digitalmars.D.learn - Inside the switch statement

Sam Hu (35/35) Jun 08 2009 To save time ,just get to my point:I do not understand why such code bel...

BCS (5/45) Jun 08 2009 This is a classic C thing. The short explanation is that a switch is a c...

Ellery Newcomer (3/20) Jun 08 2009 I would guess from the D specs
Sam Hu (5/6) Jun 08 2009 It is from the spec:

grauzone (3/5) Jun 09 2009 of C is that switches don't break automatically before each case label.

BCS (3/12) Jun 09 2009 I'm sorry, you don't have my sympathy on this one. There are to many pla...

grauzone (14/28) Jun 09 2009 What kind of fall-throughs were these?

BCS (7/39) Jun 09 2009 I don't do that, I go with this form:
Ary Borenszweig (17/50) Jun 09 2009 The solution is to forbid fallthrough, and change the switch syntax:

Saaa (2/35) Jun 09 2009 Doesn't support B :)

Ary Borenszweig (4/47) Jun 09 2009 The idea is that it not supporting B is something good.

Saaa (6/14) Jun 09 2009 I know this is your idea, but as BCS doesn't support this idea.

Ary Borenszweig (3/20) Jun 09 2009 A warning *is* nothing. :-P

Saaa (2/3) Jun 09 2009 Your idea was to give an error on the case (B) he uses. Or did I miss

Ary Borenszweig (3/7) Jun 09 2009 You missed the alternative syntax to get the same behaviour. But it's a

Saaa (2/4) Jun 09 2009 Do you mean the multiple cases?

Ary Borenszweig (3/8) Jun 09 2009 Yes, but make the "multiple cases" the *only* way to make case

Saaa (1/3) Jun 09 2009 That is the same as giving an error on case B, right?

Saaa (1/1) Jun 09 2009 I mean, the syntax stays the same.
Ary Borenszweig (4/8) Jun 09 2009 Well, yes, but you also have to prepare your mind for the change. This

Saaa (1/3) Jun 09 2009 ;)
Saaa (3/3) Jun 09 2009 (nah, just kidding, you're right, I didn't realize it :-P)

Ary Borenszweig (2/6) Jun 09 2009 Write a lexer and a parser.

Saaa (1/2) Jun 09 2009 how do you mean?

BCS (9/19) Jun 09 2009 that /does not/ support the use case I have used:

Steve Schveighoffer (14/56) Jun 10 2009 case value1:

Derek Parnell (53/67) Jun 09 2009 It is not an either-or situation.

BCS (5/37) Jun 09 2009 C# sort of takes this approach (by using goto case EXP;) but doesn't hav...

Kagamin (12/45) Jun 11 2009 The Duff's device is said to be an optimization, but I get blunt device ...

bearophile (4/5) Jun 11 2009 Compilers already know about the Duff's device, and sometimes they use i...
Joel C. Salomon (6/7) Jun 15 2009 Read Duff’s post: “this loop was the bottleneck in a real-time anima...

Sam Hu <samhudotsamhu gmail.com> writes:

To save time ,just get to my point:I do not understand why such code below
inside the switch ... case statement is allowed ,what is the point of do ...
while...:

This example uses a mixin to implement a generic Duff's device for an arbitrary
statement (in this case, the arbitrary statement is in bold). A nested function
is generated as well as a delegate literal, these can be inlined by the
compiler: 
template duffs_device(alias id1, alias id2, alias s)
{
    void duff_loop()
    {
	if (id1 < id2)
	{
	    typeof(id1) n = (id2 - id1 + 7) / 8;
	    switch ((id2 - id1) % 8)
	    {
		case 0:        do {  s();
		case 7:              s();
		case 6:              s();
		case 5:              s();
		case 4:              s();
		case 3:              s();
		case 2:              s();
		case 1:              s();
				  } while (--n > 0);
	    }
	}
    }
}

void foo() { writefln("foo"); }

void test()
{
    int i = 1;
    int j = 11;

    mixin duffs_device!(i, j, delegate { foo(); } );
    duff_loop();	// executes foo() 10 times
}

Thanks in advance.

Regards,
Sam

Jun 08 2009

BCS <none anon.com> writes:

Hello Sam,

 To save time ,just get to my point:I do not understand why such code
 below inside the switch ... case statement is allowed ,what is the
 point of do ... while...:

This is a classic C thing. The short explanation is that a switch is a
conditional, 
many way goto and the labels can be *anywhere* in the body (except inside 
another switch). For the more in depth version Google "duff's device".

 
 This example uses a mixin to implement a generic Duff's device for an
 arbitrary statement (in this case, the arbitrary statement is in
 bold).

BTW: where is this from?

