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std::lexicographical_compare

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
lexicographical_compare
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Defined in header <algorithm>
template< class InputIt1, class InputIt2 >

bool lexicographical_compare( InputIt1 first1, InputIt1 last1,

                              InputIt2 first2, InputIt2 last2 );
(1)
template< class ExecutionPolicy, class InputIt1, class InputIt2 >

bool lexicographical_compare( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,

                              InputIt2 first2, InputIt2 last2 );
(2) (since C++17)
template< class InputIt1, class InputIt2, class Compare >

bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
                              InputIt2 first2, InputIt2 last2,

                              Compare comp );
(3)
template< class ExecutionPolicy, class InputIt1, class InputIt2, class Compare >

bool lexicographical_compare( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,
                              InputIt2 first2, InputIt2 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

Lexicographical comparison is a 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.

Contents

[edit] 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);

The signature does not need to have const &, but the function object must not modify the objects passed to it.
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 InputIterator.

[edit] Return value

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

[edit] Complexity

At most 2·min(N1, N2) applications of the comparison operation, where N1 = std::distance(first1, last1) and N2 = std::distance(first2, last2).

[edit] 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 three 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.

[edit] Possible implementation

First version
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);
}
Second version
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);
}

[edit] Example

#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>
 
int main()
{
    std::vector<char> v1 {'a', 'b', 'c', 'd'};
    std::vector<char> v2 {'a', 'b', 'c', 'd'};
 
    std::srand(std::time(0));
    while (!std::lexicographical_compare(v1.begin(), v1.end(),
                                         v2.begin(), v2.end())) {
        for (auto c : v1) std::cout << c << ' ';
        std::cout << ">= ";
        for (auto c : v2) std::cout << c << ' ';
        std::cout << '\n';
 
        std::random_shuffle(v1.begin(), v1.end());
        std::random_shuffle(v2.begin(), v2.end());
    }
 
    for (auto c : v1) std::cout << c << ' ';
    std::cout << "< ";
    for (auto c : v2) std::cout << c << ' ';
    std::cout << '\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

[edit] See also

determines if two sets of elements are the same
(function template) [edit]
parallelized version of std::lexicographical_compare
(function template) [edit]