From cppreference.com
Defined in header
template<classInputIt1,classInputIt2>boollexicographical_compare(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2); (1)(constexpr since C++20)template<classExecutionPolicy,classForwardIt1,classForwardIt2>boollexicographical_compare(ExecutionPolicy&&policy,ForwardIt1first1,ForwardIt1last1,ForwardIt2first2,ForwardIt2last2); (2) (since C++17)template<classInputIt1,classInputIt2,classCompare>boollexicographical_compare(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2,Comparecomp); (3)(constexpr since C++20)template<classExecutionPolicy,classForwardIt1,classForwardIt2,classCompare>boollexicographical_compare(ExecutionPolicy&&policy,ForwardIt1first1,ForwardIt1last1,ForwardIt2first2,ForwardIt2last2,Comparecomp); (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 participate in overload resolution only if the value of the following expression is true:
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(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 pair of iterators defining the first
of elements to examine first2, last2 - the pair of iterators defining the second
of elements to examine policy - the
to use comp - comparison function object (i.e. an object that satisfies the requirements of
) which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following:
boolcmp(constType1&a,constType2&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
(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
. -ForwardIt1, ForwardIt2 must meet the requirements of
. -Compare must meet the requirements of
. 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
,
is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory,
is thrown.
Possible implementation
template<classInputIt1,classInputIt2>boollexicographical_compare(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2){for(;(first1!=last1)&&(first2!=last2);++first1,(void)++first2){if(*first1<*first2)returntrue;if(*first2<*first1)returnfalse;}return(first1==last1)&&(first2!=last2);}
template<classInputIt1,classInputIt2,classCompare>boollexicographical_compare(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2,Comparecomp){for(;(first1!=last1)&&(first2!=last2);++first1,(void)++first2){if(comp(*first1,*first2))returntrue;if(comp(*first2,*first1))returnfalse;}return(first1==last1)&&(first2!=last2);}Example
Run this code
#include<algorithm>#include<iostream>#include<random>#include<vector>voidprint(conststd::vector<char>&v,autosuffix){for(charc:v)std::cout<<c<<' ';std::cout<<suffix;}intmain(){std::vector<char>v1{'a','b','c','d'};std::vector<char>v2{'a','b','c','d'};for(std::mt19937g{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
C++98 at most min(N1,N2) comparisons were allowed, but that
is not possible (equivalence is determined by 2 comparisons) doubled the limit
C++98 the results of lexicographical comparisons involving empty ranges were unclear made clear See also