Can someone explain why lower_bound is working this way? - c++

Question

lower_bound function of C++ returns a pointer to the first array element that is at least equal to x (the third argument passed to the function). Here is the code which I compiled online using an online compiler for C++.
#include <bits/stdc++.h>
using namespace std;
int main()
{
vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(1);
v.push_back(2);
cout<<lower_bound(v.begin(),v.end(),2)-v.begin()<<endl;
return 0;
}
Output I expected was 1 but actual result says 3. Can someone please explain why this is so?

Answer 1

https://en.cppreference.com/w/cpp/algorithm/lower_bound
The range [first, last) must be partitioned with respect to the expression element < value or comp(element, value), i.e., all elements for which the expression is true must precede all elements for which the expression is false. A fully-sorted range meets this criterion.
This order:
v.push_back(1);
v.push_back(2);
v.push_back(1);
v.push_back(2);
does not fulfill that criterion.

Answer 2

The range [first, last) must be partitioned with respect to the expression element < value or comp(element, value), i.e., all elements for which the expression is true must precede all elements for which the expression is false. A fully-sorted range meets this criterion.
https://en.cppreference.com/w/cpp/algorithm/lower_bound
Your input does not satisfy this requirement (not sorted), therefore the output is undefined.

Answer 3

std::lower_bound
// lower_bound/upper_bound example
#include <iostream> // std::cout
#include <algorithm> // std::lower_bound, std::upper_bound, std::sort
#include <vector> // std::vector
int main () {
int myints[] = {10,20,30,30,20,10,10,20};
std::vector<int> v(myints,myints+8); // 10 20 30 30 20 10 10 20
std::sort (v.begin(), v.end()); // 10 10 10 20 20 20 30 30
std::vector<int>::iterator low,up;
low=std::lower_bound (v.begin(), v.end(), 20); // ^
up= std::upper_bound (v.begin(), v.end(), 20); // ^
std::cout << "lower_bound at position " << (low- v.begin()) << '\n';
std::cout << "upper_bound at position " << (up - v.begin()) << '\n';
return 0;
}
std::sort (v.begin(), v.end()); is important, the number list has to be in ascend order.