Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1a 12/4/83; site rlgvax.UUCP
Path: utzoo!linus!vaxine!wjh12!genrad!decvax!ittvax!dcdwest!sdcsvax!sdcrdcf!hplabs!hao!seismo!rlgvax!guy
From: guy@rlgvax.UUCP
Newsgroups: net.lang
Subject: Re: Lexical analyzers and parsers
Message-ID: <1900@rlgvax.UUCP>
Date: Mon, 7-May-84 15:11:36 EDT
Article-I.D.: rlgvax.1900
Posted: Mon May  7 15:11:36 1984
Date-Received: Fri, 11-May-84 00:14:36 EDT
References: <1632@tekig1.UUCP> <174@ccieng2.UUCP>
Organization: CCI Office Systems Group, Reston, VA
Lines: 54

While we're on the subject of Fortran and lexical analysis, here's the worst
consequence of the "blanks are ignored" rule I've seen yet, which appeared
recently in net.unix-wizards:

> Newsgroups: net.unix-wizards,net.bugs.usg,net.lang.f77
> Subject: weird f77 bug; anyone seen this one?

> We are running USG 5.0 on a VAX 11/780.  Here is part of a subroutine
> from an f77 program that compiles properly:

> 	subroutine scale (buff2,f2,ctscl2,type2)

> 	character buff2*132,cont2*2,type2*4
> 	integer a2,d,e2,f2,ctscl2

> 	cont2 = ' 	'
> 	a2 = index (buff2,'scale')
> 	if (a2 .ne. 0) then
> 	  do 100 d = a2+5,len(buff2)
> 	    if (f2 .eq. 1) goto 200
> 	     if ((buff2(d,d) .eq. ' ') .or.(buff2(d,d) .eq. '	'))
>      &		go to 100
> 	     e2=d
> 	     f2=1
> 	     go to 100
> 200	    if ((buff2(d,d) .ne. ' ') .or. (buff2(d,d) .ne. '	'))
>      &		go to 100
> c	    i = strtok (ctscl2,buff2,cont2,'i')
> 100	  continue
> 	endif


> If, however, you change the name of the variable 'd' to 'd2' throughout,
> the compiler flags a syntax error on the 'do 100 ...' statement.

> Any guesses?

> Newsgroups: net.unix-wizards,net.bugs.usg,net.lang.f77
> Subject: weird f77 bug solved....

> The reason for this is the f77 extension which allows a double
> precision exponent, which is a multiplier base 10.  In other words,
> '1d2' equates to '100' (1 * 10^2).  f77, on the statement above,
> was squeezing out the space between the '100' and the 'd2' and
> parsing it as a double precision constant '100d2'; thus the
> syntax error.

How'd ya like that one, folks?  A program that, by all rights, *should*
be perfectly legal is made illegal by an unfortunate (but otherwise legal!)
choice of variable name.  ("e2" would have been just as bad, for the same
reason.)

	Guy Harris
	{seismo,ihnp4,allegra}!rlgvax!guy