Cookbook系列之Cpp：數值計算

問題01：如何将字元串形式的數值轉換為諸如int或float之類的數值類型

    定義在<cstdlib>中的strtol、strtod和strtoul函數可以将以null結尾的字元串轉換為long int、double或unsigned long類型的數值。你可以使用他們把任意進制的字元串轉換為數值（相比之下，atoi函數隻能轉換十進制數值，并且沒有錯誤傳回）。

 
 #include <iostream>                                                                      
      
 #include <string>                                                                        
      
 #include <cstdlib>                                                                       
          
 using namespace std;                                                                     
          
 int main()                                                                               
      
 {                                                                                        
      
     char *offset;     // 接收解析結束位址       
          
     string s1 = "520";                                                                   
      
     cout << strtol(s1.c_str(), &offset, 0) << endl;                                      
          
     string s2 = "0xA";                                                                   
      
     cout << strtol(s2.c_str(), &offset, 16) << endl;                                     
          
     return 0;                                                                            
      
 }        
     
    

問題02：如何将int或float類型的數值轉換為某種格式的字元串

    使用stringstream類來存儲字元串資料。stringstream是一種把資料轉換成字元串的簡便方法，因為它允許使用由标準輸入輸出流類提供的格式化工具。

 
 #include <iostream>                                                                      
      
 #include <string>                                                                        
      
 #include <sstream>                                                                       
      
 #include <iomanip>                                                                       
          
 using namespace std;                                                                     
          
 int main()                                                                               
      
 {                                                                                        
      
     stringstream ss;                                                                     
          
     ss << 9;                                                                             
      
     cout << ss.str() << endl;                                                            
          
     ss.str("");                                                                          
      
     ss << showbase << hex << 16;                                                         
      
     cout << ss.str() << endl;                                                            
          
     ss.str("");                                                                          
      
     ss << setprecision(3) << 3.1415;                                                       
      
     cout << ss.str() << endl;                                                            
          
     return 0;                                                                            
      
 }        
     
    

問題03：如何把用科學計數法表示的數值字元串存儲到double變量中

    要解析用科學計數法表示的數值，最直接的方法是使用C++函數庫在<sstream>中内置的streamstring類。

 
 #include <iostream>                                                                      
      
 #include <sstream>                                                                       
      
 #include <string>                                                                        
          
 using namespace std;                             
          
 int main()                                                                               
      
 {                                                                                        
      
     stringstream ss("1.234e5");                                                          
      
     double d = 0;                                                                        
      
     ss >> d;                                                                             
          
     if(ss.fail()) {                                                                      
      
         string e = "Unable to format";                                                   
      
         throw(e);                                                                        
      
     }                                                                                    
          
     cout << d << endl;                                                                   
          
     return 0;                                                                            
      
 }        
     
    

 
 #include <iostream>                                                                      
      
 #include <limits>                                                                        
          
 using namespace std;                                                                     
          
 int main()                                                                               
      
 {                                                                                        
      
     cout << numeric_limits<int>::min() << endl;                                          
      
     cout << numeric_limits<int>::max() << endl;                                          
          
     return 0;                                                                            
      
 }