Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!rutgers!princeton!allegra!alice!ark
From: ark@alice.UUCP
Newsgroups: comp.sys.cbm
Subject: Re: C-Power bug ???
Message-ID: <6448@alice.uUCp>
Date: Mon, 8-Dec-86 12:33:42 EST
Article-I.D.: alice.6448
Posted: Mon Dec  8 12:33:42 1986
Date-Received: Mon, 8-Dec-86 21:28:55 EST
References: <1688@i.cc.purdue.edu>
Distribution: net
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 37
Keywords: C-Power, bug

In article <1688@i.cc.purdue.edu>, aig@i.cc.purdue.edu.UUCP writes:
> 
> 	I am writing a game in C-Power (please hold all applause until the end
> of the article) and have run into a bit of a problem.

The example given includes the following code:

> 		if ((j&1 == 1) && ((*pl1->y) > 50)) (*pl1->y)--;  /* up    */
> 		if ((j&2 == 2) && ((*pl1->y) < 226)) (*pl1->y)++; /* down  */
> 		if ((j&4 == 4) && ((*pl1->x) > 24)) (*pl1->x)--;  /* left  */
> 		if ((j&8 == 8) && ((*pl1->x) < 250)) (*pl1->x)++; /* right */

The trouble with this code is that the expression

		j&1 == 1

doesn't do what you think, because == binds more tightly than &.
Thus, this is equivalent to

		j&(1 == 1)

which, unfortunately, has the same effect for the wrong reason.
The trouble is that we have actually written

		j&1

because 1==1 evaluates to 1, so when we write

		j&2 == 2

we have actually written EXACTLY THE SAME THING.  No wonder
the results are confusing!  Might I suggest a rewrite:

	if ((j&1) && *pl1->y > 50) --*pl1->y;  /* up    */
	if ((j&2) && *pl1->y < 226) ++*pl1->y; /* down  */
	if ((j&4) && *pl1->x > 24) --*pl1->x;  /* left  */
	if ((j&8) && *pl1->x < 250) ++*pl1->x; /* right */