Why doesn't this code for a binary search function in c++ work?

Question

#include <iostream>

int binary_search(int arr[], int size, int target)
{
    int first = 0;
    int last = size - 1; 
    int midpoint = (last + first) / 2;
    
    while(first <= last)
    {
        if(arr[midpoint] == target)
        {
            return midpoint;
        }
        else if(arr[midpoint] < target)
        {
            first = midpoint + 1;
        }
        else if(arr[midpoint] > target)
        {
            last = midpoint - 1;
            
        }
    }
    
    return 0;
}

int main()
{
    int arr[] = {4, 12, 23, 43, 50, 60, 230, 290};
    int size = sizeof(arr)/sizeof(arr[0]);
    int target = 12;
    std::cout << binary_search(arr, size, target);
}

if the midpoint value is lesser than the target it increases 'first' and if it's greater than the target it instead decreases 'last'. This continues until 'first' is equal to 'last'. I saw another program where they called the function recursively but i wanted to make it work without it, what exactly am i doing wrong?

Answer 1

You basically forgot to update midpoint in each iteration. An updated version of your code here (not using those "C" style arrays).It was also not clear if you meant to return the found value or the index at which it was found.

#include <iostream>
#include <vector>

auto binary_search(const std::vector<int>& values, int value)
{
    std::size_t first = 0;
    std::size_t last = values.size() - 1; // use vector it keeps track of its size
    
    while (first <= last)
    {
        std::size_t midpoint  = (last + first) / 2; // you forgot to update midpoint each loop

        if (values[midpoint] == value)
        {
            return values[midpoint]; // <== do you want to return the position or the value?
        }
        
        if (values[midpoint] < value)
        {
            first = midpoint + 1;
        }
        else if (values[midpoint] > value)
        {
            last = midpoint - 1;
        }
    }

    return 0;
}

int main()
{
    std::vector<int> values{ 4, 12, 23, 43, 50, 60, 230, 290 };
    std::cout << binary_search(values, 12);
    return 0;
}