Cookbook系列之Cpp：数值计算中的字符串转换方法

问题01&＃xff1a;如何将字符串形式的数值转换为诸如int或float之类的数值类型

    定义在中的strtol、strtod和strtoul函数可以将以null结尾的字符串转换为long int、double或unsigned long类型的数值。你可以使用他们把任意进制的字符串转换为数值&＃xff08;相比之下&＃xff0c;atoi函数只能转换十进制数值&＃xff0c;并且没有错误返回&＃xff09;。

1. #include                                                                
2. #include                                                                  
3. #include                                                                 
4.                                                                                   
5. using namespace std;                                                              
6.                                                                                   
7. int main()                                                                        
8. {                                                                                 
9.     char *offset;     // 接收解析结束地址
10.                                                                                   
11.     string s1 &＃61; "520";                                                            
12.     cout << strtol(s1.c_str(), &offset, 0) << endl;                               
13.                                                                                   
14.     string s2 &＃61; "0xA";                                                            
15.     cout << strtol(s2.c_str(), &offset, 16) << endl;                              
16.                                                                                   
17.     return 0;                                                                     
18. } 

问题02&＃xff1a;如何将int或float类型的数值转换为某种格式的字符串

    使用stringstream类来存储字符串数据。stringstream是一种把数据转换成字符串的简便方法&＃xff0c;因为它允许使用由标准输入输出流类提供的格式化工具。

1. #include                                                                
2. #include                                                                  
3. #include                                                                 
4. #include                                                                 
5.                                                                                   
6. using namespace std;                                                              
7.                                                                                   
8. int main()                                                                        
9. {                                                                                 
10.     stringstream ss;                                                              
11.                                                                                   
12.     ss << 9;                                                                      
13.     cout << ss.str() << endl;                                                     
14.                                                                                   
15.     ss.str("");                                                                   
16.     ss << showbase << hex << 16;                                                  
17.     cout << ss.str() << endl;                                                     
18.                                                                                   
19.     ss.str("");                                                                   
20.     ss << setprecision(3) << 3.1415;                                                
    
21.     cout << ss.str() << endl;                                                     
22.                                                                                   
23.     return 0;                                                                     
24. } 

问题03&＃xff1a;如何把用科学计数法表示的数值字符串存储到double变量中

    要解析用科学计数法表示的数值&＃xff0c;最直接的方法是使用C&＃43;&＃43;函数库在中内置的streamstring类。

1. #include                                                                
2. #include                                                                 
3. #include                                                                  
4.                                                                                   
5. using namespace std;                      
   
6.                                                                                   
7. int main()                                                                        
8. {                                                                                 
9.     stringstream ss("1.234e5");                                                   
10.     double d &＃61; 0;                                                                 
11.     ss >> d;                                                                      
12.                                                                                   
13.     if(ss.fail()) {                                                               
14.         string e &＃61; "Unable to format";                                            
15.         throw(e);                                                                 
16.     }                                                                             
17.                                                                                   
18.     cout << d << endl;                                                            
19.                                                                                   
20.     return 0;                                                                     
21. } 

问题04&＃xff1a;如何获得某种数值类型的最值

    使用中的numeric_limits类模板可以知道某个数值类型所能表示的最大值和最小值&＃xff0c;min和max是numeric_limits的静态成员函数。

1. #include                                                                
2. #include                                                                  
3.                                                                                   
4. using namespace std;                                                              
5.                                                                                   
6. int main()                                                                        
7. {                                                                                 
8.     cout << numeric_limits<int>::min() << endl;                                   
9.     cout << numeric_limits<int>::max() << endl;                                   
10.                                                                                   
11.     return 0;                                                                     
12. }