switch case programming practice

问题

enum SQLErrorCode{
      OK = 0,
      PARTIAL_OK = 1,
      SOMEWHAT_OK = 2,
      NOT_OK = 3,
};

Code 1:

int error = getErrorCode();
if((error == SQLErrorCode.PARTIAL_OK) ||
  (error == SQLErrorCode.SOMEWHAT_OK) ||
  (error == SQLErrorCode.NOT_OK) ||
  (error < 0))
   callFunction1();
else
    callFunction2();

Code 2:

switch(error){
       case SQLErrorCode.PARTIAL_OK: 
                                    callFunction1();
                                    break;
        case SQLErrorCode.SOMEWHAT_OK:
                                    callFunction1();
                                    break;
        case SQLErrorCode.NOT_OK: 
                                    callFunction1();
                                    break;
        default:
                                    callFunction2();
                                    break;
}

Which method should I prefer. As far as performance is considered there should not be much difference. How to handle error < 0 condition in switch case.

EDIT: Joel's solution:

switch(error) {
     case SQLErrorCode.PARTIAL_OK: 
     case SQLErrorCode.SOMEWHAT_OK:
     case SQLErrorCode.NOT_OK: 
         callFunction1();
         break;
     case SQLErrorCode.OK:
         callFunction2();
         break;
     default:     // error < 0 is handled
         callFunction1();
         break;
}

Q. error < 0 is handled. If I have to handle other numbers in error which do not belong to any of the cases here including default.

回答1:

Not that it matters much for such a small number of cases but switch is in fact faster for integers: it can be, and often is, implemented as a jump table instead of a series of conditional checks.

As a comparison up the number of different cases to 10:

enum SQLErrorCode{
    CODE0 = 0,
    CODE1 = 1,
    CODE2 = 2,
    CODE3 = 3,
    CODE4 = 4,
    CODE5 = 5,
    CODE6 = 6,
    CODE7 = 7,
    CODE8 = 8,
    CODE9 = 9
};

enum SQLErrorCode getErrorCode();

void run()
{
    int error = getErrorCode();
#ifdef CASE1
    if((error == CODE0) ||       
       (error == CODE1) ||
       (error == CODE2) ||
       (error == CODE3) ||
       (error == CODE4) ||
       (error == CODE5) ||
       (error == CODE6) ||
       (error == CODE7) ||
       (error == CODE8) ||
       (error == CODE9) ||
       (error < 0))
        callFunction1();
    else
        callFunction2();
#endif
#ifdef CASE2
    switch(error)
    {
        case CODE0:
            callFunction1();
            break;
    case CODE1:
        callFunction1();
        break;
    case CODE2:
        callFunction1();
        break;
    case CODE3:
        callFunction1();
        break;
    case CODE4:
        callFunction1();
        break;
    case CODE5:
        callFunction1();
        break;
    case CODE6:
        callFunction1();
        break;
    case CODE7:
        callFunction1();
        break;
    case CODE8:
        callFunction1();
        break;
    case CODE9:
        callFunction1();
        break;
    default:
        callFunction2();
        break;
}
#endif

}

Now look at the assembly generated by doing the first case vs. the second, when built on Linux using GCC.

If you look at the assembly you ll see a significant difference (for larger statements): the series of ||s (or if/else if you do it that way) is a series of branches taken one at a time. The switch turns into a big table: it takes up more code but can mean that it can be handled in one jump.

(Incidentally, we are talking about C here right? Not C#? The code you have won't compile: in C enumerators don't use the enumeration name as a prefix. So it's PARTIAL_OK without the SQLErrorCode.)

Code 1: cc -DCASE1 -s switch.s switch.c

        .file   "1241256.c"
        .text
.globl run
        .type   run, @function
run:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        call    getErrorCode
        movl    %eax, -4(%ebp)
        cmpl    $0, -4(%ebp)
        je      .L2
        cmpl    $1, -4(%ebp)
        je      .L2
        cmpl    $2, -4(%ebp)
        je      .L2
        cmpl    $3, -4(%ebp)
        je      .L2
        cmpl    $4, -4(%ebp)
        je      .L2
        cmpl    $5, -4(%ebp)
        je      .L2
        cmpl    $6, -4(%ebp)
        je      .L2
        cmpl    $7, -4(%ebp)
        je      .L2
        cmpl    $8, -4(%ebp)
        je      .L2
        cmpl    $9, -4(%ebp)
        je      .L2
        cmpl    $0, -4(%ebp)
        jns     .L13
.L2:
        call    callFunction1
        jmp     .L15
.L13:
        call    callFunction2
.L15:
        leave
        ret
        .size   run, .-run
        .ident  "GCC: (GNU) 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"
        .section        .note.GNU-stack,"",@progbits

