How to Use binary_search from STL in C++: Syntax and Example

Use std::binary_search from the <algorithm> header to check if an element exists in a sorted container. It returns true if the element is found, otherwise false. Make sure the container is sorted before calling binary_search.

Syntax

The std::binary_search function has two main forms:

• bool binary_search(RandomIt first, RandomIt last, const T& value);
• bool binary_search(RandomIt first, RandomIt last, const T& value, Compare comp);

Here:

• first and last are iterators marking the range to search.
• value is the element to find.
• comp is an optional comparison function for custom sorting.

The container must be sorted according to the same comparison before calling binary_search.

cpp

template<class RandomIt, class T>
bool binary_search(RandomIt first, RandomIt last, const T& value);

template<class RandomIt, class T, class Compare>
bool binary_search(RandomIt first, RandomIt last, const T& value, Compare comp);

Example

This example shows how to use std::binary_search to check if a number exists in a sorted vector.

cpp

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> numbers = {1, 3, 5, 7, 9};

    // The vector must be sorted for binary_search to work correctly
    std::sort(numbers.begin(), numbers.end());

    int target = 5;
    if (std::binary_search(numbers.begin(), numbers.end(), target)) {
        std::cout << target << " found in the vector.\n";
    } else {
        std::cout << target << " not found in the vector.\n";
    }

    target = 6;
    if (std::binary_search(numbers.begin(), numbers.end(), target)) {
        std::cout << target << " found in the vector.\n";
    } else {
        std::cout << target << " not found in the vector.\n";
    }

    return 0;
}

Output

5 found in the vector.
6 not found in the vector.

Common Pitfalls

Common mistakes when using std::binary_search include:

• Not sorting the container before searching. binary_search requires a sorted range.
• Using binary_search on unsorted data leads to incorrect results.
• Using a different comparison function for sorting and searching causes wrong behavior.

Always ensure the data is sorted with the same criteria before calling binary_search.

cpp

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> numbers = {3, 1, 7, 5, 9}; // Not sorted

    int target = 5;
    // Wrong: binary_search on unsorted vector
    bool found_wrong = std::binary_search(numbers.begin(), numbers.end(), target);

    // Correct: sort first
    std::sort(numbers.begin(), numbers.end());
    bool found_right = std::binary_search(numbers.begin(), numbers.end(), target);

    std::cout << "Wrong result (unsorted): " << (found_wrong ? "found" : "not found") << "\n";
    std::cout << "Correct result (sorted): " << (found_right ? "found" : "not found") << "\n";

    return 0;
}

Output

Wrong result (unsorted): not found
Correct result (sorted): found

Quick Reference

Tips for using std::binary_search:

• Always include <algorithm> header.
• Ensure the container is sorted before searching.
• Use iterators to specify the search range.
• Returns true if element is found, false otherwise.
• For custom sorting, provide the same comparison function to binary_search.

Key Takeaways

std::binary_search checks if an element exists in a sorted range and returns true or false.

Always sort your container before using binary_search to get correct results.

Use the same comparison function for sorting and searching if you customize the order.

binary_search works with iterators, so it can be used with many container types.

Include and use std::binary_search with care to avoid logic errors.