The binary search is a search algorithm that finds the position of a target value within a sorted array. Binary search compares the target value to the middle element of the array. If they are not equal, the half in which the target cannot lie is eliminated and the search continues on the remaining half, again taking the middle element to compare to the target value, and repeating this until the target value is found. If the search ends with the remaining half being empty, the target is not in the array.

C++ Binary Search Algorithm

#include <iostream>

/** binary_search function
 * \param [in] a array to sort
 * \param [in] r right hand limit = \f$n-1\f$
 * \param [in] key value to find
 * \returns index if T is found
 * \return -1 if T is not found
 */
int binary_search(int a[], int r, int key) {
    int l = 0;

    while (l <= r) {
        int m = l + (r - l) / 2;
        if (key == a[m])
            return m;
        else if (key < a[m])
            r = m - 1;
        else
            l = m + 1;
    }
    return -1;
}

/** main function */
int main(int argc, char const* argv[]) {
    int n, key;
    std::cout << "Enter size of array: ";
    std::cin >> n;
    std::cout << "Enter array elements: ";

    int* a = new int[n];

    // this loop use for store value in Array
    for (int i = 0; i < n; i++) {
        std::cin >> a[i];
    }

    std::cout << "Enter search key: ";
    std::cin >> key;

    // this is use for find value in given array
    int res = binary_search(a, n - 1, key);
    if (res != -1)
        std::cout << key << " found at index " << res << std::endl;
    else
        std::cout << key << " not found" << std::endl;

    delete[] a;
    return 0;
}

Output:

Enter size of array: 5
Enter array elements: 10 30 40 50 20
Enter search key: 50
50 found at index 3