5-4. 问号(?)操作符

1. 问号(?)操作符是条件语句的简写形式，这是一个三元操作符、

   (condition)?value1:value2
   

   如果condition为真，那么取value1值，如果condition为假，那么取value2值

2. 举例1（将任意的条件语句转换到？的形式）

   	int m = 3;
   	int n = 6;
   	string s;
   	/*
   	s = (m < n) ? "Hellow" : "World";
   	*/
   	if (m < n)
   	{
   		s = "Hello";
   	}
   	else
   	{
   		s = "World";
   	}
   	cout << "s = " << s << endl;
   

3. 举例2

   	int m = 3;
   	int n = 6;
   string s;
   	/*
   	if (m == n)
   	{
   		s = "m == n";
   	}
   	else if (m < n)
   	{
   		s = "m < n";
   	}
   	else
   	{
   		s = "m > n";
   	}
   	cout << s << endl;
   	*/
   	s = (m == n) ? "m == n" : (m < n) ? "m < n" : "m > n";
   	cout << s << endl;
   

4. 举例3

   int m = 3;
   	int n = 6;
   	string s;
   	/*
   	if (m < n)
   	{
   		if (m == 3)
   		{
   			s = "m<n&&m==3";
   		}
   		else
   		{
   			s = "m<n&&m!=3";
   		}
   	}
   	else
   	{
   		s = "m>=n";
   	}
   	cout << s << endl;
   	*/
   	s = (m < n) ? (m == 3) ? "m<n&&m==3" : "m<n&&m!=3" : "m>=n";
   	cout << s << endl;
   

5. 注意：
   在问号操作符中，value1或者value2语句中不可以定义变量，会报错

   	int i = 1;
   	s = (m < n) ? (m == 3) ? (cout<<i<<endl,"m<n&&m==3" ): "m<n&&m!=3" : "m>=n";
   	cout << s << endl;
   

   一般把变量定义放在外面。