std::lexico...cal_compare(3)

🎀 NAME 🎀

okay bestie, this is the std::lexico...cal_compare(3) page from the C++ Standard Libary manual — yes, that spelling is in the header, don’t come for me 💅 — and the actual icon we’re talking about is std::lexicographical_compare. It’s giving “compare two ranges like a boss,” and we’re gonna get into every detail. periodt. ✨

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✨ Synopsis ✨

Defined in header <algorithm>.

(1) — constexpr since C++20:

template< class InputIt1, class InputIt2 >
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
                              InputIt2 first2, InputIt2 last2 );

(2) — since C++17:

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
bool lexicographical_compare( ExecutionPolicy&& policy,
                              ForwardIt1 first1, ForwardIt1 last1,
                              ForwardIt2 first2, ForwardIt2 last2 );

(3) — constexpr since C++20:

template< class InputIt1, class InputIt2, class Compare >
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
                              InputIt2 first2, InputIt2 last2,
                              Compare comp );

(4) — 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 );

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 participate in overload resolution only if

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.

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🧚 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.

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🌈 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.

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💖 Return value 💖

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

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🔥 Complexity 🔥

Given N_1 as std::distance(first1, last1) and N_2 as std::distance(first2, last2):

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

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😭 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.

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🛠️ 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);
}

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🦋 Example 🦋

// Run this code

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

void print(const std::vector<char>& 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

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🩹 Defect reports 🩹

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

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

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👀 See also 👀

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

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okay that’s the whole page, bestie 🫶 — source/version/date from the footer: http://cppreference.com, 2024.06.10, manual tag std::lexico...cal_compare(3). mwah 💖