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C++ 算法中的 equal_range函数

C++算法 equal_range()函数是二进制搜索的版本。此函数用于返回子参数的下限和上限，该子参数包括与[first，last)参数内的val等效的所有元素。

equal_range - 语法

default (1)      template <class ForwardIterator, class T>
                       pair<ForwardIterator,ForwardIterator>
                         equal_range (ForwardIterator first, ForwardIterator last, const T& val);

custom (2)     template <class ForwardIterator, class T, class Compare>
                      pair<ForwardIterator,ForwardIterator>
                       equal_range (ForwardIterator first, ForwardIterator last, const T& val,
                          Compare comp);

equal_range - 参数

first:一个正向迭代器，指向要搜索参数内的第一个元素。

last:前向迭代器，指向要搜索参数内的最后一个最后一个元素。

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。

val :比较参数内元素的上限值。

equal_range - 返回值

它返回两个迭代器，一个指向不小于val的第一个元素，另一个指向大于val的第一个元素。

如果找不到这样的元素，则最后返回。

equal_range - 例子1

让我们看一个简单的示例来演示equal_range()的用法:

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#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
  vector<int> v = {3, 1, 4, 2, 5};

  sort(v.begin(), v.end());

  auto result = equal_range(v.begin(), v.end(), 3);

  cout << "Lower Bound of 3 is: "<<*result.first << endl;
  cout << "Upper Bound of 3 is: "<<*result.second << endl;
  
  return 0;
}

输出:

Lower Bound of 3 is: 3
Upper Bound of 3 is: 4

equal_range - 例子2

让我们看另一个简单的示例，使用operator <来比较元素：

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#include <algorithm>
#include <vector>
#include <iostream>
 
using namespace std;
 
struct S
{
    int number;
    char name;
 
    S ( int number, char name  )
        : number ( number ), name ( name )
    {}
 
   //only the number is relevant with this comparison
    bool operator< ( const S& s ) const
    {
        return number < s.number;
    }
};
 
 
int main()
{
   //note: not ordered, only partitioned w.r.t. S defined below
    vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} };
 
    S value ( 2, '?' );
 
    auto p = equal_range(vec.begin(),vec.end(),value);
 
    for ( auto i = p.first; i != p.second; ++i )
        cout << i->name << ' ';
        
        return 0;
}

输出:

B C D

在上面的示例中，运算符<用于比较元素并返回等于2的范围内的所有元素。

equal_range - 例子3

让我们看另一个使用比较函数比较元素的简单示例:

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#include <algorithm>
#include <vector>
#include <iostream>

using namespace std;
 
struct S
{
    int number;
    char name;
 
    S ( int number, char name  )
        : number ( number ), name ( name )
    {}
 
   //only the number is relevant with this comparison
    bool operator< ( const S& s ) const
    {
        return number < s.number;
    }
};
 
struct Comp
{
    bool operator() ( const S& s, int i )
    {
        return s.number < i;
    }
 
    bool operator() ( int i, const S& s )
    {
        return i < s.number;
    }
};
 
int main()
{
   //note: not ordered, only partitioned w.r.t. S defined below
    vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} };
 
    auto p = equal_range(vec.begin(),vec.end(),2,Comp());
 
    for ( auto i = p.first; i != p.second; ++i )
        cout << i->name << ' ';
        
        return 0;
}

输出:

B C D

equal_range - 例子4

让我们看另一个简单的例子:

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#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)<<" ";
 
  pair<vector<int>::iterator, vector<int>::iterator> bounds;
 
  bounds = equal_range(v.begin(), v.end(), 3);
  if (bounds.first != v.end())
    cout <<"\nLower bound of 3 in v = "<<*bounds.first;
  if (bounds.first != v.end())
    cout <<"\nUpper bound of 3 in v = "<<*bounds.second;
 
  bounds = equal_range(v.begin(), v.end(), 4);
  if (bounds.first != v.end())
    cout <<"\nLower bound of 4 in v = "<<*bounds.first;
  if (bounds.first != v.end())
    cout <<"\nUpper bound of 4 in v = "<<*bounds.second;
 
  bounds = equal_range(v.begin(), v.end(), 5);
  if (bounds.first != v.end())
    cout <<"\nLower bound of 5 in v = "<<*bounds.first;
  if (bounds.first != v.end())
    cout <<"\nUpper bound of 5 in v = "<<*bounds.second;
 
  bounds = equal_range(v.begin(), v.end(), 7);
  if (bounds.first != v.end())
    cout <<"\nLower bound of 7 in v = "<<*bounds.first;
  cout <<"\nThis is the first of the three 7's, since the value "
         "before this 7 is "<<*(bounds.first-1)<<".";
  if (bounds.first != v.end())
    cout <<"\nUpper bound of 7 in v = "<<*bounds.second;
 
  bounds = equal_range(v.begin(), v.end(), 0);
  if (bounds.first != v.end())
    cout <<"\nLower bound of 0 in v = "<<*bounds.first;
  if (bounds.first != v.end())
    cout <<"\nUpper bound of 0 in v = "<<*bounds.second;
 
  bounds = equal_range(v.begin(), v.end(), 15);
  if (bounds.first != v.end())
    cout <<"\nLower bound of 15 in v = "<<*bounds.first;
  if (bounds.first != v.end())
    cout <<"\nUpper bound of 15 in v = "<<*bounds.second;
  cout <<"\nNote that both the lower and upper bound locations "
         "\nof 15 are the 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 
Lower bound of 3 in v = 3
Upper bound of 3 in v = 5
Lower bound of 4 in v = 5
Upper bound of 4 in v = 5
Lower bound of 5 in v = 5
Upper bound of 5 in v = 6
Lower bound of 7 in v = 7
This is the first of the three 7's, since the value before this 7 is 6.
Upper bound of 7 in v = 8
Lower bound of 0 in v = 2
Upper bound of 0 in v = 2
Note that both the lower and upper bound locations 
of 15 are the end (one-past-the-last) vector position.

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