📅  最后修改于: 2020-10-17 06:34:05             🧑  作者: Mango
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。
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);
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。
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。
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。
访问范围为[first,last)的对象。
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。
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
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
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.
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'.