[C++] STL algorithm ⑨ (predefined function object example - sort container elements from large to small | sort sorting algorithm | greater＜T＞ predefined function object)

1. Example of predefined function object - sort container elements from large to small

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1. sort sorting algorithm

The C++ Standard Template Library (STL, Standard Template Library) provides the sort algorithm function, which is defined in the <algorithm> header file and is a generic algorithm;

The sort algorithm is used to sort elements in a container . This algorithm is very efficient and can sort elements within a given iterator range , and the sorting order can be defined based on a user-specified comparison function;

The user-specified comparison function is a binary predicate;

The function prototype of the default sorting rule of the std::sort algorithm is as follows:

template< class RandomIt >  
void sort( RandomIt first, RandomIt last );  

• RandomIt first, RandomIt last parameters: This function accepts two random access iterators first and last , which define the sequence range that needs to be sorted. Note: This range is a front-closed and then-open interval;
• Default comparison rules: Elements in this range will be compared and sorted using the < operator by default . If a custom class does not implement the < operator overloaded function, an error may be reported;

The function prototype of the std::sort algorithm custom sorting rule is as follows:

template< class RandomIt, class Compare >  
void sort( RandomIt first, RandomIt last, Compare comp );

• RandomIt first, RandomIt last parameters: This function accepts two random access iterators first and last , which define the sequence range that needs to be sorted; Note: This range is a front-closed and then-open interval;
• Custom comparison rule Compare comp parameter: This parameter is a binary predicate, that is, a function object that receives 2 parameters and returns a bool value; the elements in this range will be sorted using this binary predicate rule;

2. greater predefined function object

The C++ Standard Template Library (STL, Standard Template Library) provides the greater<T> predefined function object , which is a binary predicate . With the help of this function object, two values ​​can be compared in a convenient way to determine the first value. Is it greater than the second value;

This function object is mainly used in STL algorithms to control sorting order, search conditions and other scenarios;

The greater<T> function object is defined in the <functional> header file. It accepts a generic type parameter T, which specifies the element type to be compared;

The greater<T> function object internally overloads the function call operator operator(), so that instance objects of this class can be called like ordinary functions;

2. Code examples - predefined function objects

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1. Code examples

Code example:

#include "iostream"
using namespace std;
#include <vector>
#include <algorithm>
#include "functional"

int main() {
    
    

	// 创建一个 set 集合容器
	vector<int> myVector;

	// 向容器中插入元素
	myVector.push_back(9);
	myVector.push_back(5);
	myVector.push_back(2);
	myVector.push_back(7);

	// 向 foreach 循环中传入 Lambda 表达式
	for_each(myVector.begin(), myVector.end(), [](int a) {
    
    
		std::cout << a << " ";
		});
	cout << endl;


	// 将 myVector 容器中的元素按照从大到小的顺序排列
	sort(myVector.begin(), myVector.end(), greater<int>());

	// 向 foreach 循环中传入 Lambda 表达式
	for_each(myVector.begin(), myVector.end(), [](int a) {
    
    
		std::cout << a << " ";
		});
	cout << endl;

	
	// 控制台暂停 , 按任意键继续向后执行
	system("pause");
	return 0;
};

2. Execution results

Results of the :

9 5 2 7
9 7 5 2
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