读《C程序设计语言》笔记4

练习2-1：

　　编写一个程序以确定分别由signed及unsigned限定的char、short、int及long类型变量的取值范围。采用打印标准头文件中的相应值以及直接计算两种方式实现：

　　采用打印标准头文件中的相应值：　

 
  1 
  #include  
  < 
  stdio.h 
  > 
  
 
  2 
  #include  
  < 
  limits.h 
  > 
  
 
  3 
   
  // 
  determine ranges of types 
  
 
  4 
   
  int 
   main()
 
  5 
  {
 
  6 
  printf( 
  " 
  采用打印标准头文件limits.h中的相应值：\n 
  " 
  );
 
  7 
   
  // 
  signed types 
  
 
  8 
   
  printf( 
  " 
  signed char min = %d\n 
  " 
  , SCHAR_MIN);
 
  9 
  printf( 
  " 
  signed char max = %d\n 
  " 
  , SCHAR_MAX);
 
  10 
  printf( 
  " 
  signed short min = %d\n 
  " 
  , SHRT_MIN);
 
  11 
  printf( 
  " 
  signed short max = %d\n 
  " 
  , SHRT_MAX);
 
  12 
  printf( 
  " 
  signed int min = %d\n 
  " 
  , INT_MIN);
 
  13 
  printf( 
  " 
  signed int max = %d\n 
  " 
  , INT_MAX);
 
  14 
  printf( 
  " 
  signed long min = %ld\n 
  " 
  , LONG_MIN);
 
  15 
  printf( 
  " 
  signed long max = %ld\n 
  " 
  , LONG_MAX);
 
  16 
   
  // 
  unsigned types 
  
 
  17 
   
  printf( 
  " 
  unsigned char max = %u\n 
  " 
  , UCHAR_MAX);
 
  18 
  printf( 
  " 
  unsigned short max = %u\n 
  " 
  , USHRT_MAX);
 
  19 
  printf( 
  " 
  unsigned int max = %u\n 
  " 
  , UINT_MAX);
 
  20 
  printf( 
  " 
  unsigned long max = %lu\n 
  " 
  , ULONG_MAX);
 
  21 
  system( 
  " 
  pause 
  " 
  );
 
  22 
   
  return 
    
  0 
  ;
 
  23 
  } 
 

　　执行结果：

　　这种实现方式没有什么需要讨论的，只需要知道相应的头文件和变量名称。

直接计算：

　　

 
  1 
  #include  
  < 
  stdio.h 
  > 
  
 
  2 
   
  // 
  determine ranges of types 
  
 
  3 
   
  int 
   main()
 
  4 
  {
 
  5 
  printf( 
  " 
  采用直接计算的方式：\n 
  " 
  );
 
  6 
   
  // 
  signed types 
  
 
  7 
   
  printf( 
  " 
  signed char min = %d\n 
  " 
  ,  
  - 
  ( 
  char 
  )((unsigned  
  char 
  ) 
  ~ 
  0 
  >> 
  1 
  ) 
  - 
  1 
  );
 
  8 
  printf( 
  " 
  signed char max = %d\n 
  " 
  , ( 
  char 
  )((unsigned  
  char 
  ) 
  ~ 
  0 
  >> 
  1 
  ));
 
  9 
  printf( 
  " 
  signed short min = %d\n 
  " 
  ,  
  - 
  ( 
  short 
  )((unsigned  
  short 
  ) 
  ~ 
  0 
  >> 
  1 
  ) 
  - 
  1 
  );
 
  10 
  printf( 
  " 
  signed short max = %d\n 
  " 
  , ( 
  short 
  )((unsigned  
  short 
  ) 
  ~ 
  0 
  >> 
  1 
  ));
 
  11 
  printf( 
  " 
  signed int min = %d\n 
  " 
  ,  
  - 
  ( 
  int 
  )((unsigned  
  int 
  ) 
  ~ 
  0 
  >> 
  1 
  ) 
  - 
  1 
  );
 
  12 
  printf( 
  " 
  signed int max = %d\n 
  " 
  , ( 
  int 
  )((unsigned  
  int 
  ) 
  ~ 
  0 
  >> 
  1 
  ));
 
  13 
  printf( 
  " 
  signed long min = %ld\n 
  " 
  ,  
  - 
  ( 
  long 
  )((unsigned  
  long 
  ) 
  ~ 
  0 
  >> 
  1 
  ) 
  - 
  1 
  );
 
  14 
  printf( 
  " 
  signed long max = %ld\n 
  " 
  , ( 
  long 
  )((unsigned  
  long 
  ) 
  ~ 
  0 
  >> 
  1 
  ));
 
  15 
   
  // 
  unsigned types 
  
 
  16 
   
  printf( 
  " 
  unsigned char max = %u\n 
  " 
  , (unsigned  
  char 
  ) 
  ~ 
  0 
  );
 
  17 
  printf( 
  " 
  unsigned short max = %u\n 
  " 
  , (unsigned  
  short 
  ) 
  ~ 
  0 
  );
 
  18 
  printf( 
  " 
  unsigned int max = %u\n 
  " 
  , (unsigned  
  int 
  ) 
  ~ 
  0 
  );
 
  19 
  printf( 
  " 
  unsigned long max = %lu\n 
  " 
  , (unsigned  
  long 
  ) 
  ~ 
  0 
  );
 
  20 
  system( 
  " 
  pause 
  " 
  );
 
  21 
   
  return 
    
  0 
  ;
 
  22 
  } 
 

　执行结果：

　　

小结：

　　利用按位运算符进行计算。表达式：(char)((unsigned char)~0>>1)

　　先把数字0的各个二进制位全部转换为1：取反，~0

　　然后，将结果值转换为unsigned char类型：(unsigned char)~0

　　再把这个unsigned char类型值右移一位以清除符号位：(unsigned char)~0 >> 1

　　最后，把它转换为char类型：(char)((unsigned char)~0>>1)

　　这一系列操作的最终结果就得到了signed类型字符的最大值。

　　而其signed类型字符的最小值加负号后还需减去1才是最小值。