# std::equal_range

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt, class T > std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value ); |  | (until C++20)
template< class ForwardIt, class T > constexpr std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value ); |  | (since C++20)
| (2) |
template< class ForwardIt, class T, class Compare > std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp ); |  | (until C++20)
template< class ForwardIt, class T, class Compare > constexpr std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp ); |  | (since C++20)

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

The range `[first, last)` must be at least partially ordered with respect to `value`, i.e. it must satisfy all of the following requirements:

- partitioned with respect to element < value or comp(element, value) (that is, all elements for which the expression is true precedes all elements for which the expression is false)
- 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`.

### 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);

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 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 LegacyForwardIterator.
- `Compare` must meet the requirements of BinaryPredicate. it is not required to satisfy Compare

### 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

### 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-LegacyRandomAccessIterators, the number of iterator increments is linear.

### 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));
    }

### Example

Run this code

    #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

### 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

### See also

lower_bound | returns an iterator to the first element not less than the given value (function template)
upper_bound | returns an iterator to the first element greater than a certain value (function template)
binary_search | determines if an element exists in a certain range (function template)
partition | divides a range of elements into two groups (function template)

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