C++ - 算法(algorithm) 的 谓词(predicate) 详解

算法(algorithm) 的 谓词(predicate) 详解

 

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本文地址: http://blog.csdn.net/caroline_wendy/article/details/15378055

 

算法谓词, 即标准库算法传递的参数, 可以指定算法的操作, 如std::sort, 默认是从小到大, 通过谓词可以修改从大到小.

本文包含基本的5种谓词模式: 函数,函数指针,lambda表达式,函数对象,库定义的函数对象.

 

1. 函数(function)谓词

通过传递函数名, 匹配二元谓词(binary predicates), 根据函数提供的策略, 输出值;

代码:

/*Function Predicate*/  bool isLarger (const std::string &s1, const std::string &s2) {  	return s1.size() > s2.size();  }  ......  std::stable_sort(sv.begin(), sv.end(), isLarger);

 

2. 函数指针(function pointer)谓词

建立一个函数指针, 传入算法, 使用指针代替函数名, 用法类似函数谓词.

代码:

bool (*pf) (const std::string &s1, const std::string &s2);  	pf = &isLarger;  	std::stable_sort(sv.begin(), sv.end(), *pf);

 

3. Lambda表达式(lambda expression)谓词

Lambda表达式格式: [capture list] (parameter list) -> return type { function body }

需要匹配谓词数, 一元(unary) 或 二元(binary), 也可以通过[capture list]传递函数的变量;

代码:

std::stable_sort(sv.begin(), sv.end(),  	[](const std::string& s1, const std::string& s2){ return s1.size()>s2.size(); });

 

4. 函数对象(Function Object)谓词

类中重载函数的调用"()", 使类可以被调用, 并且传入算法谓词中, 进行使用.

代码:

/*Function Object*/  class LargerString {  public:  	bool operator() (const std::string& a, const std::string& b) {  		return a.size() > b.size();  	}  };  ......  std::stable_sort(sv.begin(), sv.end(), LargerString());

 

 

5. 库定义的函数对象(Library-Defined Function Object)谓词

使用标准库定义的函数对象, 充当算法中的谓词, 包含在#include<functional>,包含基本的算法和逻辑操作.

代码:

std::stable_sort(sv.begin(), sv.end(), std::less<std::string>());

 

所有方法代码(Eclipse CDT; GCC 4.7.1):

/*   * CppPrimer.cpp   *   *  Created on: 2013.11.11   *      Author: Caroline   */    /*eclipse cdt*/    #include <iostream>  #include <string>  #include <vector>  #include <algorithm>  #include <functional>    using namespace std;    class PrintString {  public:  	PrintString (std::ostream &o = std::cout, char c = ' ') : os(o), sep(c) { }  	void operator() (const std::string &s) const { os << s << sep; }  private:  	std::ostream &os;  	char sep;  };    /*Function Predicate*/  bool isLarger (const std::string &s1, const std::string &s2) {  	return s1.size() > s2.size();  }    /*Function Object*/  class LargerString {  public:  	bool operator() (const std::string& a, const std::string& b) {  		return a.size() > b.size();  	}  };    int main (void) {    	std::vector<std::string> sv = {"Beauty", "Girl", "Lady", "Women", "Pretty"};    	std::stable_sort(sv.begin(), sv.end(), isLarger);  	std::cout << "Function Predicate : ";  	std::for_each(sv.begin(), sv.end(), PrintString(std::cout));  	std::cout << std::endl;    	std::stable_sort(sv.begin(), sv.end(),  			[](const std::string& s1, const std::string& s2){ return s1.size()>s2.size(); });  	std::cout << "Lambda Expression Predicate : ";  	std::for_each(sv.begin(), sv.end(), PrintString(std::cout));  	std::cout << std::endl;    	bool (*pf) (const std::string &s1, const std::string &s2);  	pf = &isLarger;  	std::stable_sort(sv.begin(), sv.end(), *pf);  	std::cout << "Function Pointer Predicate : ";  	std::for_each(sv.begin(), sv.end(), PrintString(std::cout));  	std::cout << std::endl;    	std::stable_sort(sv.begin(), sv.end(), LargerString());  	std::cout << "Function Object Predicate : ";  	std::for_each(sv.begin(), sv.end(), PrintString(std::cout));  	std::cout << std::endl;      	std::stable_sort(sv.begin(), sv.end(), std::larger<std::string>());  	std::cout << "Library-Defined Function Object Predicate : ";  	std::for_each(sv.begin(), sv.end(), PrintString(std::cout));  	std::cout << std::endl;        return 0;    }      

输出:

Function Predicate : Beauty Pretty Women Girl Lady   Lambda Expression Predicate : Beauty Pretty Women Girl Lady   Function Pointer Predicate : Beauty Pretty Women Girl Lady   Function Object Predicate : Beauty Pretty Women Girl Lady   Library-Defined Function Object Predicate : Women Pretty Lady Girl Beauty