equal_range()

Library:  Algorithms

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Algorithm that finds the largest subrange in a range of values into which a given value can be inserted without violating the ordering of the values

#include <algorithm>

namespace std {
  template <class ForwardIterator, class T>
  pair<ForwardIterator, ForwardIterator>
  equal_range(ForwardIterator start, ForwardIterator finish,
              const T& value);

  template <class ForwardIterator, class T, class Compare>
  pair<ForwardIterator, ForwardIterator>
  equal_range(ForwardIterator start, ForwardIterator finish,
              const T& value, Compare comp);
}

The equal_range() algorithm performs a binary search on an ordered range to determine where the element value can be inserted without violating the range's ordering. The library includes two versions of the algorithm. The first version uses operator<() to search for the valid insertion range, and assumes that the sequence was sorted using the same ordering. The second version allows you to specify a function object of type Compare, and assumes that Compare was the function used to sort the sequence. The function object must be a binary predicate.

equal_range() returns a pair of iterators, i and j, that define a range containing elements equivalent to value, in other words, the first and last valid insertion points for value. If value is not an element in the container, i and j are equal. Otherwise, i points to the first element not "less" than value, and j points to the first element greater than value. In the second version, the ordering is defined by the comparison object. Formally, equal_range() returns a sub-range [i, j) such that value can be inserted at any iterator k within the range without violating the ordering. Depending upon the version of the algorithm used, k must satisfy one of the following conditions:

!(*k < value) && !(value < *k)

or

comp(*k,value) == false && comp(value, *k) == false

The range [start,finish) is assumed to be sorted. Type T must be LessThanComparable.

equal_range() performs at most 2 * log(finish - start) + 1 comparisons.

//
// eqlrange.cpp
//

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



int main ()
{
    // Typedef for convenience.
    typedef std::vector<int, std::allocator<int> > vector;

    vector::value_type arr[] = {
        0, 1, 2, 2, 3, 4, 2, 2, 2, 6, 7 
    };

    // Set up a vector.
    vector v1 (arr, arr + sizeof arr / sizeof *arr);

    // Try equal_range variants.
    std::pair<vector::iterator, vector::iterator> p1 =
        std::equal_range (v1.begin (), v1.end (), 3);

    std::pair<vector::iterator, vector::iterator> p2 =
        std::equal_range (v1.begin (), v1.end (), 2,
                     std::less<vector::value_type> ()); 

    // Output results.
    std::cout << "\nThe equal range for 3 is: "
              << "(" << *p1.first << " , " 
              << *p1.second << ")" << std::endl;

    std::cout << "\nThe equal range for 2 is: "
              << "(" << *p2.first << " , " 
              << *p2.second << ")" << std::endl; 

    return 0;
}


Program Output

The equal range for 3 is: (3 , 4)

The equal range for 2 is: (2 , 3)

binary_function, lower_bound(), upper_bound()

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.3.3.3