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digitalmars.D.learn - switch ( Expression ) ScopeStatement

reply Derek Parnell <derek psych.ward> writes:
I see that the syntax for 'switch' is ...
 
   switch ( Expression ) ScopeStatement

and ScopeStatement is either a BlockStatement or a NonEmptyStatement.

Meaning that 

  switch (i) j = k;

is valid syntax! Why is that?

-- 
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell
Jul 06 2009
next sibling parent Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
On Mon, Jul 6, 2009 at 8:18 PM, Derek Parnell<derek psych.ward> wrote:

 I see that the syntax for 'switch' is ...

 =A0 switch ( Expression ) ScopeStatement

 and ScopeStatement is either a BlockStatement or a NonEmptyStatement.

 Meaning that

 =A0switch (i) j =3D k;

 is valid syntax! Why is that?


So you can write this, of course!

switch(x) case 1: break;

I mean, you sometimes need a one-case switch, yes?  ;)
Jul 06 2009
prev sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Derek Parnell:

 is valid syntax! Why is that?


To allow train-wrecks like this one:

version(Tango) import tango.stdc.stdio;
void main(char[][] args) {
  if (args.length > 1)
    switch (args[1]) {
      int x = 1; // This initialization to 1 doesn't happen,
                 // it's the same as int x = void;
      case "1": printf("1! x=%d\n", x); break;
      case "2": printf("2! x=%d\n", x); break;
    }
}

Bye,
bearophile
Jul 06 2009
next sibling parent reply Derek Parnell <derek psych.ward> writes:
On Mon, 06 Jul 2009 22:10:20 -0400, bearophile wrote:


 Derek Parnell:

 is valid syntax! Why is that?


 
 To allow train-wrecks like this one:
 
 version(Tango) import tango.stdc.stdio;
 void main(char[][] args) {
   if (args.length > 1)
     switch (args[1]) {
       int x = 1; // This initialization to 1 doesn't happen,
                  // it's the same as int x = void;
       case "1": printf("1! x=%d\n", x); break;
       case "2": printf("2! x=%d\n", x); break;
     }
 }


No quite the same thing, actually. You have highlighted another quirk with
switch though. 

My question was really about why is it that the first token after the "(
Expression )" did not have to be a left-brace, but could be any statement
at all. 

  switch(x) funcA();
  switch(x) switch (y) {};

Just seems wrong somehow but I'm positive that the experts have a perfectly
rational answer. I just hope I can understand it.

-- 
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell
Jul 06 2009
parent reply Tim Matthews <tim.matthews7 gmail.com> writes:
Derek Parnell wrote:

 On Mon, 06 Jul 2009 22:10:20 -0400, bearophile wrote:
 

 Derek Parnell:

 is valid syntax! Why is that?


 To allow train-wrecks like this one:

 version(Tango) import tango.stdc.stdio;
 void main(char[][] args) {
   if (args.length > 1)
     switch (args[1]) {
       int x = 1; // This initialization to 1 doesn't happen,
                  // it's the same as int x = void;
       case "1": printf("1! x=%d\n", x); break;
       case "2": printf("2! x=%d\n", x); break;
     }
 }


 
 No quite the same thing, actually. You have highlighted another quirk with
 switch though. 
 
 My question was really about why is it that the first token after the "(
 Expression )" did not have to be a left-brace, but could be any statement
 at all. 
 
   switch(x) funcA();
   switch(x) switch (y) {};
 
 Just seems wrong somehow but I'm positive that the experts have a perfectly
 rational answer. I just hope I can understand it.
 


Switch is really a neat form of comparison and gotos but with actual 
labels replaced with a case statement. A block statement is usually used 
to have more than one case statement valid. This is valid code but 
uncomment the next line and it becomes invalid:

module test;

import tango.io.Stdout;

void main(char[][] args)
{
   if (args.length > 1)
     switch (args[1])
       case "1": Stdout("1"); break;
       //case "2": Stdout("2"); break; //uncomment and error

}
Jul 06 2009
parent reply Derek Parnell <derek psych.ward> writes:
On Tue, 07 Jul 2009 14:57:39 +1200, Tim Matthews wrote:


 Switch is really a neat form of comparison and gotos but with actual 
 labels replaced with a case statement. A block statement is usually used 
 to have more than one case statement valid. This is valid code but 
 uncomment the next line and it becomes invalid:
 
 module test;
 
 import tango.io.Stdout;
 
 void main(char[][] args)
 {
    if (args.length > 1)
      switch (args[1])
        case "1": Stdout("1"); break;
        //case "2": Stdout("2"); break; //uncomment and error
 
 }


Actually that is not valid code. The first "break;" breaks the program.

