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Difference between revisions of "cpp/algorithm/lexicographical compare"

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< cpp‎ | algorithm
m ({{range}})
 
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{{dcl begin}}
 
{{dcl begin}}
 
{{dcl header|algorithm}}
 
{{dcl header|algorithm}}
−
{{dcl rev multi|num=1|anchor=1|until1=c++20|dcl1=
+
{{dcla|num=1|notes={{mark constexpr since c++20}}|
 
template< class InputIt1, class InputIt2 >
 
template< class InputIt1, class InputIt2 >
 
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
 
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
 
                               InputIt2 first2, InputIt2 last2 );
 
                               InputIt2 first2, InputIt2 last2 );
−
|dcl2=
 
−
template< class InputIt1, class InputIt2 >
 
−
constexpr bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
 
−
                                        InputIt2 first2, InputIt2 last2 );
 
 
}}
 
}}
−
{{dcl|since=c++17|num=2|1=
+
{{dcl|num=2|since=c++17|
−
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
+
template< class ExecutionPolicy,
 +
          class ForwardIt1, class ForwardIt2 >
 
bool lexicographical_compare( ExecutionPolicy&& policy,
 
bool lexicographical_compare( ExecutionPolicy&& policy,
 
                               ForwardIt1 first1, ForwardIt1 last1,
 
                               ForwardIt1 first1, ForwardIt1 last1,
 
                               ForwardIt2 first2, ForwardIt2 last2 );
 
                               ForwardIt2 first2, ForwardIt2 last2 );
 
}}
 
}}
−
{{dcl rev multi|num=3|anchor=3|until1=c++20|dcl1=
+
{{dcla|num=3|notes={{mark constexpr since c++20}}|
 
template< class InputIt1, class InputIt2, class Compare >
 
template< class InputIt1, class InputIt2, class Compare >
 
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
 
bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
−
                               InputIt2 first2, InputIt2 last2, Compare comp );
+
                               InputIt2 first2, InputIt2 last2,
−
|dcl2=
+
                              Compare comp );
−
template< class InputIt1, class InputIt2, class Compare >
+
−
constexpr bool lexicographical_compare( InputIt1 first1, InputIt1 last1,
+
−
                                        InputIt2 first2, InputIt2 last2,
+
−
                                        Compare comp );
+
 
}}
 
}}
−
{{dcl|since=c++17|num=4|1=
+
{{dcl|num=4|since=c++17|
 
template< class ExecutionPolicy,
 
template< class ExecutionPolicy,
 
           class ForwardIt1, class ForwardIt2, class Compare >
 
           class ForwardIt1, class ForwardIt2, class Compare >
Line 40: Line 33:
 
Checks if the first range {{range|first1|last1}} is lexicographically ''less'' than the second range {{range|first2|last2}}.  
 
Checks if the first range {{range|first1|last1}} is lexicographically ''less'' than the second range {{range|first2|last2}}.  
  
−
@1@ Elements are compared using {{tt|operator<}}.
+
@1@ Elements are compared using {{c/core|operator<}}.
 
@3@ Elements are compared using the given binary comparison function {{c|comp}}.
 
@3@ Elements are compared using the given binary comparison function {{c|comp}}.
 
@2,4@ Same as {{v|1,3}}, but executed according to {{c|policy}}. {{cpp/algorithm/parallel overload precondition|plural=true}}
 
@2,4@ Same as {{v|1,3}}, but executed according to {{c|policy}}. {{cpp/algorithm/parallel overload precondition|plural=true}}
Line 61: Line 54:
 
{{par req named|InputIt1, InputIt2|InputIterator}}
 
{{par req named|InputIt1, InputIt2|InputIterator}}
 
{{par req named|ForwardIt1, ForwardIt2|ForwardIterator}}
 
{{par req named|ForwardIt1, ForwardIt2|ForwardIterator}}
 +
{{par req named|Compare|Compare}}
 
{{par end}}
 
{{par end}}
  
Line 67: Line 61:
  
 
===Complexity===
 
===Complexity===
 +
Given {{mathjax-or|\(\scriptsize N_1\)|N{{su|b=1}}}} as {{c|std::distance(first1, last1)}} and {{mathjax-or|\(\scriptsize N_2\)|N{{su|b=2}}}} as {{c|std::distance(first2, last2)}}:
  
−
At most {{math|2&middot;min(N1, N2)}} applications of the comparison operation, where {{c|1=N1 = std::distance(first1, last1)}} and {{c|1=N2 = std::distance(first2, last2)}}.
+
@1,2@ At most {{mathjax-or|\(\scriptsize 2\min(N_1,N_2)\)|2min({{su|b=1}},N{{su|b=2}})}} comparisons using {{c/core|operator<}}.
 +
 
 +
@3,4@ At most {{mathjax-or|\(\scriptsize 2\min(N_1,N_2)\)|2min(N{{su|b=1}},N{{su|b=2}})}} applications of the comparison function {{c|comp}}.
  
