Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!ncar!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.fortran
Subject: Re: Dubious Fortran Construct
Keywords: DO loops; transfer of control
Message-ID: <14994@mimsy.UUCP>
Date: 14 Dec 88 08:26:11 GMT
References: <22994@sgi.SGI.COM> <3672@s.cc.purdue.edu> <1740@devsys.oakhill.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 52

Summary of problem:

	DO 10 I=A,B
	IF (cond) GOTO 10
	DO 10 J=C,D
	code
   10	CONTINUE
	more code

Background: the problem arises only because the scope of a DO loop
is named by a label that may be shared.  In other programming languages
one escapes by, in essence, forcing programmers to write the inner
loop using a different label.  Often the label is omitted altogether:

	do i in [a..b]
	    if cond then
		do j in [c..d]
		    code
		od
	    fi
	od

Claim: There is a simple and consistent way to view this problem that
makes the `goto' illegal.

Approach: since `the range of a DO loop includes the final statement',
one should view the code as having an invisible loop delimiter that
appears just *after* the labelled statement.  Thus the example transforms
to:

	DO I=A,B
	IF (cond) GOTO 10
	DO J=C,D
	code
   10	CONTINUE
	end do J
	end do I
	more code

That is, because the DO J loop used label 10 as its final statement,
its end appears just *after* the continue statement.  The `IF' therefore
jumps into its loop.

Note that this interpretation of DO-loops gives consistently correct
compilations of all other legal constructs.  It also seems to me
`intuitively proper'.  One cannot end the loop *before* the labelled
statement (since it must be executed if it in fact does something),
and it seems nonsensical to say that the loop ends somehow `inside'
the labelled statement, therefore it must end just after---`between
the lines', as it were.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris