C++ upper_bound()
C++算法upper_bound()
C++算法 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来比较元素和第二个版本使用给定的比较函数，即comp。
语法
default (1) template <class ForwardIterator, class T>
ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last,
const T& val);
custom (2) template <class ForwardIterator, class T, class Compare>
ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last,
const T& val, Compare comp);
参数
first: 指向要搜索范围中第一个元素的前向迭代器。
last : 前向迭代器，指向要搜索范围内的最后一个元素。
comp : 用户定义的二进制谓词函数，它接受两个参数，并且如果两个参数顺序正确，则返回true，否则返回false。
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，则返回最后一个。
复杂度
平均而言，复杂度在第一个和最后一个之间的距离是对数的: 最多执行log2(N)+ 1个元素比较，其中N =最后一个-第一个。
数据竞争
访问[first，last)范围内的对象。
异常
如果以下情况，该函数将引发异常元素比较或对迭代器的操作都会引发异常。
注意: 无效的参数会导致未定义的行为。
示例1
让我们看一个简单的示例来演示upper_bound()的用法:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
vector<int> 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:
示例2
让我们看另一个简单的示例:
#include <iostream> // std::cout
#include <algorithm> // std::lower_bound, std::upper_bound, std::sort
#include <vector> // std::vector
using namespace std;
int main () {
int myints[] = {10,20,30,30,20,10,10,20};
vector<int> 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<int>::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
示例3
让我们看另一个简单的示例:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
int a[] = {2, 3, 5, 6, 7, 7, 7, 8, 9, 10};
vector<int> v(a, a+10);
cout <<"\nHere are the contents of v:\n";
for (vector<int>::size_type i=0; i<v.size(); i++)
cout <<v.at(i)<<" ";
vector<int>::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.
示例4
让我们看另一个简单的示例:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
bool ignore_case(char a, char b) {
return(tolower(a) == tolower(b));
}
int main(void) {
vector<char> 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'.
