Grouping switch statement cases together?

问题

I may be over looking something but is there a simple way in C++ to group cases together instead of writing them out individually? I remember in basic I could just do:

SELECT CASE Answer
CASE 1, 2, 3, 4

Example in C++ (For those that need it):

#include <iostream.h>
#include <stdio.h>
int main()
{
   int Answer;
   cout << "How many cars do you have?";
   cin >> Answer;
   switch (Answer)                                      
      {
      case 1:
      case 2:
      case 3:
      case 4:
         cout << "You need more cars. ";
         break;                                        
      case 5:
      case 6:
      case 7:
      case 8:
         cout << "Now you need a house. ";
         break;                                        
      default:
         cout << "What are you? A peace-loving hippie freak? ";
      }
      cout << "\nPress ENTER to continue... " << endl;
      getchar();
      return 0;
}

回答1:

No, but you can with an if-else if-else chain which achieves the same result:

if (answer >= 1 && answer <= 4)
  cout << "You need more cars.";
else if (answer <= 8)
  cout << "Now you need a house.";
else
  cout << "What are you? A peace-loving hippie freak?";

You may also want to handle the case of 0 cars and then also the unexpected case of a negative number of cars probably by throwing an exception.

PS: I've renamed Answer to answer as it's considered bad style to start variables with an uppercase letter.

As a side note, scripting languages such as Python allow for the nice if answer in [1, 2, 3, 4] syntax which is a flexible way of achieving what you want.

回答2:

AFAIK all you can do is omit the returns to make things more compact in C++:

switch(Answer)
{
    case 1: case 2: case 3: case 4:
        cout << "You need more cars.";
        break;
    ...
}

(You could remove the other returns as well, of course.)

回答3:

Sure you can.

You can use case x ... y for the range

Example:

#include <iostream.h>
#include <stdio.h>
int main()
{
   int Answer;
   cout << "How many cars do you have?";
   cin >> Answer;
   switch (Answer)                                      
      {
      case 1 ... 4:
         cout << "You need more cars. ";
         break;                                        
      case 5 ... 8:
         cout << "Now you need a house. ";
         break;                                        
      default:
         cout << "What are you? A peace-loving hippie freak? ";
      }
      cout << "\nPress ENTER to continue... " << endl;
      getchar();
      return 0;
}

Make sure you have "-std=c++0x" flag enabled within your compiler

回答4:

You can't remove keyword case. But your example can be written shorter like this:

switch ((Answer - 1) / 4)                                      
{
   case 0:
      cout << "You need more cars.";
      break;                                        
   case 1:
      cout << "Now you need a house.";
      break;                                        
   default:
      cout << "What are you? A peace-loving hippie freak?";
}

回答5:

Your example is as concise as it gets with the switch construct.

回答6:

You can use like this:

case 4: case 2:
 {
   //code ...
 }

For use 4 or 2 switch case.

回答7:

gcc has a so-called "case range" extension:

http://gcc.gnu.org/onlinedocs/gcc-4.2.4/gcc/Case-Ranges.html#Case-Ranges

I used to use this when I was only using gcc. Not much to say about it really -- it does sort of what you want, though only for ranges of values.

The biggest problem with this is that only gcc supports it; this may or may not be a problem for you.

(I suspect that for your example an if statement would be a more natural fit.)

回答8:

If you're willing to go the way of the preprocessor abuse, Boost.Preprocessor can help you.

    #include <boost/preprocessor/seq/for_each.hpp>

    #define CASE_case(ign, ign2, n) case n:

    #define CASES(seq) \
        BOOST_PP_SEQ_FOR_EACH(CASE_case, ~, seq)

    CASES((1)(3)(15)(13))

Running this through gcc with -E -P to only run the preprocessor, the expansion of CASES gives:

    case 1: case 3: case 15: case 13:

Note that this probably wouldn't pass a code review (wouldn't where I work!) so I recommend it be constrained to personal use.

It should also be possible to create a CASE_RANGE(1,5) macro to expand to

    case 1: case 2: case 3: case 4: case 5:

for you as well.

回答9:

No, unless you want to break compatibility and your compiler supports it.

回答10:

#include <stdio.h>
int n = 2;
int main()
{
     switch(n)
     {
          case 0: goto _4;break;
          case 1: goto _4;break;
          case 2: goto _4;break;
          case 3: goto _4;break;
          case 4:
              _4:
                printf("Funny and easy!\n");
                break;
          default:
                printf("Search on StackOverflow!\n");
                break;
     }
}

来源：https://stackoverflow.com/questions/4494170/grouping-switch-statement-cases-together

标签

c++

switch-statement