>::min();

auto first = std::cbegin(myset);
std::advance(first, lf);

auto last = std::cbegin(myset);
std::advance(last, rg + 1);

for (auto it = first; it != std::cend(myset) && it != last; ++it) {
    mx = std::max(mx, it->second);
}

Answered By - Edgar Rokjān

Answer Checked By - David Goodson (PHPFixing Volunteer)