std::ranges::lexicographical_compare() algorithm

since C++20

Simplified

// (1)
constexpr bool
    lexicographical_compare( I1 first1, S1 last1, I2 first2, S2 last2,
                             Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr bool lexicographical_compare( R1&& r1, R2&& r2, Comp comp = {},
                                        Proj1 proj1 = {}, Proj2 proj2 = {} );

The type of arguments are generic and have the following constraints:

• I1, I2 - std::input_iterator

• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>

• R1, R2 - std::ranges::input_range

• Comp:

  • (1) - std::indirect_strict_weak_order<std::projected<I1, Proj1>, std::projected<I2, Proj2>>
  • (2) - std::indirect_strict_weak_order<std::projected<ranges::iterator_t<R1>, Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>

• Proj1, Proj2 - (none)

The Proj1, Proj2 and Pred template arguments have the following default types: std::identity, ranges::less for all overloads.

Additionally, each overload has the following constraints:

• (1) - std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
• (2) - std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2>

Detailed

// (1)
template<
  std::input_iterator I1,
  std::sentinel_for<I1> S1,
  std::input_iterator I2,
  std::sentinel_for<I2> S2,
  class Proj1 = std::identity,
  class Proj2 = std::identity,
  std::indirect_strict_weak_order<std::projected<I1, Proj1>, std::projected<I2, Proj2>> Comp = ranges::less
>
constexpr bool
    lexicographical_compare( I1 first1, S1 last1, I2 first2, S2 last2,
                             Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
template<
  ranges::input_range R1,
  ranges::input_range R2,
  class Proj1 = std::identity,
  class Proj2 = std::identity,
  std::indirect_strict_weak_order<
                                  std::projected<ranges::iterator_t<R1>, Proj1>,
                                  std::projected<ranges::iterator_t<R2>, Proj2> Comp = ranges::less
>
constexpr bool lexicographical_compare( R1&& r1, R2&& r2, Comp comp = {},
                                        Proj1 proj1 = {}, Proj2 proj2 = {} );

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

• (1) Elements are compared using the given binary comparison function comp.

• (2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

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.

The function-like entities described on this page are niebloids.

Parameters

first1 last1	The first range of elements to compare.
r1	The first range of elements to compare.
first2 last2	The second range of elements to compare.
r2	The first range of elements to compare.
comp	Comparison function to apply to the projected elements.
proj1	Projection to apply to the elements of the first range.
proj2	Projection to apply to the elements of the first range.

Return value

true if the first range is lexicographically less than the second.

Complexity

Given N1 as ranges::distance(first1, last1) and N2 as ranges::distance(first2, last2).

At most 2 * min(N1, N2) applications of the comparison and corresponding projections

Exceptions

(none)

Possible implementation

ranges::lexicographical_compare

struct lexicographical_compare_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             class Proj1 = std::identity, class Proj2 = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<I1, Proj1>,
                 std::projected<I2, Proj2>> Comp = ranges::less>
    constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                              Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2)
        {
            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
                return true;

            if (std::invoke(comp, std::invoke(proj2, *first2), std::invoke(proj1, *first1)))
                return false;
        }
        return (first1 == last1) && (first2 != last2);
    }

    template<ranges::input_range R1, ranges::input_range R2,
             class Proj1 = std::identity, class Proj2 = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R1>, Proj1>,
                 std::projected<ranges::iterator_t<R2>, Proj2>> Comp = ranges::less>
    constexpr bool operator()(R1&& r1, R2&& r2, Comp comp = {},
                              Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::ref(comp), std::ref(proj1), std::ref(proj2));
    }
};

inline constexpr lexicographical_compare_fn lexicographical_compare;

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
#include <vector>

int main()
{
    std::vector<char> v1 {'a', 'b', 'c', 'd'};
    std::vector<char> v2 {'a', 'b', 'c', 'd'};

    namespace ranges = std::ranges;
    auto os = std::ostream_iterator<char>(std::cout, " ");

    std::mt19937 g {std::random_device {}()};
    while (not ranges::lexicographical_compare(v1, v2))
    {
        ranges::copy(v1, os);
        std::cout << ">= ";
        ranges::copy(v2, os);
        std::cout << '\n';

        ranges::shuffle(v1, g);
        ranges::shuffle(v2, g);
    }

    ranges::copy(v1, os);
    std::cout << "<  ";
    ranges::copy(v2, os);
    std::cout << '\n';
}

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