Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!amdahl!pyramid!decwrl!labrea!polya!crew
From: crew@polya.STANFORD.EDU (Roger Crew)
Newsgroups: comp.lang.modula2
Subject: Re: The semantics of the for-loop
Message-ID: <2633@polya.STANFORD.EDU>
Date: 17 Apr 88 21:34:52 GMT
References: <5571@uwmcsd1.UUCP> <2455@pasteur.Berkeley.Edu>
Reply-To: crew@polya.Stanford.EDU (Roger Crew)
Organization: Stanford University Computer Science Dept.
Lines: 71
Summary: FOR <> WHILE in Modula-2

In article <2455@pasteur.Berkeley.Edu> seitz@cory.Berkeley.EDU.UUCP 
(Matthew Eric Seitz) writes:
> 
> _Programming_In_Modula-2_ essentially defines the for statement as
> the following equivalency
> 
> "FOR" ident ":=" expression1 "TO" expression2 [ "BY" ConstExpression ] "DO" 
> 	StatementSequence
> "END"
> is equivalent to
> 
> ident ":=" expression1;
> "WHILE" ident "<=" expression2 "DO"
> 	StatementSequence
> 	[ident ":=" ident "+" ConstExpression (* if "BY" clause used *) |
> 	 ident ":=" ident "+ 1" (* if "BY" clause not used *) ]";"
> "END"
> (* ident := undefined *)
> 

Ouch.

If _Programming_in_Modula-2_ says this,... 
well, it's a mistake, plain and simple.

Consider:

  FOR ch := 0C TO 255C DO
     whatever
  END;

which supposedly is equivalent to

  ch := 0C;
  WHILE ch <= 255C DO
     whatever
     INC(ch);
  END;

If you don't like playing with characters, try FOR i := -2^31 TO 2^31-1
(for 32 bit integers... i.e., MININT and MAXINT, or whatever they happen 
to be called these days...).

The WHILE loop will bomb at runtime when it tries to increment 255C.
If you were doing this with integers, you won't be this lucky.

(hint:  
in C, for(ch=0;ch<=255;ch++) is an infinite loop when ch is an 8-bit char).

You really do need that second IF statement (or something similar;
if you don't like writing the statement body twice, then you have to
go through contortions like

    ident := expression1;
    IF ident <= expression2 THEN
	LOOP
	    whatever;
	    IF ident = expression2 THEN EXIT END;
	    INC(ident);
	END;
    END;

And even this general form fails miserably for

	FOR ch := 0C TO 255C BY 2 DO
	    whatever
        END;

(unless, or course, you cheat by changing 255C to 254C)

--
Roger (crew@polya.stanford.edu)		``Beam Wesley into the sun!''