This is valid ...

 module test;
 
 import tango.io.Stdout;
 
 void main(char[][] args)
 {
    if (args.length > 1)
      switch (args[1])
        case "1": Stdout("1"); // break; //uncomment and error
 }

Ok, why on earth would a one-case switch be useful? And that still doesn't
help me understand why allowing other statement types in that position is a
good idea.


-- 
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell
Jul 06 2009
next sibling parent Tim Matthews <tim.matthews7 gmail.com> writes:
Derek Parnell wrote:

 On Tue, 07 Jul 2009 14:57:39 +1200, Tim Matthews wrote:
 

 Switch is really a neat form of comparison and gotos but with actual 
 labels replaced with a case statement. A block statement is usually used 
 to have more than one case statement valid. This is valid code but 
 uncomment the next line and it becomes invalid:

 module test;

 import tango.io.Stdout;

 void main(char[][] args)
 {
    if (args.length > 1)
      switch (args[1])
        case "1": Stdout("1"); break;
        //case "2": Stdout("2"); break; //uncomment and error

 }


 
 Actually that is not valid code. The first "break;" breaks the program.


yeah I forgot to remove the break as it was no longer part of the switch 
statement.



 Ok, why on earth would a one-case switch be useful? And that still doesn't
 help me understand why allowing other statement types in that position is a
 good idea.
 
 


C compatibility. It's not really a problem to D. If there is a compiler 
that only accepted block statements for switch not many people would 
care that it doesn't conform to C spec because no one uses it like that 
anyway.
Jul 06 2009
prev sibling parent Tim Matthews <tim.matthews7 gmail.com> writes:
Derek Parnell wrote:

 On Tue, 07 Jul 2009 14:57:39 +1200, Tim Matthews wrote:
 

 Switch is really a neat form of comparison and gotos but with actual 
 labels replaced with a case statement. A block statement is usually used 
 to have more than one case statement valid. This is valid code but 
 uncomment the next line and it becomes invalid:

 module test;

 import tango.io.Stdout;

 void main(char[][] args)
 {
    if (args.length > 1)
      switch (args[1])
        case "1": Stdout("1"); break;
        //case "2": Stdout("2"); break; //uncomment and error

 }


 
 Actually that is not valid code. The first "break;" breaks the program.
 


It shouldn't have accepted it so it's here now. 
http://d.puremagic.com/issues/show_bug.cgi?id=3144
Jul 06 2009
prev sibling next sibling parent Tim Matthews <tim.matthews7 gmail.com> writes:
bearophile wrote:

 Derek Parnell:

 is valid syntax! Why is that?


 
 To allow train-wrecks like this one:
 
 version(Tango) import tango.stdc.stdio;
 void main(char[][] args) {
   if (args.length > 1)
     switch (args[1]) {
       int x = 1; // This initialization to 1 doesn't happen,
                  // it's the same as int x = void;
       case "1": printf("1! x=%d\n", x); break;
       case "2": printf("2! x=%d\n", x); break;
     }
 }
 
 Bye,
 bearophile



You are looking at something completely different but is not limited to 
switches. Switchs are a bit like gotos and gotos can achieve this same 
skip initialization errors.
Jul 06 2009
prev sibling parent BCS <none anon.com> writes:
Hello bearophile,


 Derek Parnell:
 

 is valid syntax! Why is that?
 


 To allow train-wrecks like this one:
 
 version(Tango) import tango.stdc.stdio;
 void main(char[][] args) {
 if (args.length > 1)
 switch (args[1]) {
 int x = 1; // This initialization to 1 doesn't happen,
 // it's the same as int x = void;
 case "1": printf("1! x=%d\n", x); break;
 case "2": printf("2! x=%d\n", x); break;
 }
 }
 Bye,
 bearophile


you want a train wreck?


void main()
{
    switch(5) default: { case 6: {} }
}
Jul 06 2009