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From: tomc@oakhill.UUCP (Tom Cunningham)
Newsgroups: net.lang.c
Subject: Conversion, rounding, and truncation
Message-ID: <797@oakhill.UUCP>
Date: Fri, 31-Oct-86 12:01:23 EST
Article-I.D.: oakhill.797
Posted: Fri Oct 31 12:01:23 1986
Date-Received: Sat, 1-Nov-86 04:40:11 EST
Organization: Motorola Inc. Austin, Tx
Lines: 42

Well, my query about expression sequencing generated so much excitement
and discussion (and some dross), I thought I'd throw another catalytic
cookie into the ring.  Here is a C code fragment:

	double d;
	long l;
	float f;
	double pow();

	d = pow(2.0,3.0);
	l = d;
	printf("d = %f, l = %ld\n", d, l);
	f = d;
	l = f;
	printf("f = %f, l = %ld\n", d, l);

The output from code generated by the Sun 3 compiler (4.2BSD), Microsoft C,
and VAX VMS C is:

	d = 8.000000, l = 8
	d = 8.000000, l = 8

Output from a Unix SYSV compiler is:

	d = 8.000000, l = 7
	d = 8.000000, l = 8

Now, K&R clearly state on pg. 42 that float to int causes truncation of
any fractional part, and I assume this applies to the double-to-long case
as well.  Moreover, they say that double is converted to float by
rounding, which I guess would explain the second SYSV output line.

What I don't understand is why the formatting routines appear to round
the floating point number, since it looks like the pow() in the SYSV
implementation returns something not quite 8.0.  Am I missing something
here?  If this has already been discussed ad nauseum, someone please
clue me.

Tom Cunningham     "Good, fast, cheap -- select two."
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