Why do we have only costant value's or constant value expression in SWITCH statement

[rsodimbakam]

Hi,
Can anyone provide me some information about why do we have only constant value's or constant value expression be in SWITCH case statement ?

Is there any technical reason behind or its just compiler standard ?

Thanks
Kiran

[Paul McKenzie]

Hi,
Can anyone provide me some information about why do we have only constant value's or constant value expression be in SWITCH case statement ?

Those are the rules of the language.

Regards,

Paul McKenzie

[Peter_APIIT]

If the value can be change and no case will be match after changed.

Think logically.

[Graham]

Because switch is a glorified goto and the targets are defined statically at compile time, not computed dynamically at run-time.

Or, as Paul put it, "them's the rules".

[_uj]

Can anyone provide me some information about why do we have only constant value's or constant value expression be in SWITCH case statement ?

Is there any technical reason behind or its just compiler standard ?

To my knowledge most (if not all) languages require the case labels to be compile-time constants. What differs is that in some languages the switch doesn't have to evaluate to an integer.

[_uj]

Because switch is a glorified goto

The switch offers a unique functionality namely that the case labels are evaluated without order. This cannot be mimiced by any combination of other control structures.

So the switch is not just a "glorified goto". It has a unique role among the common control structures.

[GCDEF]

Please don't use apostrophes to make words plural. I can't understand why that simple little rule confuses so many people.

[laserlight]

To my knowledge most (if not all) languages require the case labels to be compile-time constants. What differs is that in some languages the switch doesn't have to evaluate to an integer.

PHP is an exception, e.g.,

switch ($x)
{
case $a:
    echo 'a';
    break;
case $b:
    echo 'b';
    break;
case $a + $b:
    echo 'a+b';
    break;
default:
    echo 'none of the above';
}

where $x, $a and $b are variables whose values are only known at run time.

[Hobson]

Please don't use apostrophes to make words plural. I can't understand why that simple little rule confuses so many people.

Your picky :P

[GCDEF]

Your picky :P

You're picky.

Yes, I'm a grammar cop, I'll admit. Still, most people here will be out in the professional world and proper use of the written language is important.

[Graham]

The switch offers a unique functionality namely that the case labels are evaluated without order. This cannot be mimiced by any combination of other control structures.

So the switch is not just a "glorified goto". It has a unique role among the common control structures.

What?

What does "case labels are evaluated without order" mean?

The only difference between switch and FORTRAN's computed GOTO is syntactic sugar. I could do anything that switch can do in FORTRAN. The only complication is that I would need to map (the used values of) the switch variable on to a consecutive sequence of integer values starting at one. That the compiler in C++ does this for me is, as I said, syntactic sugar.

E.g.

switch (i)
{
case 4:
   // do stuff
   break;

case 73:
   // do other stuff
   break;

default:
   // more stuff
   break;
}

in FORTRAN:

   INTEGER J
    IF (I.EQ.4) THEN
	  J = 1
    ELSEIF (I.EQ.73) THEN
	  J = 2
   ELSE
	  J = 3
   ENDIF
   GOTO(100, 200, 300) J
100 do stuff
   GOTO 1000

200 do other stuff
   GOTO 1000

300 more stuff
   GOTO 1000

1000 ...

[_uj]

What?

What does "case labels are evaluated without order" mean?

The only difference between switch and FORTRAN's computed GOTO is syntactic sugar. I could do anything that switch can do in FORTRAN. The only complication is that I would need to map (the used values of) the switch variable on to a consecutive sequence of integer values starting at one. That the compiler in C++ does this for me is, as I said, syntactic sugar.

You can implement any control structure available in C++ in assembly and that would involve gotos. So if you would call all control structures "glorified gotos" I would agree, but if you single out the switch, I don't.

As a control structure the switch has a unique property. The label cases are in no specific order. That's what characterizes a switch and that's for example why an if-chain isn't functionally equivalent with a switch. That's also why a switch can be implemented in O(1) using say a table of some kind.

The fact that you (or a compiler) can accomplish the functional equivalent of a switch with gotos in some language is irrelevant. The switch has a unique property that no combination of other control structures can mimic. It's not just a "glorified goto".

[MrViggy]

PHP is an exception, e.g.,

switch ($x)
{
case $a:
    echo 'a';
    break;
case $b:
    echo 'b';
    break;
case $a + $b:
    echo 'a+b';
    break;
default:
    echo 'none of the above';
}

where $x, $a and $b are variables whose values are only known at run time.

Isn't PHP an interpreted language (like PERL)?

Viggy

[laserlight]

Isn't PHP an interpreted language (like PERL)?

Yes, PHP code is typically interpreted at run time (after all, C++ is also interpreted, but at compile time, heh).

[Speedo]

Yes, PHP code is typically interpreted at run time (after all, C++ is also interpreted, but at compile time, heh).

Heh, well, unless you're going to directly write in machine code every language has to be interpreted at some point.