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Difference between revisions of "cpp/algorithm/min element"

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< cpp‎ | algorithm
(Added LWG issue #212 DR (part 2/2).)
(Added LWG issue #2150 DR (part 12/12).)
 
(5 intermediate revisions by 3 users not shown)
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{{dcl begin}}
 
{{dcl begin}}
 
{{dcl header|algorithm}}
 
{{dcl header|algorithm}}
−
{{dcl rev multi|num=1
+
{{dcla|num=1|notes={{mark constexpr since c++17}}|
−
|until1=c++17|dcl1=
+
 
template< class ForwardIt >  
 
template< class ForwardIt >  
 
ForwardIt min_element( ForwardIt first, ForwardIt last );
 
ForwardIt min_element( ForwardIt first, ForwardIt last );
−
|dcl2=
 
−
template< class ForwardIt >
 
−
constexpr ForwardIt min_element( ForwardIt first, ForwardIt last );
 
 
}}
 
}}
−
{{dcl|since=c++17|num=2|1=
+
{{dcl|num=2|since=c++17|
 
template< class ExecutionPolicy, class ForwardIt >  
 
template< class ExecutionPolicy, class ForwardIt >  
 
ForwardIt min_element( ExecutionPolicy&& policy,  
 
ForwardIt min_element( ExecutionPolicy&& policy,  
 
                       ForwardIt first, ForwardIt last );
 
                       ForwardIt first, ForwardIt last );
 
}}
 
}}
−
{{dcl rev multi|num=3
+
{{dcla|num=3|notes={{mark constexpr since c++17}}|
−
|until1=c++17|dcl1=
+
−
template< class ForwardIt, class Compare >
+
−
ForwardIt min_element( ForwardIt first, ForwardIt last, Compare comp );
+
−
|dcl2=
+
 
template< class ForwardIt, class Compare >
 
template< class ForwardIt, class Compare >
−
constexpr ForwardIt min_element( ForwardIt first, ForwardIt last,
+
ForwardIt min_element( ForwardIt first, ForwardIt last,
−
                                Compare comp );
+
                      Compare comp );
 
}}
 
}}
−
{{dcl|since=c++17|num=4|1=
+
{{dcl|num=4|since=c++17|
 
template< class ExecutionPolicy, class ForwardIt, class Compare >
 
template< class ExecutionPolicy, class ForwardIt, class Compare >
 
ForwardIt min_element( ExecutionPolicy&& policy,  
 
ForwardIt min_element( ExecutionPolicy&& policy,  
−
                       ForwardIt first, ForwardIt last, Compare comp );
+
                       ForwardIt first, ForwardIt last,
 +
                      Compare comp );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Finds the smallest element in the range {{tt|[first, last)}}.
+
Finds the smallest element in the range {{range|first|last}}.
  
−
@1@ Elements are compared using {{tt|operator<}}.
+
@1@ Elements are compared using {{rev inl|until=c++20|{{c/core|operator<}}}}{{rev inl|since=c++20|{{c|std::less{}<!---->}}}}.
−
@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}}
+
@3@ Elements are compared using the comparison function {{c|comp}}.
 +
 
 +
@2,4@ Same as {{v|1,3}}, but executed according to {{c|policy}}.
 +
@@ {{cpp/algorithm/parallel overload precondition|plural=yes}}
  
 
===Parameters===
 
===Parameters===
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{{par|first, last|forward iterators defining the range to examine}}
 
{{par|first, last|forward iterators defining the range to examine}}
 
{{par exec pol}}
 
{{par exec pol}}
−
{{par cmp|comp|p1=ForwardIt|{{tt|a}} is ''less'' than {{tt|b}}}}  
+
{{par cmp|comp|p1=ForwardIt}}  
 
{{par hreq}}
 
{{par hreq}}
 
{{par req named|ForwardIt|ForwardIterator}}
 
{{par req named|ForwardIt|ForwardIterator}}
Line 48: Line 44:
  
 
===Return value===
 
===Return value===
−
Iterator to the smallest element in the range {{tt|[first, last)}}. If several elements in the range are equivalent to the smallest element, returns the iterator to the first such element. Returns {{c|last}} if the range is empty.
+
Iterator to the smallest element in the range {{range|first|last}}. If several elements in the range are equivalent to the smallest element, returns the iterator to the first such element. Returns {{c|last}} if the range is empty.
  
 
===Complexity===
 
===Complexity===
−
Exactly {{math|max(N-1,0)}} comparisons, where {{c|1=N = std::distance(first, last)}}.
+
Given {{mathjax-or|\(\scriptsize N\)|N}} as {{c|std::distance(first, last)}}:
 +
 
 +
@1,2@ Exactly {{mathjax-or|\(\scriptsize \max(N-1,0)\)|max(N-1,0)}} comparisons using {{rev inl|until=c++20|{{c/core|operator<}}}}{{rev inl|since=c++20|{{c|std::less{}<!---->}}}}.
 +
 
 +
@3,4@ Exactly {{mathjax-or|\(\scriptsize \max(N-1,0)\)|max(N-1,0)}} applications of the comparison function {{c|comp}}.
  
