C++进阶STL-常用的查找算法

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本文深入探讨了C++标准模板库(STL)中的多种查找算法，包括find、find_if、binary_search、adjacent_find、count及count_if的使用方法与自定义条件实现。通过具体代码示例，详细解析了如何在不同场景下应用这些算法，如查找特定对象、满足条件的第一个元素、二分查找、查找相邻重复元素以及计算符合特定条件的元素数量。

部署运行你感兴趣的模型镜像

#include <algorithm>

find

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


class Person
{
public:
	Person(int age, int id) :m_age(age), m_id(id)
	{

	}
	bool operator==(const Person& p)const  //使用find算法的时候在查找自定义对象需要 重载 == 操作符
	{
		return m_age == p.m_age&&m_id == p.m_id;
	}

	int m_age;
	int m_id;
};


int main()
{
	vector<Person> v1;
	Person p1(1, 1), p2(2, 2), p3(3, 3), p4(4, 4);
	v1.push_back(p1);
	v1.push_back(p2);
	v1.push_back(p3);
	v1.push_back(p4);

	vector<Person>::iterator it = find(v1.begin(),v1.end(), p2);
	if (it == v1.end())
	{
		cout << "not found" << endl;
	}
	else
	{
		cout << "found" << endl;
	}

    return 0;
}

find_if（会根据我们的条件（函数），返回第一个满足条件的元素的迭代器）

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


class Person
{
public:
	Person(int age, int id) :m_age(age), m_id(id)
	{

	}

	int m_age;
	int m_id;
};

struct Compare
{
	bool operator()(const Person&p1)
	{
		return p1.m_age >4;
	};
};


int main()
{
	vector<Person> v1;
	Person p1(1, 1), p2(2, 2), p3(3, 3), p4(4, 4),p5(5,5);

	v1.push_back(p1);
	v1.push_back(p2);
	v1.push_back(p3);
	v1.push_back(p4);
	v1.push_back(p4);
	v1.push_back(p5);

	vector<Person>::iterator it =find_if(v1.begin(),v1.end(),Compare());
	if (it == v1.end())
	{
		cout << "not found" << endl;
	}
	else
	{
		cout << "found:" <<(*it).m_age<< endl;
	}
	
    return 0;
}

结果：5

binary_search：二分法查找（必须有序排列的）

进行对象查找，自定义compare函数，如果是正常类型就不用提供compare，conpare可以是普通函数，也可以是函数对象（仿函数）

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


class Person
{
public:
	Person(int age, int id) :m_age(age), m_id(id)
	{
	}
	int m_age;
	int m_id;
};

struct Compare
{
	bool operator()(const Person&p1, const Person& p2)
	{
		return p1.m_age < p2.m_age;
	};
};

int main()
{
	vector<Person> v1;
	Person p1(1, 1), p2(2, 2), p3(3, 3), p4(4, 4),p5(5,5);

	v1.push_back(p1);
	v1.push_back(p2);
	v1.push_back(p3);
	v1.push_back(p4);

	bool tmp=binary_search(v1.begin(),v1.end(), p5, Compare());
	if (tmp == false)
	{
		cout << "not found" << endl;
	}
	else
	{
		cout << "found" << endl;
	}

    return 0;
}

结果：not found

adjacent_find：查找相邻重复的元素

如果进行自定义类型查找，要么重载 == 操作符，要么需要给adjacent_find提供函数对象（仿函数）或者普通函数

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


class Person
{
public:
	Person(int age, int id) :m_age(age), m_id(id)
	{

	}
	bool operator==(const Person& p)const
	{
		return m_age == p.m_age&&m_id == p.m_id;
	}

	int m_age;
	int m_id;
};

//struct Compare
//{
//	bool operator()(const Person&p1, const Person& p2)
//	{
//		return p1.m_age == p2.m_age && p1.m_id==p2.m_id;
//	};
//};



int main()
{
	vector<Person> v1;
	Person p1(1, 1), p2(2, 2), p3(3, 3), p4(4, 4),p5(5,5);

	v1.push_back(p1);
	v1.push_back(p2);
	v1.push_back(p3);
	v1.push_back(p4);
	v1.push_back(p4);
	v1.push_back(p5);


	vector<Person>::iterator it =adjacent_find(v1.begin(),v1.end());
	if (it == v1.end())
	{
		cout << "not found" << endl;
	}
	else
	{
		cout << "found:" <<(*it).m_age<< endl;
	}

    return 0;
}

结果：4

count：计算符合条件（可以自定义）的个数

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


class Person
{
public:
	Person(int age, int id) :m_age(age), m_id(id)
	{

	}
	bool operator == (const Person& p2) const
	{
		return m_age == p2.m_age && m_id == p2.m_id;
	}
	int m_age;
	int m_id;
};

struct Compare
{
	bool operator()(const Person&p1)
	{
		return p1.m_age ==4;
	};
};


int main()
{
	vector<Person> v1;
	Person p1(1, 1), p2(2, 2), p3(3, 3), p4(4, 4),p5(5,5);

	v1.push_back(p4);
	v1.push_back(p5);
	v1.push_back(p1);
	v1.push_back(p2);
	v1.push_back(p3);
	v1.push_back(p4);
	v1.push_back(p4);
	v1.push_back(p5);


	int num =count(v1.begin(),v1.end(),p4);

	cout << num << endl;

    return 0;
}

结果：3

count_if：返回满足条件（可自定义）的元素个数

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


class Person
{
public:
	Person(int age, int id) :m_age(age), m_id(id)
	{

	}
	bool operator == (const Person& p2) const
	{
		return m_age == p2.m_age && m_id == p2.m_id;
	}
	int m_age;
	int m_id;
};

struct Compare
{
	bool operator()(const Person&p1)
	{
		return p1.m_age >3;
	};
};


int main()
{
	vector<Person> v1;
	Person p1(1, 1), p2(2, 2), p3(3, 3), p4(4, 4),p5(5,5);


	v1.push_back(p4);
	v1.push_back(p5);
	v1.push_back(p1);
	v1.push_back(p2);
	v1.push_back(p3);
	v1.push_back(p4);
	v1.push_back(p4);
	v1.push_back(p5);


	int num =count_if(v1.begin(),v1.end(),Compare());

	cout << num << endl;

    return 0;
}