Code 2: cc -DCASE2 -s switch.s switch.c

        .text
.globl run
        .type   run, @function
run:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        call    getErrorCode
        movl    %eax, -4(%ebp)
        cmpl    $9, -4(%ebp)
        ja      .L2
        movl    -4(%ebp), %eax
        sall    $2, %eax
        movl    .L13(%eax), %eax
        jmp     *%eax
        .section        .rodata
        .align 4
        .align 4
.L13:
        .long   .L3
        .long   .L4
        .long   .L5
        .long   .L6
        .long   .L7
        .long   .L8
        .long   .L9
        .long   .L10
        .long   .L11
        .long   .L12
        .text
.L3:
        call    callFunction1
        jmp     .L15
.L4:
        call    callFunction1
        jmp     .L15
.L5:
        call    callFunction1
        jmp     .L15
.L6:
        call    callFunction1
        jmp     .L15
.L7:
        call    callFunction1
        jmp     .L15
.L8:
        call    callFunction1
        jmp     .L15
.L9:
        call    callFunction1
        jmp     .L15
.L10:
        call    callFunction1
        jmp     .L15
.L11:
        call    callFunction1
        jmp     .L15
.L12:
        call    callFunction1
        jmp     .L15
.L2:
        call    callFunction2
.L15:
        leave
        ret
        .size   run, .-run
        .ident  "GCC: (GNU) 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"
        .section        .note.GNU-stack,"",@progbits

回答2:

Without expressing preference for which is best, here is another possibility:

switch(error){
    case SQLErrorCode.PARTIAL_OK: 
    case SQLErrorCode.SOMEWHAT_OK:
    case SQLErrorCode.NOT_OK: 
                                callFunction1();
                                break;
    default:
                                callFunction2();
                                break;
}

回答3:

Why not...

switch(error) {
    case SQLErrorCode.PARTIAL_OK: 
    case SQLErrorCode.SOMEWHAT_OK:
    case SQLErrorCode.NOT_OK: 
         callFunction1();
         break;
    case SQLErrorCode.OK:
         callFunction2();
         break;
    default:
         if (error < 0)
              callFunction1();
         else
              callFunction2();
         break;
}

Easier to write than your switch, and easier to read than your if. Yet it still handles error < 0.

EDIT:

Richard brings up a good point. I've edited to handle positive and negative errors outside the known range.

回答4:

Assuming getErrorCode() returns one of your enumerated values or something less than 0, how about

int error = getErrorCode();
if (error == SQLErrorCode.OK)
  callFunction2(); // Good path
else
  callFunction1(); // Error / not good enough path

Obviously, if your code needs to callFunction2() on error > 3, then this doesn't work.

回答5:

You could also write a function that will determine what an OK error or not OK error code is:

bool isOK(int code)
{
  return code == SQLErrorCode.OK;
}

and your code could become:

if (isOk(getErrorCode()))
{
  callFunction2;
}
else
{
  callFunction1;
}

回答6:

I haven't touched C in awhile, but doesn't it have fall through? So you could write the second chuck as such...

switch(error){
   case SQLErrorCode.PARTIAL_OK: 
    case SQLErrorCode.SOMEWHAT_OK:
    case SQLErrorCode.NOT_OK: 
                                callFunction1();
                                break;
    default:
                                callFunction2();
                                break;

}

回答7:

Whenever you have multiple ways to get the same effect, you're causing confusion. Since, in the switch version, you've got different cases for SqlErrorCode.PARTIAL_OK and SqlErrorCode.SOMEWHAT_OK, the implication is that they have different processing. It takes some study to see what is happening (and it isn't completely compatible with your if-statement handling, which probably means it confused you).

In this case, I'd use an if statement, since the idea is to use one function or another.

来源：https://stackoverflow.com/questions/1241256/switch-case-programming-practice

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c++

if-statement

switch-statement