[C++STL] The principle and use of traversal algorithm (for_each)

1. STLAlgorithm Overview

1. <algorithm> <functional> <numeric>The algorithm is mainly contained in the header file
2. <algorithm>It is the largest of all STL header files and involves: traversal, comparison, search, exchange, copy, modification and other operations
3. <functional>Defines some template classes for declaring function objects
4. <numeric>Minimal, containing only a few template functions that perform simple mathematical operations on sequences

Algorithms are divided into: qualitative change algorithm and non-qualitative change algorithm

• Qualitative change algorithm : The content of the elements in the interval will be changed during the operation. For example, copy, replace, delete, etc.
• Non-qualitative algorithm : the content of elements in the interval will not be changed during the operation process, such as searching, counting, traversing, finding extreme values, etc.

2. for_each

Function: Traverse all elements in the container

• for_each()The algorithm is very flexible, allowing each element to be accessed, processed, and modified in different ways. Therefore, for_each()it is both a qualitative change algorithm and a non-qualitative algorithm.

Function prototype:	for_each(iterator beg, iterator end, func)	explain
Parameter 1	iterator beg	start iterator
Parameter 2	iterator end	end iterator
Parameter 3	func	function or function object

accomplish:

//for_each()的实现如下
template <typename Iterator, typename Operation>
    Operation for_each (Iterator beg, Iterator end, Operation op)
    {
    
    
        while(beg != end)
        {
    
    
            op(*beg);
            ++beg;
        }
        return op;
    }

Example:

#include <iostream>
#include <algorithm> //必须包含该头文件
#include <vector>
using namespace std;

//普通函数
void printA(int value)
{
    
    
	cout << value << " ";
}

//函数对象(仿函数)
class printB
{
    
    
public:
	void operator()(int value)
	{
    
    
		cout << value << " ";
	}
};

class printC
{
    
    
public:
	void operator()(int& value)
	{
    
    
		value = value + 1;    //修改入参的值
		cout << value << " ";
	}
};

void test01() 
{
    
    
	vector<int> vec;
	for (int i = 0; i < 10; i++)
	{
    
    
		vec.push_back(i);
	}

	//普通函数遍历算法
	cout << "for_each printA：";
	for_each(vec.begin(), vec.end(), printA);
	cout << endl;

	//函数对象(仿函数)遍历算法-非质变算法
	cout << "for_each printB()：";
	for_each(vec.begin(), vec.end(), printB());
	cout << endl;

	//函数对象(仿函数)遍历算法-质变算法
	cout << "for_each printC()：";
	for_each(vec.begin(), vec.end(), printC());
	cout << endl;
}

int main()
{
    
    
	test01();
	system("pause");
	return 0;
}

//result
for_each printA：0 1 2 3 4 5 6 7 8 9
for_each printB()：0 1 2 3 4 5 6 7 8 9
for_each printC()：1 2 3 4 5 6 7 8 9 10

Note: Since C++11, range-based for循环a more convenient and natural traversal method is provided. Therefore, for_each()the importance of algorithms may gradually decrease.

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