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std::equal_range

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
equal_range
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 ForwardIt, class T >

std::pair<ForwardIt,ForwardIt>
    equal_range( ForwardIt first, ForwardIt last,

                 const T& value );
(1)
template< class ForwardIt, class T, class Compare >

std::pair<ForwardIt,ForwardIt>
    equal_range( ForwardIt first, ForwardIt last,

                 const T& value, Compare comp );
(2)

Returns a range containing all elements equivalent to value in the range [first, last).

The range [first, last) must be partitioned with respect to comparison with value, i.e. it must satisfy all of the following requirements:

  • partitioned with respect to element < value or comp(element, value)
  • partitioned with respect to !(value < element) or !comp(value, element)
  • for all elements, if element < value or comp(element, value) is true then !(value < element) or !comp(value, element) is also true

A fully-sorted range meets these criteria.

The returned range is defined by two iterators, one pointing to the first element that is not less than value and another pointing to the first element greater than value. The first iterator may be alternatively obtained with std::lower_bound(), the second - with std::upper_bound().

The first version uses operator< to compare the elements, the second version uses the given comparison function comp.


Contents

[edit] Parameters

first, last - the range of elements to examine
value - value to compare the elements to
comp - binary predicate which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type1 &a, const Type2 &b);

The signature does not need to have const &, but the function must not modify the objects passed to it.
The types Type1 and Type2 must be such that an object of type T can be implicitly converted to both Type1 and Type2, and an object of type ForwardIt can be dereferenced and then implicitly converted to both Type1 and Type2. ​

Type requirements
-
ForwardIt must meet the requirements of ForwardIterator.
-
Compare must meet the requirements of BinaryPredicate. it is not required to satisfy Compare

[edit] Return value

std::pair containing a pair of iterators defining the wanted range, the first pointing to the first element that is not less than value and the second pointing to the first element greater than value.

If there are no elements not less than value, last is returned as the first element. Similarly if there are no elements greater than value, last is returned as the second element

[edit] Complexity

The number of comparisons performed is logarithmic in the distance between first and last (At most 2 * log
2
(last - first) + O(1)
comparisons). However, for non-RandomAccessIterators, the number of iterator increments is linear.

[edit] Possible implementation

First version
template<class ForwardIt, class T>
std::pair<ForwardIt,ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last,
                const T& value)
{
    return std::make_pair(std::lower_bound(first, last, value),
                          std::upper_bound(first, last, value));
}
Second version
template<class ForwardIt, class T, class Compare>
std::pair<ForwardIt,ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last,
                const T& value, Compare comp);
{
    return std::make_pair(std::lower_bound(first, last, value, comp),
                          std::upper_bound(first, last, value, comp));
}

[edit] Example

#include <algorithm>
#include <vector>
#include <iostream>
 
struct S
{
    int number;
    char name;
    // note: name is ignored by this comparison operator
    bool operator< ( const S& s ) const { return number < s.number; }
};
 
int main()
{
    // note: not ordered, only partitioned w.r.t. S defined below
    std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} };
 
    S value = {2, '?'};
 
    auto p = std::equal_range(vec.begin(), vec.end(), value);
 
    for ( auto i = p.first; i != p.second; ++i )
        std::cout << i->name << ' ';
 
 
    // heterogeneous comparison:
    struct Comp
    {
        bool operator() ( const S& s, int i ) const { return s.number < i; }
        bool operator() ( int i, const S& s ) const { return i < s.number; }
    };
 
    auto p2 = std::equal_range(vec.begin(),vec.end(), 2, Comp{});
 
    for ( auto i = p2.first; i != p2.second; ++i )
        std::cout << i->name << ' ';
}

Output:

B C D B C D

[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 270 C++98 Compare was required to be a strict weak ordering only a partitioning is needed; heterogeneous comparisons permitted


[edit] See also

returns an iterator to the first element not less than the given value
(function template) [edit]
returns an iterator to the first element greater than a certain value
(function template) [edit]
determines if an element exists in a certain range
(function template) [edit]
divides a range of elements into two groups
(function template) [edit]