 A nested function is generated as well as a delegate literal,
 these can be inlined by the compiler:
 template duffs_device(alias id1, alias id2, alias s)
 {
 void duff_loop()
 {
 if (id1 < id2)
 {
 typeof(id1) n = (id2 - id1 + 7) / 8;
 switch ((id2 - id1) % 8)
 {
 case 0:        do {  s();
 case 7:              s();
 case 6:              s();
 case 5:              s();
 case 4:              s();
 case 3:              s();
 case 2:              s();
 case 1:              s();
 } while (--n > 0);
 }
 }
 }
 }
 void foo() { writefln("foo"); }
 
 void test()
 {
 int i = 1;
 int j = 11;
 mixin duffs_device!(i, j, delegate { foo(); } );
 duff_loop();	// executes foo() 10 times
 }

Jun 08 2009

Ellery Newcomer <ellery-newcomer utulsa.edu> writes:

BCS wrote:
 Hello Sam,
 
 To save time ,just get to my point:I do not understand why such code
 below inside the switch ... case statement is allowed ,what is the
 point of do ... while...:

 
 This is a classic C thing. The short explanation is that a switch is a
 conditional, many way goto and the labels can be *anywhere* in the body
 (except inside another switch). For the more in depth version Google
 "duff's device".
 
 This example uses a mixin to implement a generic Duff's device for an
 arbitrary statement (in this case, the arbitrary statement is in
 bold).

 
 BTW: where is this from?

I would guess from the D specs

http://www.digitalmars.com/d/1.0/template-mixin.html

Jun 08 2009

Sam Hu <samhudotsamhu gmail.com> writes:

Thanks so much for all your help!I am studying hard to *loop unrolling*.

 BTW: where is this from?

It is from the spec:
http://www.digitalmars.com/d/1.0/template-mixin.html

BTW,is this Duff?
http://groups.google.com/group/net.lang.c/msg/66008138e07aa94c

Jun 08 2009

grauzone <none example.net> writes:

 http://groups.google.com/group/net.lang.c/msg/66008138e07aa94c

Many people (even Brian Kernighan?) have said that the worst feature 

of C is that switches don't break automatically before each case label.

Oh god, that's from 1984, and even today we're struggling with this 
bullshit in the most modern dialect of C, D.

Jun 09 2009

BCS <none anon.com> writes:

Hello grauzone,

 http://groups.google.com/group/net.lang.c/msg/66008138e07aa94c
 

 Many people (even Brian Kernighan?) have said that the worst feature
 of C is that switches don't break automatically before each case
 label.
 
 Oh god, that's from 1984, and even today we're struggling with this
 bullshit in the most modern dialect of C, D.
 

I'm sorry, you don't have my sympathy on this one. There are to many place 
I've used fall throught to chuck it out.

Jun 09 2009

grauzone <none example.net> writes:

BCS wrote:
 Hello grauzone,
 
 http://groups.google.com/group/net.lang.c/msg/66008138e07aa94c

 Many people (even Brian Kernighan?) have said that the worst feature
 of C is that switches don't break automatically before each case
 label.

 Oh god, that's from 1984, and even today we're struggling with this
 bullshit in the most modern dialect of C, D.

 
 I'm sorry, you don't have my sympathy on this one. There are to many 
 place I've used fall throught to chuck it out.

What kind of fall-throughs were these?

A:

case value1:
case value2:
case valueN:
	code1();
	break;

B:

case value1:
	code1();
case value2:
	code2();
	break;

Jun 09 2009

BCS <none anon.com> writes:

Hello grauzone,

 BCS wrote:
 
 Hello grauzone,
 
 http://groups.google.com/group/net.lang.c/msg/66008138e07aa94c
 

 Many people (even Brian Kernighan?) have said that the worst feature
 of C is that switches don't break automatically before each case
 label.
 
 Oh god, that's from 1984, and even today we're struggling with this
 bullshit in the most modern dialect of C, D.
 

 I'm sorry, you don't have my sympathy on this one. There are to many
 place I've used fall throught to chuck it out.
 

 What kind of fall-throughs were these?
 
 A:
 
 case value1:
 case value2:
 case valueN:
 code1();
 break;

I don't do that, I go with this form:

case value1, value2, valueN:
  code1();
  break;


case B, The most usefull case was where I used the switch as a "jump into 
the middle of this block of code" device.

 B:
 
 case value1:
 code1();
 case value2:
 code2();
 break;

Jun 09 2009

Ary Borenszweig <ary esperanto.org.ar> writes:

grauzone wrote:
 BCS wrote:
 Hello grauzone,

 http://groups.google.com/group/net.lang.c/msg/66008138e07aa94c

 Many people (even Brian Kernighan?) have said that the worst feature
 of C is that switches don't break automatically before each case
 label.

 Oh god, that's from 1984, and even today we're struggling with this
 bullshit in the most modern dialect of C, D.

 I'm sorry, you don't have my sympathy on this one. There are to many 
 place I've used fall throught to chuck it out.

 
 What kind of fall-throughs were these?
 
 A:
 
 case value1:
 case value2:
 case valueN:
     code1();
     break;
 
 B:
 
 case value1:
     code1();
 case value2:
     code2();
     break;

The solution is to forbid fallthrough, and change the switch syntax:

switch(value) {
   case 1:
   case 2:
	// something
	break;
}

gives: Error, missing break at the end of case1.

But:

switch(value) {
   case 1, 2:
	// something
	break;
}

works as expected.

What's wrong with that?

Jun 09 2009