how to break out early of a case ?

[MarkMLl]

Once we have support for compile time functions (the current term for them is “pure functions”) you can add that yourself

:-) OTOH I'd suggest that that specific case might be worth examining whenever anybody complains that his desired initialisation can't be coded... I'm not suggesting that some implementation be added prematurely, only that it might turn out to be a common "pattern".

MarkMLl

[PascalDragon]

Once we have support for compile time functions (the current term for them is “pure functions”) you can add that yourself

:-) OTOH I'd suggest that that specific case might be worth examining whenever anybody complains that his desired initialisation can't be coded... I'm not suggesting that some implementation be added prematurely, only that it might turn out to be a common "pattern".

No, it's not worth examining, because then the next one will come with the next function and so on. Better focus this into a more general approach.

Also if all you need is the size in bits then you can always use BitSizeOf.

[MarkMLl]

No, it's not worth examining, because then the next one will come with the next function and so on. Better focus this into a more general approach.

Also if all you need is the size in bits then you can always use BitSizeOf.

I didn't mean examining as in "we'll add a function" but as in "if you did it like this you wouldn't be asking us to add a function" :-)

MarkMLl

[Kaller]

Grave digging here but in simple cases where the condition is not deeply embedded then

Code: Pascal  [Select][+][-]

1. case I of
2.   somevalue :
3.   begin
4.     statement ...
5.     statement...
6.  
7.     if someconditionhere then
8.    begin
9.      ; // Do nothing
10.    else
11.   begin
12.     // Carry on
13.   end;
14.  
15.     statement ...
16.     statement....
17.   end;
18.  
19.   <more cases here>
20. end; { case }
21.  

Otherwise you could do something using functions, and/or raise an exception to the same effect.

[440bx]

After asking the question I figured a very simple way that is even more powerful and flexible than the C switch.  Something like this:

Code: Pascal  [Select][+][-]

1. while TRUE do
2. begin
3.   case SelectorVariable of
4.     1 :
5.     begin
6.       <statement>
7.       <statement>
8.  
9.       if somecondition then break;  { exits the case early }
10.  
11.       <statement>
12.  
13.       SelectorVariable := 2; { to cause fall through as in C }
14.     end;
15.  
16.     2 :  
17.     begin
18.       <bunch of statements here>
19.  
20.       break; { exit case statement }
21.     end;
22.  
23.     <more cases if needed >
24.  
25.     otherwise
26.     begin
27.       <statements>
28.  
29.       break;
30.     end;
31.   end; { case }
32. end; { while }
33.  

Note that instead of using TRUE, the while loop could use the SelectorVariable too and exit when it becomes a specific value, this could make some executions simpler.

As shown above, any case can carry out any test it wants and execute "break" which will end the loop, therefore the "case" too since they are now tied together.

The first case shows how to accomplish the C fall through.  Using the while loop enables full control of the execution of the each case, the sequence in which they are executed, fall through and, the number of time each case is executed.

Move over C switch, go to the corner and talk to Beethoven.

[MarkMLl]

Code: Pascal  [Select][+][-]

1.       SelectorVariable := 2; { to cause fall through as in C }
2.     end;
3.  
4.     2 :  
5.  

Does that actually work, and is it documented behaviour? My understanding was that the selector was evaluated only at the head of the case statement.

MarkMLl

[Eugene Loza]

the selector was evaluated only at the head of the case statement.

Indeed, that's my understanding too, and that's how it works for me in objfpc mode:

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. var
4.   A: Integer;
5. begin
6.   A := 1;
7.   case A of
8.     1: begin
9.          WriteLn('1');
10.          A := 3;
11.        end;
12.     2: WriteLn('2');
13.     3: begin
14.          WriteLn('3');
15.          A := 2;
16.       end;
17.   end;
18.   WriteLn('Done');
19.   ReadLn;
20. end.  

[Nitorami]

You are right, that puzzled me as well because the code of 440bx does work, surprisingly. The trick is in the "while true" loop which repeats the case selection.

[MarkMLl]

You are right, that puzzled me as well because the code of 440bx does work, surprisingly. The trick is in the "while true" loop which repeats the case selection.

The problem is that he's changing the selector, then implying that it falls straight into 2: in the style of C. I agree that wrapping the case in a while works (which is, after all, standard state-machine practice) but I'm very dubious about that claimed fallthrough.

MarkMLl

[Zoran]

You are right, that puzzled me as well because the code of 440bx does work, surprisingly. The trick is in the "while true" loop which repeats the case selection.

The problem is that he's changing the selector, then implying that it falls straight into 2: in the style of C. I agree that wrapping the case in a while works (which is, after all, standard state-machine practice) but I'm very dubious about that claimed fallthrough.

MarkMLl

You didn't understand the point -- in case "1", the selector variable is set to "2", then while loop continues and case "2" is executed in next iteration -- that is emulated fall-through; and that is what 440bx meant, not a real fall-through.

[Zoran]

Actually, I realize that I used a very similar trick (currently lines 148-182) to what 440bx shows, though I have never thought about it as a kind of "fall-through".

[Joanna]

This discussion seems very long 

I think having a lot of code inside a case statement can get kind of messy. Here is what I would do. I would move the code out of case into procedures.

Code: Pascal  [Select][+][-]

1. Case condition of
2. 1:procedure1;
3. 2:procedure2;
4. End;
5.  

Procedures are easy to break out of and doing so would make you exit the case statement as well.

[MarkMLl]

You didn't understand the point -- in case "1", the selector variable is set to "2", then while loop continues and case "2" is executed in next iteration -- that is emulated fall-through; and that is what 440bx meant, not a real fall-through.

Well he ought to write what he means then (no disrespect intended). A constant stream of "of course Pascal can do that..." statements which turn out to be not quite accurate does nothing to promote the language when read by the broader community.

MarkMLl

[MarkMLl]

This discussion seems very long 

I think having a lot of code inside a case statement can get kind of messy. Here is what I would do. I would move the code out of case into procedures.

Code: Pascal  [Select][+][-]

1. Case condition of
2. 1:procedure1;
3. 2:procedure2;
4. End;
5.  

Procedures are easy to break out of and doing so would make you exit the case statement as well.

That's fine in theory but not in practice. Consider the situation where the case statement is in a loop, and each case can do one of

* exit the current procedure

* break the loop

* continue at the head of the loop

* continue at the end of the case statement

As soon as you have to accommodate those you're back at the situation of having multiple lines of code in each case, you've really gained nothing except a few lines of boilerplate elsewhere.

MarkMLl

[Joanna]

i tried this and the continue seems to work fine from inside case

Code: Pascal  [Select][+][-]

1. h:= 1;
2.   while h < 5 do
3.        begin
4.        case h of
5.             1:begin ShowMessage('continue'); continue; end;
6.             2:break;
7.            END;
8.        inc(h);
9.        END;  

do you have any code examples to illustrate what you are saying ?