Copyright: code word is not easy, please indicate the source: https://blog.csdn.net/qq_24309981/article/details/90581412
Theory 1
Include files: #include <algorithm>
Namespace: a using namespace std;
function template:
template <class ForwardIterator, class T>
ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val);
template <class ForwardIterator, class T, class Compare>
ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val, Compare comp);
Function: the Return to Iterator Lower bound (return ordered section the first element is not less than the specified iterator value; upper_bound, return an ordered section first iterator is greater than the value specified element)
- Returns an iterator pointing to the first element in the range [first,last) which does not compare less than val, If all the element in the range compare less than val, the function returns last;
- The elements are compared using operator< for the first version, and comp for the second;
- The elements in the range shall already be sorted according to this same criterion (operator< or comp);
- The function optimizes the number of comparisons performed by comparing non-consecutive elements of the sorted range, which is specially efficient for random-access iterators;
- Unlike upper_bound, the value pointed by the iterator returned by this function may also be equivalent to val, and not only greater.
time complexity:
- On average, logarithmic in the distance between first and last: Performs approximately log2(N)+1 element comparisons (where N is this distance).
- On non-random-access iterators, the iterator advances produce themselves an additional linear complexity in N on average.
The behavior of this function template is equivalent to:
template <class ForwardIterator, class T>
ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val)
{
ForwardIterator it;
iterator_traits<ForwardIterator>::difference_type count, step;
count = distance(first,last);
while (count>0)
{
it = first; step=count/2; advance (it,step);
if (*it<val)
{ // or: if (comp(*it,val)), for version (2)
first=++it;
count-=step+1;
}
else
{
count=step;
}
}
return first;
}
Example 2
// lower_bound/upper_bound example
#include <iostream> // std::cout
#include <algorithm> // std::lower_bound, std::upper_bound, std::sort
#include <vector> // std::vector
int main () {
int myints[] = {10,20,30,30,20,10,10,20};
std::vector<int> v(myints,myints+8); // 10 20 30 30 20 10 10 20
std::sort (v.begin(), v.end()); // 10 10 10 20 20 20 30 30
std::vector<int>::iterator low,up;
low=std::lower_bound (v.begin(), v.end(), 20); // ^
up= std::upper_bound (v.begin(), v.end(), 20); // ^
std::cout << "lower_bound at position " << (low- v.begin()) << '\n';
std::cout << "upper_bound at position " << (up - v.begin()) << '\n';
return 0;
}
//输出
lower_bound at position 3
upper_bound at position 6