std::lexicographical_compare

 

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Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Non-modifying sequence operations     Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)(C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)     stable_partition is_partitioned (C++11)     partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection          set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11)	Execution policies (C++17) is_execution_policy (C++17) execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)(C++17)(C++17)(C++20) execution-policy (exposition only*)(C++26) execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20) Numeric operations iota (C++11) inner_product      reduce (C++17) accumulate adjacent_difference transform_reduce (C++17) partial_sum inclusive_scan (C++17)     exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17) Specialized <memory> algorithms Specialized <random> algorithms ranges::generate_random (C++26) C library qsort bsearch

 

Defined in header <algorithm>
template< class InputIt1, class InputIt2 > bool lexicographical_compare( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2 );	(1)	(constexpr since C++20)
template< class InputIt1, class InputIt2, class Compare > bool lexicographical_compare( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp );	(2)	(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > bool lexicographical_compare( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2 );	(3)	(since C++17)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class Compare > bool lexicographical_compare( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, Compare comp );	(4)	(since C++17)

See More

Checks if the first target range [first1, last1) is lexicographically less than the second target range [first2, last2).

1) Elements are compared using operator<.

2) Elements are compared using the given binary comparison function comp.

3,4) Same as (1,2), but executed according to policy.

These overloads participate in overload resolution only if the value of the following expression is true:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>	(since C++20)

Lexicographical comparison is an operation with the following properties:

• Two empty ranges are lexicographically equal.
• An empty range is lexicographically less than any non-empty range.
• Non-empty ranges are compared element by element:
  • The first pair of mismatching elements defines which range is lexicographically less or greater than the other.
  • If two ranges have equivalent elements and are of the same length, then the ranges are lexicographically equal.
  • If one range is a prefix of another, the shorter range is lexicographically less than the other.

Parameters

first1, last1	-	the pair of iterators defining the first target range
first2, last2	-	the pair of iterators defining the second target range
comp	-	comparison function object (i.e. an object that satisfies the requirements of Compare) 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); While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2.
policy	-	the execution policy to use
Type requirements
- InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
- ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
- Compare must meet the requirements of Compare.

Return value

true if the first target range is lexicographically less than the second target range, otherwise false.

Complexity

Given

• \(\scriptsize N_1\)N1 as std::distance(first1, last1),
• \(\scriptsize N_2\)N2 as std::distance(first2, last2):

1) At most \(\scriptsize 2\min(N_1,N_2)\)2min(1,N2) comparisons using operator<.

2) At most \(\scriptsize 2\min(N_1,N_2)\)2min(N1,N2) applications of the comparator comp.

3) \(\scriptsize \mathcal{O}(\min(N_1,N_2))\)𝓞(min(1,N2)) comparisons using operator<.

4) \(\scriptsize \mathcal{O}(\min(N_1,N_2))\)𝓞(min(1,N2)) applications of the comparator comp.

Exceptions

3,4) During the execution process:

• If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
• If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Possible implementation

lexicographical_compare (1)
template<class InputIt1, class InputIt2> bool lexicographical_compare(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2) { for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2) { if (*first1 < *first2) return true; if (*first2 < *first1) return false; } return (first1 == last1) && (first2 != last2); }
lexicographical_compare (2)
template<class InputIt1, class InputIt2, class Compare> bool lexicographical_compare(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, Compare comp) { for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2) { if (comp(*first1, *first2)) return true; if (comp(*first2, *first1)) return false; } return (first1 == last1) && (first2 != last2); }

Example

Run this code

#include <algorithm>
#include <array>
#include <print>
#include <random>

int main()
{
    std::array v1{'a', 'b', 'c', 'd'},
               v2{'a', 'b', 'c', 'd'};

    std::mt19937 g{std::random_device{}()};

    for (int repeat{4}; repeat; --repeat)
    {
        std::shuffle(v1.begin(), v1.end(), g);
        std::shuffle(v2.begin(), v2.end(), g);

        const bool is_less = std::lexicographical_compare(v1.begin(), v1.end(),
                                                          v2.begin(), v2.end());

        std::println("{:n:} {} {:n:}", v1, is_less ? " <  " : " >= ", v2);
    }
}

Possible output:

a, b, c, d  >=  a, b, c, d
b, d, a, c  <   d, c, a, b
b, c, d, a  <   c, b, a, d
b, d, a, c  >=  a, b, d, c

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 142	C++98	at most \(\scriptsize \min(N_1,N_2)\)min(N1,N2) comparisons were allowed, but that is not possible (equivalence is determined by 2 comparisons)	doubled the limit
LWG 1205	C++98	the results of lexicographical comparisons involving empty ranges were unclear	made clear

See also

ranges::lexicographical_compare (C++20)	compares two ranges lexicographically (algorithm function object)
equal	determines if two sets of elements are the same (function template & algorithm function object)
ranges::equal (C++20)
lexicographical_compare_three_way (C++20)	compares two ranges using three-way comparison (function template)