Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site randvax.ARPA
Path: utzoo!linus!philabs!seismo!hao!hplabs!sdcrdcf!randvax!obrien
From: obrien@randvax.ARPA (Michael O'Brien)
Newsgroups: net.lang.c
Subject: problem in type conversion
Message-ID: <1603@randvax.ARPA>
Date: Fri, 30-Dec-83 20:20:11 EST
Article-I.D.: randvax.1603
Posted: Fri Dec 30 20:20:11 1983
Date-Received: Mon, 2-Jan-84 02:33:57 EST
Organization: Rand Corp., Santa Monica
Lines: 31

We think there may be a serious problem with type conversion.  Consider the
following program:

int i = 6, j = 6;
main() {
	i *= .5;
	j = j * .5;
	printf ("%d, %d\n", i, j);
}

This program will show i = 0 and j = 3.  The problem is that the ".5" in
the first line is converted to an integer before the multiplication takes
place, and in the second line, the multiplication is done in double precision
before the integer conversion occurs.

This seems to violate Kernighan and Ritchie, which states that

	E1 op= E2

may be regarded as identical to

	E1 = E1 op (E2);

when E1 and E2 are of arithmetic type, except that E1 is evaluated only
once in the first case.  That single difference does not seem to imply such
an egregious difference in type conversion.

We see this both in 4.1 and 4.2 BSD PCC, and in the Ritchie compiler for V7
on the PDP-11.  We don't have System III or System V compilers handy.

Any comments?