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digitalmars.D.learn - switch statement with variable branches

reply Yuxuan Shui <yshuiv7 gmail.com> writes:
Somehow I can't use ubyte variables behind 'case', but ulong 
works fine. Why is that?

void main() {
	alias TestType = ulong; // won't compile if = ubyte
	import std.stdio;
	TestType a,b,c;
	readf("%s %s %s ", &a, &b, &c);
	switch(c){
		case a: writeln("a");break;
		case b: writeln("b");break;
		default: assert(false);
	}
}
Jan 18 2017
next sibling parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 01/18/2017 05:22 PM, Yuxuan Shui wrote:

 Somehow I can't use ubyte variables behind 'case', but ulong works fine.
 Why is that?

 void main() {
     alias TestType = ulong; // won't compile if = ubyte
     import std.stdio;
     TestType a,b,c;
     readf("%s %s %s ", &a, &b, &c);
     switch(c){
         case a: writeln("a");break;
         case b: writeln("b");break;
         default: assert(false);
     }
 }


case expressions must be constants:

   "The case expressions must all evaluate to a constant value or
    array, or a runtime initialized const or immutable variable of
    integral type."

   https://dlang.org/spec/statement.html#SwitchStatement

The fact that it compiles for ulong looks like a bug to me. It compiles 
probably because switch is most likely implemented in terms of a chained 
if-else-if statements by the compiler and it just works because there is 
no explicit check whether they are constant or not.

Ali
Jan 18 2017
parent Yuxuan Shui <yshuiv7 gmail.com> writes:
On Thursday, 19 January 2017 at 02:00:10 UTC, Ali Çehreli wrote:

 On 01/18/2017 05:22 PM, Yuxuan Shui wrote:

 Somehow I can't use ubyte variables behind 'case', but ulong 
 works fine.
 Why is that?


 case expressions must be constants:

   "The case expressions must all evaluate to a constant value or
    array, or a runtime initialized const or immutable variable 
 of
    integral type."

   https://dlang.org/spec/statement.html#SwitchStatement

 The fact that it compiles for ulong looks like a bug to me. It 
 compiles probably because switch is most likely implemented in 
 terms of a chained if-else-if statements by the compiler and it 
 just works because there is no explicit check whether they are 
 constant or not.

 Ali


If you try:

void main() {
     alias TestType = ulong; // won't compile if = ubyte
     import std.stdio;
     TestType a,b,c;
     readf("%s %s %s ", &a, &b, &c);
     final switch(c){
         case a: writeln("a");break;
         case b: writeln("b");break;
         default: assert(false);
     }
}

Then the error message:

test.d(7): Error: case variables not allowed in final switch 
statements
test.d(8): Error: case variables not allowed in final switch 
statements

Makes it looks like that "case variable" is an intended feature.
Jan 18 2017
prev sibling parent Stefan Koch <uplink.coder googlemail.com> writes:
On Thursday, 19 January 2017 at 01:22:56 UTC, Yuxuan Shui wrote:

 Somehow I can't use ubyte variables behind 'case', but ulong 
 works fine. Why is that?

 void main() {
 	alias TestType = ulong; // won't compile if = ubyte
 	import std.stdio;
 	TestType a,b,c;
 	readf("%s %s %s ", &a, &b, &c);
 	switch(c){
 		case a: writeln("a");break;
 		case b: writeln("b");break;
 		default: assert(false);
 	}
 }


It is a bug that this code compiled.
Case Variables can only be used on const values, to prevent 
mutation of them inside the switch itself.

try to make the type a const ubyte.
Jan 19 2017