📜  C++ STL-algorithm.upper_bound()函数

📅  最后修改于: 2020-10-17 06:34:05             🧑  作者: Mango

C++ STL algorithmupper_bound()

C++ STL algorithm.upper_bound()函数是二进制搜索的版本。此函数用于返回一个迭代器,该迭代器指向[first,last)范围内大于指定值val的第一个元素。

第一个版本使用运算符<比较元素,第二个版本使用给定的比较函数,即comp。

句法

default (1)      template 
                          ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last,
                               const T& val); 

custom (2)      template 
                          ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last,
                               const T& val, Compare comp);

参数

first:指向要搜索范围内第一个元素的前向迭代器。

last:一个正向迭代器,指向要搜索范围内的过去最后一个元素。

comp:用户定义的二进制谓词函数,该函数接受两个参数,如果两个参数顺序正确,则返回true,否则返回false。它遵循严格的弱排序来对元素进行排序。

val:比较范围内元素的上限值。

返回值

它返回一个指向范围内第一个大于val的元素的迭代器,如果找不到该元素,则返回last。

复杂度

平均而言,复杂度是first和last之间的距离的对数:最多执行log2(N)+ 1个元素比较,其中N = last-first。

数据竞争

访问范围为[first,last)的对象。

异常处理

如果元素比较或迭代器上的操作引发异常,则此函数引发异常。

注意:无效的参数会导致未定义的行为。

例子1

让我们看一个简单的示例来演示upper_bound()的用法:

#include 
#include 
#include 

using namespace std;

int main()
{

  vector v = {3, 1, 4, 6, 5};
  
  decltype(v)::iterator it = upper_bound(v.begin(), v.end(), 3);
  cout<<"Upper bound of 3 is: ";
  cout << *it << endl;
  
  return 0;
}

输出:

Upper bound of 3 is: 4

例子2

让我们看另一个简单的例子:

#include      // std::cout
#include     // std::lower_bound, std::upper_bound, std::sort
#include        // std::vector

using namespace std;

int main () {
  int myints[] = {10,20,30,30,20,10,10,20};
  vector v(myints,myints+8);           // 10 20 30 30 20 10 10 20

  sort (v.begin(), v.end());                // 10 10 10 20 20 20 30 30

  vector::iterator low,up;
  low=lower_bound (v.begin(), v.end(), 20); //          ^
  up= upper_bound (v.begin(), v.end(), 20); //                   ^

  cout << "lower_bound at position " << (low- v.begin()) << '\n';
  cout << "upper_bound at position " << (up - v.begin()) << '\n';

  return 0;
}

输出:

lower_bound at position 3
upper_bound at position 6

例子3

让我们看另一个简单的例子:

#include 
#include 
#include 
 
using namespace std;
 
int main()
{
  int a[] = {2, 3, 5, 6, 7, 7, 7,  8, 9, 10};
  vector v(a, a+10);
  cout <<"\nHere are the contents of v:\n";
  for (vector::size_type i=0; i::iterator upper;
 
  upper = upper_bound(v.begin(), v.end(), 3);
  if (upper != v.end())
    cout <<"\nUpper bound of 3 in v = "<<*upper;
 
  upper = upper_bound(v.begin(), v.end(), 4);
  if (upper != v.end())
    cout <<"\nUpper bound of 4 in v = "<<*upper;
 
  upper = upper_bound(v.begin(), v.end(), 5);
  if (upper != v.end())
    cout <<"\nUpper bound of 5 in v = "<<*upper;
 
  upper = upper_bound(v.begin(), v.end(), 7);
  if (upper != v.end())
    cout <<"\nUpper bound of 7 in v = "<<*upper;
 
  upper = upper_bound(v.begin(), v.end(), 0);
  if (upper != v.end())
    cout <<"\nUpper bound of 0 in v = "<<*upper;
 
  upper = upper_bound(v.begin(), v.end(), 15);
  if (upper != v.end())
    cout <<"\nUpper bound of 15 in v = "<<*upper;
  cout <<"\n\nNote that the upper bound location of 15 is \nthe end (one-past-the-last) vector position.";
 
  return 0;
}

输出:

Here are the contents of v:
2 3 5 6 7 7 7 8 9 10 
Upper bound of 3 in v = 5
Upper bound of 4 in v = 5
Upper bound of 5 in v = 6
Upper bound of 7 in v = 8
Upper bound of 0 in v = 2

Note that the upper bound location of 15 is 
the end (one-past-the-last) vector position.

例子4

让我们看另一个简单的例子:

#include 
#include 
#include 

using namespace std;

bool ignore_case(char a, char b) {
   return(tolower(a) == tolower(b));
}

int main(void) {
   vector v = {'A', 'b', 'C', 'd', 'E'};
   auto it = upper_bound(v.begin(), v.end(), 'C');

   cout << "Upper bound of \'C\' is " << *it << endl;

   it = upper_bound(v.begin(), v.end(), 'C', ignore_case);

   cout << "Upper bound of \'C\' is " << *it << endl;

   it = upper_bound(v.begin(), v.end(), 'z', ignore_case);

   cout << "All elements are less than \'z\'." << endl;

   return 0;
}

输出:

Upper bound of 'C' is d
Upper bound of 'C' is C
All elements are less than 'z'.