 
===Exceptions===
 
===Exceptions===
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===Possible implementation===
 
===Possible implementation===
−
{{eq fun
+
{{eq impl
−
|1=
+
|title1=min_element (1)|ver1=1|1=
 
template<class ForwardIt>
 
template<class ForwardIt>
 
ForwardIt min_element(ForwardIt first, ForwardIt last)
 
ForwardIt min_element(ForwardIt first, ForwardIt last)
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     if (first == last)
 
     if (first == last)
 
         return last;
 
         return last;
−
 
+
   
 
     ForwardIt smallest = first;
 
     ForwardIt smallest = first;
−
    ++first;
 
 
      
 
      
−
     for (; first != last; ++first)
+
     while (++first != last)
 
         if (*first < *smallest)
 
         if (*first < *smallest)
 
             smallest = first;
 
             smallest = first;
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     return smallest;
 
     return smallest;
 
}
 
}
−
|2=
+
|title2=min_element (3)|ver2=3|2=
 
template<class ForwardIt, class Compare>
 
template<class ForwardIt, class Compare>
 
ForwardIt min_element(ForwardIt first, ForwardIt last, Compare comp)
 
ForwardIt min_element(ForwardIt first, ForwardIt last, Compare comp)
Line 82: Line 81:
 
      
 
      
 
     ForwardIt smallest = first;
 
     ForwardIt smallest = first;
−
    ++first;
 
 
      
 
      
−
     for (; first != last; ++first)
+
     while (++first != last)
 
         if (comp(*first, *smallest))
 
         if (comp(*first, *smallest))
 
             smallest = first;
 
             smallest = first;
Line 102: Line 100:
 
int main()
 
int main()
 
{
 
{
−
     std::vector<int> v{3, 1, 4, 1, 5, 9};
+
     std::vector<int> v{3, 1, -4, 1, 5, 9};
−
+
   
 
     std::vector<int>::iterator result = std::min_element(v.begin(), v.end());
 
     std::vector<int>::iterator result = std::min_element(v.begin(), v.end());
−
     std::cout << "min element at: " << std::distance(v.begin(), result);
+
     std::cout << "min element has value " << *result << " and index ["
 +
              << std::distance(v.begin(), result) << "]\n";
 
}
 
}
 
|output=
 
|output=
−
min element at: 1
+
min element has value -4 and index [2]
 
}}
 
}}
  
 
===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=lwg|dr=212|std=C++98|before=the return value was not specified if {{c|1=first == last}}|after=returns {{c|last}} in this case}}
+
{{dr list item|wg=lwg|dr=212|std=C++98|before=the return value was not specified if {{range|first|last}} is empty|after=returns {{c|last}} in this case}}
 +
{{dr list item|wg=lwg|dr=2150|std=C++98|before=the iterator to the first non-greatest element was returned|after=corrected the return value}}
 
{{dr list end}}
 
{{dr list end}}
  

Latest revision as of 18:09, 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)
min_element
(C++11)

Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
template< class ForwardIt >
ForwardIt min_element( ForwardIt first, ForwardIt last );
 (1) (constexpr since C++17)
template< class ExecutionPolicy, class ForwardIt >

ForwardIt min_element( ExecutionPolicy&& policy,

                       ForwardIt first, ForwardIt last );
 (2) (since C++17)
template< class ForwardIt, class Compare >

ForwardIt min_element( ForwardIt first, ForwardIt last,

                       Compare comp );
 (3) (constexpr since C++17)
template< class ExecutionPolicy, class ForwardIt, class Compare >

ForwardIt min_element( ExecutionPolicy&& policy,
                       ForwardIt first, ForwardIt last,

                       Compare comp );
 (4) (since C++17)

Finds the smallest element in the range [first, last).

1) Elements are compared using operator<(until C++20)std::less{}(since C++20).
3) Elements are compared using the 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)

Contents

[edit] Parameters

first, last - forward iterators defining the range 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 an object of type ForwardIt can be dereferenced and then implicitly converted to both of them.

Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator.

[edit] Return value

Iterator to the smallest element in the range [first, last). If several elements in the range are equivalent to the smallest element, returns the iterator to the first such element. Returns last if the range is empty.

[edit] Complexity

Given N as std::distance(first, last):

1,2) Exactly max(N-1,0) comparisons using operator<(until C++20)std::less{}(since C++20).
3,4) Exactly max(N-1,0) 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

min_element (1) 
template<class ForwardIt>
ForwardIt min_element(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return last;
 
    ForwardIt smallest = first;
 
    while (++first != last)
        if (*first < *smallest)
            smallest = first;
 
    return smallest;
}
min_element (3) 
template<class ForwardIt, class Compare>
ForwardIt min_element(ForwardIt first, ForwardIt last, Compare comp)
{
    if (first == last)
        return last;
 
    ForwardIt smallest = first;
 
    while (++first != last)
        if (comp(*first, *smallest))
            smallest = first;
 
    return smallest;
}

[edit] Example

Run this code
#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{3, 1, -4, 1, 5, 9};
 
    std::vector<int>::iterator result = std::min_element(v.begin(), v.end());
    std::cout << "min element has value " << *result << " and index ["
              << std::distance(v.begin(), result) << "]\n";
}

Output: 

min element has value -4 and index [2]

[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 212 C++98 the return value was not specified if [first, last) is empty returns last in this case
LWG 2150 C++98 the iterator to the first non-greatest element was returned corrected the return value

[edit] See also

 returns the largest element in a range
(function template) [edit]
(C++11)
 returns the smallest and the largest elements in a range
(function template) [edit]
returns the smaller of the given values
(function template) [edit]
(C++20)
 returns the smallest element in a range
(niebloid)[edit]
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