lower_bound

Algorithm

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• Complexity
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Determine the first valid position for an element in a sorted container.

None

template <class ForwardIterator, class T>
 ForwardIterator lower_bound(ForwardIterator first, 
                             ForwardIterator last,
                             const T& value);

 template <class ForwardIterator, class T, class Compare>
  ForwardIterator lower_bound(ForwardIterator first, 
                              ForwardIterator last,
                              const T& value, Compare comp);

The lower_bound algorithm compares a supplied value to elements in a sorted container and returns the first position in the container that value can occupy without violating the container's ordering. There are two versions of the algorithm. The first uses the less than operator (operator<) to perform the comparison, and assumes that the sequence has been sorted using that operator. The second version lets you include a function object of type Compare, and assumes that Compare is the function used to sort the sequence. The function object must be a binary predicate.

lower_bound's return value is the iterator for the first element in the container that is greater than or equal to value, or, when the comparison operator is used, the first element that does not satisfy the comparison function. Formally, the algorithm returns an iterator i in the range [first, last) such that for any iterator j in the range [first, i) the following corresponding conditions hold:

*j  <  value

or

 comp(*j,  value) == true

lower_bound performs at most log(last - first) + 1 comparisons.

//
// ul_bound.cpp
//
 #include <vector>
 #include <algorithm>
 #include <iostream.h>
 
 int main()
 {
   typedef vector<int>::iterator iterator;
   int d1[11] = {0,1,2,2,3,4,2,2,2,6,7};

   // Set up a vector
   vector<int> v1(d1,d1 + 11);

   // Try lower_bound variants
   iterator it1 = lower_bound(v1.begin(),v1.end(),3);
   // it1 = v1.begin() + 4

   iterator it2 = 
       lower_bound(v1.begin(),v1.end(),2,less<int>());
   // it2 = v1.begin() + 4

   // Try upper_bound variants
   iterator it3 = upper_bound(v1.begin(),v1.end(),3);
   // it3 = vector + 5

   iterator it4 = 
      upper_bound(v1.begin(),v1.end(),2,less<int>());
   // it4 = v1.begin() + 5 

   cout << endl << endl
        << "The upper and lower bounds of 3: ( "
        << *it1 << " , " << *it3 << " ]" << endl;

   cout << endl << endl
        << "The upper and lower bounds of 2: ( "
        << *it2 << " , " << *it4 << " ]" << endl;

   return 0;
 }

Output :
The upper and lower bounds of 3: ( 3 , 4 ]
The upper and lower bounds of 2: ( 2 , 3 ]

If your compiler does not support default template parameters then you need to always supply the Allocator template argument. For instance you'll have to write:

vector<int,allocator<int> >

instead of:

vector<int>

upper_bound, equal_range

©Copyright 1996, Rogue Wave Software, Inc.