NAME¶

std::lexicographical_compare - std::lexicographical_compare

Synopsis¶

Defined in header <algorithm>
template< class InputIt1, class InputIt2 >

bool lexicographical_compare( InputIt1 first1, InputIt1
last1, (1) (constexpr since C++20)

InputIt2 first2, InputIt2
last2 );
template< class ExecutionPolicy,

class ForwardIt1, class ForwardIt2 >
bool lexicographical_compare( ExecutionPolicy&& policy,
ForwardIt1 first1, (2) (since C++17)
ForwardIt1 last1,

ForwardIt2 first2,
ForwardIt2 last2 );
template< class InputIt1, class InputIt2, class Compare
>

bool lexicographical_compare( InputIt1 first1, InputIt1
last1, (3) (constexpr since C++20)
InputIt2 first2, InputIt2
last2,

Compare comp );
template< class ExecutionPolicy,

class ForwardIt1, class ForwardIt2, class
Compare >
bool lexicographical_compare( ExecutionPolicy&& policy,
ForwardIt1 first1, (4) (since C++17)
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.

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.
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);
comp -
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 \(\scriptsize N_1\)N
1 as std::distance(first1, last1) and \(\scriptsize N_2\)N
2 as std::distance(first2, last2):

1,2) At most \(\scriptsize 2\min(N_1,N_2)\)2min(
1,N
2) comparisons using operator<.
3,4) At most \(\scriptsize 2\min(N_1,N_2)\)2min(N
1,N
2) 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¶

// 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

Defect reports

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

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

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
(C++20) less than another
(niebloid)