custom comparator in lower_bound() stl function in c++

Question

#include<iostream>
#include<algorithm>
using namespace std;
bool compare(int a,int b){
    cout<<a<<" "<<b<<endl;
    return a<=b;
}

int main()
{    int n;
    cin>>n;
    int a[n]={1,2,5,10,50,100,200,500,2000};
    int money=100;
    int it =lower_bound(a,a+n,money,compare)-a;
    cout<<a[it];
    return 0;
}

In this code, I made a custom comparator for lower_bound() , so it is supposed to output 100 in the end (as in the lst compare 100 <=100 is true). But it gives the next index (200) as an output... why?

Answer 1

Right, just as Yadav said. You can also see cppreference about compare requirement:

comp - binary predicate which returns true if the first argument is less than (ie is ordered before) the second.

https://en.cppreference.com/w/cpp/algorithm/lower_bound