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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
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
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
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