How to apply binary search for searching points/pairs in c++? - c++

Question

I want to search if the pair exists in the vector of pairs using binary search.
Here is my code: This code is only looking for the first value in pair:
Could you modify this code that it will be looking for the exact pair?
#include <bits/stdc++.h>
using namespace std;
struct compare {
bool operator()(const pair<int, int>& value,
const int& key)
{
return (value.first < key);
}
bool operator()(const int& key,
const pair<int, int>& value)
{
return (key < value.first);
}
};
int main()
{
vector<pair<int, int> > vect;
vect.push_back(make_pair(1, 20));
vect.push_back(make_pair(3, 42));
vect.push_back(make_pair(4, 36));
vect.push_back(make_pair(2, 80));
vect.push_back(make_pair(7, 50));
vect.push_back(make_pair(9, 20));
vect.push_back(make_pair(3, 29));
sort(vect.begin(), vect.end());
// printing the sorted vector
cout << "KEY" << '\t' << "ELEMENT" << endl;
for (pair<int, int>& x : vect)
cout << x.first << '\t' << x.second << endl;
// searching for the key element 3
cout << "search for key 3 in vector" << endl;
if (binary_search(vect.begin(), vect.end(),
3, compare()))
cout << "Element found";
else
cout << "Element not found";
return 0;
}

Answer 1

If you want to look for an exact pair, you need to provide the pair to binary_search, like this
if (std::binary_search(vect.begin(), vect.end(), std::pair{3, 42}))
// ... found
Note that you don't need the custom compare function here. The default comparator does the right thing. (In fact, you should use the same comparator as used to sort the elements in the first place, otherwise binary_search will be broken).
Note that pre c++17, you need to provide the template arguments to pair, like this,
if (std::binary_search(vect.begin(), vect.end(), std::pair<int,int>{3, 42}))
// ... found
If you want to find the position of the found element, you can use lower_bound like so,
auto lb = std::lower_bound(vect.begin(), vect.end(), std::pair<int,int>{3, 42});
if (lb != vect.end())
std::cout << "Element found at position "
<< std::distance(vect.begin(), lb);
Also, please don't use #include <bits/stdc++.h> or using namespace std;