Binary Search in C++ Standard Template Library (STL)

Binary search is a widely used searching algorithm that requires the array to be sorted before search is applied. The main idea behind this algorithm is to keep dividing the array in half (divide and conquer) until the element is found, or all the elements are exhausted.

It works by comparing the middle item of the array with our target, if it matches, it returns true otherwise if the middle term is greater than the target, the search is performed in the left sub-array.
If the middle term is less than target, the search is performed in the right sub-array.

 
The prototype for binary search is :

binary_search(startaddress, endaddress, valuetofind)

startaddress: the address of the first element of the array.
endaddress: the address of the last element of the array.
valuetofind: the target value which we have to search for.
#include <iostream>
#include <algorithm>

using namespace std;

void show(int a[], int arraysize)
{
    for(int i = 0; i < arraysize; ++i)
        cout << a[i] <<" ";
}


int main()
{
    int a[]= {1, 5, 8, 9, 6, 7, 3, 4, 2, 0};
    int asize = sizeof(a) / sizeof(a[0]);
    cout << "\n The array is : ";
    show(a, asize);

    cout << "\n\nLet's say we want to search for 2 in the array";
    cout << "\n So, we first sort the array";
    sort(a, a + 10);
    cout << "\n\n The array after sorting is : ";
    show(a, asize);
    cout << "\n\nNow, we do the binary search";
    if (binary_search(a, a + 10, 2)) 
       cout << "\nElement found in the array";
    else 
       cout << "\nElement not found in the array";

    cout << "\n\nNow, say we want to search for 10";
     if (binary_search(a, a + 10, 10)) 
       cout << "\nElement found in the array";
    else 
       cout << "\nElement not found in the array";

    return 0;

}

 
The output of the above program is :

The array is : 1 5 8 9 0 6 7 3 4 2 0

Let's say we want to search for 2 in the array
 So, we first sort the array

The array after sorting is : 0 1 2 3 4 5 6 7 8 9

Now, we do the binary search
Element found in the array

Now, say we want to search for 10
Element not found in the array

 

Related Article: std::bsearch in C++
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