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.

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

Return value

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

Complexity

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

1,2) At most 2min(1,N2) comparisons using operator<.

3,4) At most 2min(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

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);
}

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(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');
}

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.

See also