C++ Algorithm equal_range()

C++ Algorithm equal_range() function is the version of binary search. This function is used to return the lower bound and upper bound of the sub range that includes all the elements equivalent to val in the range [first, last).

Where sub range is defined by two iterators, one pointing to the first element that is not less than val and another pointing to the first element greater than val.

• The first version uses operator < to compare the elements and the second version uses the given comparison function i.e. comp.
• The range [first, last) must be partitioned with respect to comparison with val, i.e. it must satisfy all of the following conditions:
  • partitioned with respect to element < val or comp(element, val)
  • partitioned with respect to !(val < element) or !comp(val, element)
  • for all elements, if element < val or comp(element, val) is true then !(val < element) or !comp(val, element) is also true.

Syntax

Parameter

first: A forward iterator pointing to the first element in the range to be searched.

last: A forward iterator pointing to the past last element in the range to be searched.

comp: A user-defined binary predicate function that accepts two arguments and returns true if the two arguments are in order otherwise, it returns false. It follows the strict weak ordering to order the elements.

Lower Bound of 3 is: 3
Upper Bound of 3 is: 4

B C D

B C D

Here are the contents of v:
2 3 5 6 7 7 7 8 9 10 
Lower bound of 3 in v = 3
Upper bound of 3 in v = 5
Lower bound of 4 in v = 5
Upper bound of 4 in v = 5
Lower bound of 5 in v = 5
Upper bound of 5 in v = 6
Lower bound of 7 in v = 7
This is the first of the three 7's, since the value before this 7 is 6.
Upper bound of 7 in v = 8
Lower bound of 0 in v = 2
Upper bound of 0 in v = 2
Note that both the lower and upper bound locations 
of 15 are the end (one-past-the-last) vector position.