C++ Primer Binary Search not working. Can not understand why

Question

void binary_search_vec(std::vector<int> a, int finder) {    

    auto full_beg = a.begin();
    auto beg = a.begin();
    auto end = a.end();
    auto mid = beg + (end - beg) / 2;


    while (*mid != finder && mid != end) {

        if (finder > *mid) {
          beg = mid + 1;

        }
        else {  //(*mid > finder) 


            end = mid;
        }
        mid = beg + (end - beg) / 2;

        if (finder == *mid) { // Here is the problem leading to undefined behavior. I am negating the condition in the While Loop. Credit to Bob___ for the fix. And thank you to the community.

         std::cout << "Num found: " << finder << " at position: " << std::distance(full_beg, mid) << 
         std::endl;
         break;
        
        }

    }
    
}

Edit: The code in question is from C++ Primer 5th edition. The binary search in question simply does not return the num found. I Can not understand why. Compared line by line with other users. Theirs work mine does not.

This code from another user works. I do not understand the difference 2nd edit: @Bob___ fix code:

auto full_beg = a.begin();
auto beg = a.begin();
auto end = a.end();
auto mid = beg + (end - beg) / 2;


while (*mid != finder && mid != end) {

if (finder > * mid) {
      beg = mid + 1;

    }
    else {  //(*mid > finder) 


        end = mid;
    }
    mid = beg + (end - beg) / 2;

}
 if (finder == *mid) {
    std::cout << "Num found: " << finder << " at position Index: " << std::distance(full_beg, mid) << std::endl;

}
else std::cout << "Not Found!" << std::endl;

}

Answer 1

Move the last if branch out of the while loop and remove the break statement otherwise it won't work if the finder is the same as the mid .

The following code works:

#include <iostream>
#include <vector>


void binary_search_vec(std::vector<int> a, int finder) {    

    auto full_beg = a.begin();
    auto beg = a.begin();
    auto end = a.end();
    auto mid = beg + (end - beg) / 2;

    std::cout << "MID AT THE BEGINNING: " << *mid << std::endl;
    while (mid != end && *mid != finder) {

        if (finder > *mid) {
          std::cout << "Finder > mid" << std::endl;
          beg = mid + 1;

        }
        else {  //(*mid > finder) 
          std::cout << "Finder > mid" << std::endl;
          end = mid;
        }
        mid = beg + (end - beg) / 2;
        std::cout << "Setting mid to " << *mid << std::endl;
    }
    
    if (finder == *mid) {
      std::cout << "Num found: " << finder << " at position: " << std::distance(full_beg, mid) << std::endl;
    }
}

int main() {
  std::cout << "Hello World!\n";
  std::vector<int> v = std::vector<int>();
  v.push_back(0);
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);

  binary_search_vec(v, 1);
}

Using your function, the previous code does not work when finder = 2 .

Also check the comment from @Bob__: the order of the condition evaluation inside the while is extremely important!

Answer 2

Here is the fix and updated code. Courtesy of Bob___

void binary_search_vec(std::vector a, int finder) {

auto full_beg = a.begin();
auto beg = a.begin();
auto end = a.end();
auto mid = beg + (end - beg) / 2;


while (*mid != finder && mid != end) {

if (finder > * mid) {
      beg = mid + 1;

    }
    else {  //(*mid > finder) 


        end = mid;
    }
    mid = beg + (end - beg) / 2;

}
 if (finder == *mid) {
    std::cout << "Num found: " << finder << " at position Index: " << std::distance(full_beg, mid) << std::endl;

}
else std::cout << "Not Found!" << std::endl;

}