Using std:: lower_bound

Question

// 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;
}

In this code can somebody please explain why we need to do (low - v.begin()) and (up - v.begin()) on the third and fourth lines from the bottom.

If I do

cout << low << endl;

I get the follwoing error which I dont understand

  cannot bind ‘std::ostream {aka std::basic_ostream}’ lvalue to ‘std::basic_ostream&&’ 

Answer 1

*low *is an iterator as you declared. It holds the memory address that generates by your PC and you don't need any memory address to return . By writing

low- v.begin()

You make a command to your program to return the actual position of your searching query as an answer.

That's why it returns a value the

Address Position - Beginning of the Vector Position 

Suppose your vector starting memory address is FFF1 and your searching value is at FFF8 ... Then it return FFF8 - FFF1 = 7 .. ( Example is just to illustrate )

That's how I understand .

Answer 2

The two subtractions are calculating the offset of the elements found at the positions low and up in the vector by calculating the delta between the iterator v.begin() and the iterators low and up , respectively. To make this code clearer, it probably would have been better to use std::distance .

When you tried to use cout << low << endl; you were trying to print out the value of the iterator. That's very different. Oh, and you're missing the std:: namespace reference around cout and endl .