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Path: utzoo!watmath!clyde!burl!hou3c!hocda!houxm!hogpc!houti!ariel!vax135!floyd!cmcl2!rocky2!cubsvax!peters
From: peters@cubsvax.UUCP
Newsgroups: net.lang.c,net.bugs.4bsd
Subject: Re: i = i * f   vs.  i *= f
Message-ID: <205@cubsvax.UUCP>
Date: Sun, 15-Apr-84 15:12:31 EST
Article-I.D.: cubsvax.205
Posted: Sun Apr 15 15:12:31 1984
Date-Received: Tue, 17-Apr-84 08:00:26 EST
References: <866@elsie.UUCP>
Organization: Columbia Univ Biology, New York City
Lines: 28

Looks like you found a real bug!  on the 4.1bsd compiler, the following
program -- which compares i = i / f with i /= f -- gives the results indicated.

Obviously, the problem is that when the compiler encounters
"int op= float;" it casts the float to an int before doing the evaluation,
instead of promoting the int to a float, then casting the result.

According to K&R, p191, the two expressions "may be taken as equivalent"
(slight paraphrase), which is *very* misleading if this isn't a bug.  They
then say, "... however, [for "op=" ] E1 [left-hand side] is evaluated only
once."  This is opaque to me.  Can someone explain it?  I. e., does that
mean it's not a bug?  It's egregious [look it up!] in any case.

	main()
	{
		int	i;

		i = 100;
		i = i / 3.3;
		printf("i = i/3.3 gives: %d\n", i); /* gives 30, as expected */
		i = 100;
		i /= 3.3;
		printf("i /= 3.3 gives: %d\n", i);  /* gives 33!!!!   */
	}

{philabs,cmcl2!rocky2}!cubsvax!peters            Peter S. Shenkin 
Dept of Biol. Sci.;  Columbia Univ.;  New York, N. Y.  10027;  212-280-5517
"In accordance with the recent proclivity for clever mottos, this is mine."