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# std::equal_range

From cppreference.com
< cpp‎ | algorithm

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, as does a range resulting from a call to std::partition.

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`.

### [edit] Parameters

first, last | - | the range of elements to examine
value | - | value to compare the elements to
comp | - | comparison function 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); |  |
The signature does not need to have const &, but the function must not modify the objects passed to it. `cmp` will be called as both `cmp(value, *iterator)` and `cmp(*iterator, value)`.
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.

### [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-`RandomAccessIterator`s, 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

Run this code

    #include <algorithm>
    #include <vector>
    #include <iostream>

    struct S
    {
        int number;
        char name;

        S ( int number, char name  )
            : number ( number ), name ( name )
        {}

        // only the number is relevant with this comparison
        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 << ' ';
    }

Output:

    B C D

### [edit] Example With Comparator

Run this code

    #include <algorithm>
    #include <vector>
    #include <iostream>

    struct S
    {
        int number;
        char name;

        S ( int number, char name  )
            : number ( number ), name ( name )
        {}

        // only the number is relevant with this comparison
        bool operator< ( const S& s ) const
        {
            return number < s.number;
        }
    };

    struct Comp
    {
        bool operator() ( const S& s, int i )
        {
            return s.number < i;
        }

        bool operator() ( int i, const S& s )
        {
            return i < 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'} };

        auto p = std::equal_range(vec.begin(),vec.end(),2,Comp());

        for ( auto i = p.first; i != p.second; ++i )
            std::cout << i->name << ' ';
    }

Output:

    B C D

### [edit] See also

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

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