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std::lexicographical_compare

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

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

1) Elements are compared using operator<.
3) Elements are compared using the given binary comparison function comp.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

 (until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

 (since C++20)

Lexicographical comparison is an operation with the following properties:

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

Parameters

first1, last1 - the first range of elements to examine
first2, last2 - the second range of elements to examine
policy - the execution policy to use. See execution policy for details.
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.
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 range is lexicographically less than the second, otherwise false.

Complexity

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

1,2) At most \(\scriptsize min(N_1,N_2)\)min(N1,N2) comparisons using operator<.
3,4) At most \(\scriptsize min(N_1,N_2)\)min(N1,N2) applications of the comparison function comp.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

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 (3) 
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

#include <algorithm>
#include <iostream>
#include <random>
#include <vector>
 
void print(std::vector<char> const& v, auto suffix)
{
    for (char c : v)
        std::cout << c << ' ';
    std::cout << suffix;
}
 
int main()
{
    std::vector<char> v1{'a', 'b', 'c', 'd'};
    std::vector<char> v2{'a', 'b', 'c', 'd'};
 
    for (std::mt19937 g{std::random_device{}()};
         !std::lexicographical_compare(v1.begin(), v1.end(),
                                       v2.begin(), v2.end());)
    {
        print(v1, ">= ");
        print(v2, '\n');
 
        std::shuffle(v1.begin(), v1.end(), g);
        std::shuffle(v2.begin(), v2.end(), g);
    }
 
    print(v1, "<  ");
    print(v2, '\n');
}

Possible output:

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

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 (equality 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

determines if two sets of elements are the same
(function template)
(C++20)
compares two ranges using three-way comparison
(function template)
(C++20)
returns true if one range is lexicographically less than another
(niebloid)

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