Newsgroups: comp.text
Path: utzoo!sq!msb
From: msb@sq.sq.com (Mark Brader)
Subject: Re: ISC 2.0.2 troff broke? (groff as well?)
Message-ID: <1991Mar4.213407.16278@sq.sq.com>
Summary: In troff, positive, rather than nonzero, is true in .if's
Organization: SoftQuad Inc., Toronto, Canada
References: <1434@vidiot.UUCP> <1435@vidiot.UUCP> <1991Feb27.221722.2213@mtxinu.COM>
Date: Mon, 4 Mar 91 21:34:07 GMT
Lines: 38

"Vidiot" asked why

	.if \nG==1&\nI==0 ...

didn't work.  Jaap Akkerhuis diagnosed correctly that Vidiot had
forgotten troff's rather counterintuitive rule of no precedence,
and suggested the following (or intended to suggest it, anyway;
I have fixed a typo):

> What you want to do, is to put the test in parentheses as in
> 	.if (\nG==1)&(\nI==0) .tm condition is true

And this is right.  Jaap also observed that some people would say one of:

> 	.if \nI==0 .if \nG==1
> 	.if !\nI .if \nG .tm true

and that these are different if I and G are not really booleans.
However, he falls from grace in saying that the second one is
equivalent to the C code:

>	if( I != 0 && G == 0)

First, the conditions are reversed, but again, that's incidental.
The more important thing is this.  In troff, when an integer value is
treated as boolean, *only* a *positive* value is considered true.
This is counterintuitive for C programmers, who are used to any nonzero
value being considered true.

The last line of troff above is thus actually equivalent to *this* in C:

	if (I <= 0 && G > 0)
-- 
Mark Brader		"The default choice ... is in many ways the most
utzoo!sq!msb		 important thing.  ... People can get started without
msb@sq.com		 reading a big manual."		-- Brian W. Kernighan

This article is in the public domain.