 
===Exceptions===
 
===Exceptions===
−
{{cpp/algorithm/parallel_exceptions_reporting_behavior|singular=no}}
+
{{cpp/algorithm/parallel exceptions reporting behavior|singular=no}}
  
 
===Possible implementation===
 
===Possible implementation===
Line 115: Line 112:
 
#include <vector>
 
#include <vector>
  
−
void print(std::vector<char> const& v, auto suffix)
+
void print(const std::vector<char>& v, auto suffix)
 
{
 
{
 
     for (char c : v)
 
     for (char c : v)
Line 124: Line 121:
 
int main()
 
int main()
 
{
 
{
−
     std::vector<char> v1 {'a', 'b', 'c', 'd'};
+
     std::vector<char> v1{'a', 'b', 'c', 'd'};
−
     std::vector<char> v2 {'a', 'b', 'c', 'd'};
+
     std::vector<char> v2{'a', 'b', 'c', 'd'};
−
 
+
   
−
     for (std::mt19937 g {std::random_device{}()};
+
     for (std::mt19937 g{std::random_device{}()};
 
         !std::lexicographical_compare(v1.begin(), v1.end(),
 
         !std::lexicographical_compare(v1.begin(), v1.end(),
 
                                       v2.begin(), v2.end());)
 
                                       v2.begin(), v2.end());)
Line 133: Line 130:
 
         print(v1, ">= ");
 
         print(v1, ">= ");
 
         print(v2, '\n');
 
         print(v2, '\n');
−
 
+
       
 
         std::shuffle(v1.begin(), v1.end(), g);
 
         std::shuffle(v1.begin(), v1.end(), g);
 
         std::shuffle(v2.begin(), v2.end(), g);
 
         std::shuffle(v2.begin(), v2.end(), g);
 
     }
 
     }
−
 
+
   
 
     print(v1, "<  ");
 
     print(v1, "<  ");
 
     print(v2, '\n');
 
     print(v2, '\n');
Line 151: Line 148:
 
===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=lwg|dr=142|std=C++98|before=at most {{math|min(N1, N2)}} comparisons were allowed, but that<br>is not possible (equality is determined by 2 comparisons)|after=doubled the limit}}
+
{{dr list item|wg=lwg|dr=142|std=C++98|before=at most {{mathjax-or|\(\scriptsize \min(N_1,N_2)\)|min(N{{su|b=1}},N{{su|b=2}})}} comparisons were allowed, but that<br>is not possible (equivalence is determined by 2 comparisons)|after=doubled the limit}}
 +
{{dr list item|wg=lwg|dr=1205|std=C++98|before=the results of lexicographical comparisons involving empty ranges were unclear|after=made clear}}
 
{{dr list end}}
 
{{dr list end}}
  

Latest revision as of 19:26, 9 April 2024

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
lexicographical_compare
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
template< class InputIt1, class InputIt2 >

bool lexicographical_compare( InputIt1 first1, InputIt1 last1,

                              InputIt2 first2, InputIt2 last2 );
 (1) (constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2 >
bool lexicographical_compare( ExecutionPolicy&& policy,
                              ForwardIt1 first1, ForwardIt1 last1,

                              ForwardIt2 first2, ForwardIt2 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) (constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class Compare >
bool lexicographical_compare( ExecutionPolicy&& policy,
                              ForwardIt1 first1, ForwardIt1 last1,
                              ForwardIt2 first2, ForwardIt2 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 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.

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);

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.

[edit] Return value

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

[edit] 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.

[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 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

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);
}

[edit] 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

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 142 C++98 at most min(N1,N2) comparisons were allowed, but that
is not possible (equivalence is determined by 2 comparisons) 		doubled the limit
LWG 1205 C++98 the results of lexicographical comparisons involving empty ranges were unclear made clear

[edit] See also

 determines if two sets of elements are the same
(function template) [edit]
(C++20)
 compares two ranges using three-way comparison
(function template) [edit]
(C++20)
 returns true if one range is lexicographically less than another
(niebloid)[edit]
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