lower_bound in C++

The lower_bound function in C++ is used to find the iterator pointing to the first element in a sorted range that is not less than a specified value. To understand the steps involved in using lower_bound, we need to consider the following:

1. Sorted Range

lower_bound requires the input range to be sorted in ascending order. This could be a vector, array, or any other container that supports random access iterators.

2. Value to Search

lower_bound takes the value to search for as its second argument. This value should have the same type as the elements in the range.

3. Returns an Iterator

lower_bound returns an iterator pointing to the first element in the range that is not less than the specified value. If all elements in the range are less than the value, it returns an iterator pointing to one past the last element.

4. Binary Search

lower_bound uses a binary search algorithm to find the desired iterator efficiently. It repeatedly divides the range in half and compares the middle element with the search value until it finds the desired iterator.

5. Complexity

The complexity of lower_bound is logarithmic in the size of the range. This makes it an efficient way to search for values in sorted containers.

Example Usage

Here's an example usage of lower_bound:

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

int main() {
    std::vector<int> vec = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    int value = 6;

    auto it = std::lower_bound(vec.begin(), vec.end(), value);

    if (it != vec.end()) {
        std::cout << "Found at index: " << std::distance(vec.begin(), it) << std::endl;
    } else {
        std::cout << "Not found" << std::endl;
    }

    return 0;
}

In this example, lower_bound is used to find the first element in the vector vec that is not less than the value 6. The iterator it points to this element, which is 6 itself. The distance function is used to calculate the index of the element in the vector. The output of this code will be "Found at index: 5".

This is a basic explanation of the lower_bound function in C++. Let me know if you have any further questions.