std::lower_bound

 

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all_of any_of none_of (C++11) (C++11) (C++11) for_each count count_if mismatch equal	find find_if find_if_not (C++11) find_end find_first_of adjacent_find search search_n
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copy copy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n remove remove_if remove_copy remove_copy_if	replace replace_if replace_copy replace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffle shuffle   (C++11) unique unique_copy
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is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
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lower_bound upper_bound	binary_search equal_range
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C library
qsort	bsearch

 

Defined in header <algorithm>
template< class ForwardIterator, class T > ForwardIterator lower_bound( ForwardIterator first, ForwardIterator last,                              const T& value );	(1)
template< class ForwardIterator, class T, class Compare > ForwardIterator lower_bound( ForwardIterator first, ForwardIterator last,                              const T& value, Compare comp );	(2)

Returns an iterator pointing to the first element in the range [first, last) that is not less than value. The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Parameters

first, last	-	iterators defining the range to examine
value	-	value to compare the elements to
comp	-	comparison function which returns ​true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function must not modify the objects passed to it. The type  Type1 must be such that an object of type ForwardIterator can be dereferenced and then implicitly converted to  Type1. The type  Type2 must be such that an object of type T can be implicitly converted to  Type2. ​

Return value

iterator pointing to the first element that is not less than value, or last if no such element is found.

Complexity

logarithmic in the distance between first and last

Possible implementation

First version
template<class ForwardIterator, class T> ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last, const T& value) { ForwardIterator it; std::iterator_traits<ForwardIterator>::difference_type count, step; count = std::distance(first, last);   while (count > 0) { it = first; step = count / 2; std::advance(it, step); if (*it < value) { first = ++it; count -= step + 1; } else count = step; } return first; }
Second version
template<class ForwardIterator, class T, class Compare> ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last, const T& value, Compare comp) { ForwardIterator it; std::iterator_traits<ForwardIterator>::difference_type count, step; count = std::distance(first,last);   while (count > 0) { it = first; step = count / 2; std::advance(it, step); if (comp(*it, value)), first = ++it; count -= step + 1; } else count = step; } return first; }

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    std::vector<int> data = { 1, 1, 2, 3, 3, 3, 3, 4, 4, 4, 5, 5, 6 };
 
    auto lower = std::lower_bound(data.begin(), data.end(), 4);
    auto upper = std::upper_bound(data.begin(), data.end(), 4);
 
    std::copy(lower, upper, std::ostream_iterator<int>(std::cout, " "));
}

Output:

4 4 4

See also

equal_range	returns range of elements matching a specific key (function template)
upper_bound	returns an iterator to the first element greater than a certain value (function template)