binary_search()作为STL函数 (binary_search() as a STL function)
Syntax:
句法：
bool binary_search (
ForwardIterator first,
ForwardIterator last,
const T& value);
Where,
哪里，
ForwardIterator first = iterator to start of the range
ForwardIterator first =迭代器开始范围
ForwardIterator last =iterator to end of the range
ForwardIterator last =迭代器到范围的结尾
T &value = reference to searching element which is of datatype T, T can be any inbuilt datatype or user-defined data type
T＆value =对数据类型为T的搜索元素的引用， T可以是任何内置数据类型或用户定义的数据类型
Return type: bool
返回类型： bool
True - if element found in the range
True-如果在范围内找到元素
False - If the element not found in the range
False-如果在范围内找不到元素
The above syntax is used to compare elements using standard comparison operators. Two elements a & b are said to be equal if (!(a<b) && !(a>b)).
上面的语法用于使用标准比较运算符比较元素。 if(！(a <b)&&！(a> b))的两个元素a和b相等。
We can also define the user-defined comparator for comparing. The syntax with user-defined comparator is like below:
我们还可以定义用户定义的比较器进行比较。 用户定义的比较器的语法如下：
bool binary_search(
ForwardIterator first,
ForwardIterator last,
const T& val,
Comparator comp);
Using the above syntax two elemnts a & b would be equal if (!comp(a<b) && !comp(a>b)).
使用以上语法， 如果(！comp(a <b)&&！comp(a> b))，则两个元素a和b相等。
1)使用默认比较器 (1) Using default comparator)
#include <bits/stdc++.h>
using namespace std;
int main()
{
vector<int> arr{ 3, 2, 1, 4, 5, 6, 7 };
//tp perform binary search we need sorted
//input array
sort(arr.begin(), arr.end());
int search_element = 4;
//ForwardIterator first=arr.begin()
//ForwardIterator last=arr.end()
//const T& val=search_element
if (binary_search(arr.begin(), arr.end(), search_element)) {
cout << "search_element " << search_element << " found\n";
}
else
cout << "search_element" << search_element << " not found\n";
search_element = 7;
//ForwardIterator first=arr.begin()
//ForwardIterator last=arr.begin()+arr.size()-1
//const T& val=search_element
if (binary_search(arr.begin(), arr.begin() + arr.size() - 1, search_element)) {
cout << "search_element " << search_element << " found\n";
}
else
cout << "search_element " << search_element << " not found\n";
return 0;
}
Output:
输出：
search_element 4 found
search_element 7 not found
In the above program, we have checked to cases and have used the default comparator. No need to mention, since this uses the binary search algorithm for searching, we need to feed sorted array only.
在上面的程序中，我们检查了案例并使用了默认的比较器。 无需提及，因为它使用二进制搜索算法进行搜索，所以我们仅需要提供已排序的数组。
So as a pre-requisite we sorted the array. The sorted array is: [1,2,3,4,5,6,7]
因此，作为前提条件，我们对数组进行了排序。 排序后的数组是： [1,2,3,4,5,6,7]
Then for the first case, our range is the entire vector, and it returned true is since it finds the searched element 4.
那么对于第一种情况，我们的范围是整个向量，并且由于找到了要搜索的元素4而返回true。
For the second case, our range is [1-6] as we mentioned the end iterator as arr.begin()+arr.size()-1 which leaves the last element 7. So searched element 7 is not found.
对于第二种情况，我们的范围是[1-6]，因为我们提到的最终迭代器为arr.begin()+ arr.size()-1 ，它保留了最后一个元素7 。 因此未找到搜索到的元素7 。
2)使用用户定义的比较器功能 (2) Using user-defined comparator function)
Here we have taken a use case where we have student details for five students. By using the user-defined comparator we have checked whether there is a student with the specified marks or not.
在这里，我们使用了一个用例，其中有五个学生的学生详细信息。 通过使用用户定义的比较器，我们检查了是否有指定分数的学生。
#include <bits/stdc++.h>
using namespace std;
class student {
int score;
int roll;
string name;
public:
student()
{
score = 0;
roll = 0;
name = "";
}
student(int sc, int ro, string nm)
{
score = sc;
roll = ro;
name = nm;
}
int get_score()
{
return score;
}
int get_roll()
{
return roll;
}
string get_name()
{
return name;
}
};
//comparator for sorting
bool myfunc(student a, student b)
{
if (a.get_score() < b.get_score()) //no swap
return true;
else //swap
return false;
}
//comparator for binary search
//match found if !mycomp(a,b) && !mycomp(b,a)
bool mycomp(student a, student b)
{
if (a.get_score() < b.get_score())
return true;
return false;
}
int main()
{
vector<student> arr(5);
//1st student
arr[0] = student(80, 5, "XYZ"); //roll 5, marks 80
//2nd student
arr[1] = student(70, 10, "INC"); //roll 10, marks 70
//3rd student
arr[2] = student(85, 7, "HYU"); //roll 7, marks 85
//4th student
arr[3] = student(83, 1, "EFG"); //roll 1, marks 83
//5th student
arr[4] = student(81, 11, "ABC"); //roll 11, marks 81
//sort based on marks
//user-defined compartor=myfunc to sort
sort(arr.begin(), arr.end(), myfunc);
//find if any student exists who scored 81
student t1(81, -1, ""); //dummy search element just to search the student marks
if (binary_search(arr.begin(), arr.end(), t1, mycomp))
cout << "Student with marks 81 exists\n";
else
cout << "Student with marks 81 doesn't exist\n";
//find if any student exists who scored 75
student t2(75, -1, ""); //dummy search element just to search the student marks
if (binary_search(arr.begin(), arr.end(), t2, mycomp))
cout << "Student with marks 75 exists\n";
else
cout << "Student with marks 75 doesn't exist\n";
return 0;
}
Output:
输出：
Student with marks 81 exists
Student with marks 75 doesn't exist
Here we have first sorted the student vector using the user-defined comparator function. We have defined a separate comparator for binary search, though both have the same body. We have specified just to underline the factor that both comparators are not used in the same way. In case of searching, there is a match b/w T a and T b (T be the datatype) if !comp(a,b) && !comp(b, a) where a is element from the vector and b is the searching element.
在这里，我们首先使用用户定义的比较器函数对学生向量进行了排序。 我们为二进制搜索定义了一个单独的比较器，尽管两者具有相同的主体。 我们仅指定要强调两个比较器未以相同方式使用的因素。 在搜索的情况下， 如果！comp(a，b)&&！comp(b，a)匹配b / w T a和T b(T为数据类型) ，其中a是向量中的元素， b是搜索元素。
So in this article, you saw how efficiently we can use binary_search to search elements within a range, no matter what kind of object it is. Even for the user-defined datatype, we can do by using a user-defined comparator as shown above. But, the main thing to remember is that the input list must be sorted(based on the key). For example, as we were searching for the marks, we sorted the student list based on marks.
因此，在本文中，您看到了无论使用哪种对象，我们都能有效地使用binary_search搜索范围内的元素。 即使对于用户定义的数据类型，我们也可以通过使用用户定义的比较器来完成，如上所示。 但是，要记住的主要事情是必须对输入列表进行排序(基于键)。 例如，在搜索标记时，我们根据标记对学生列表进行了排序。
翻译自: https://www.includehelp.com/stl/std-binary_search-in-cpp.aspx
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