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From: roger@fluke.UUCP (Roger Ferrel)
Newsgroups: net.lang.c
Subject: Re: problem in type conversion
Message-ID: <1302@vax4.fluke.UUCP>
Date: Wed, 4-Jan-84 14:57:14 EST
Article-I.D.: vax4.1302
Posted: Wed Jan  4 14:57:14 1984
Date-Received: Fri, 6-Jan-84 04:17:32 EST
References: <1603@randvax.ARPA>
Organization: John Fluke Mfg. Co., Everett, Wash
Lines: 74

What we see here with the:

	inttype *= 0.5;
		vs
	inttype = inttype * 0.5;

is a weekness of most C compilers. It seems none of them handles float
operations very well when there are mixed types.  Of course the above is
a poor example since normally more efficent code would be generated by
using:

	intype >>= 1;  /* how is this for getting off the subject? */

Whenever coding in C I avoid floats and doubles as much as 
possible.  When using them in an expression I usually make sure to
explicitly cast everything to doubles and I tend to stay away from
the "fancy" assignment operators in mixed type expressions.

Thus for the first example I would code:

	inttype = (int) ( ((double) inttype) * ((double) 0.5));

I know it looks messy but many C compilers need the hints to generate
proper code.

Rules to follow:

	1) In an expression with mixed type explicitly cast operands to
	   biggest type in the expression since this will be the size
	   of the result. (Especially if the largest type is a double.)
	   Types largest to smallest are:

		double, long, int, short, char

	   Many C compilers convert smaller types to int before doing
	   anything with them in an expression.

	   Floats (single precision) should always be cast to double
	   since C supports double operations but not single.  To
	   find out how good your compiler is with mixed operands look
	   at the code produced for:

		 float f;
		 int   i;
		 f *= i;  /* f and i should be cast to doubles and the
			   * result reduced to single precision float
			   */

	2) Explicitly cast the result of an expression to the same type as
	   the target variable when the target is of smaller type then
	   the expression.  (This is being very conservative.)

	3) If the expression will be of smaller type then the variable
	   the result is assigned to cast each operand of the
	   expression to the type of the variable you are doing the
	   assignment to.  This will avoid unplanned trucation errors.
	   Example:

		int i;
		double d;

		d = (double) i / (double) 17;

Following these rules avoids the implicit cast bugs in many C
compilers.  In practice most compilers do a good job of casting as long
as you stick with the various types of integers.  When you start using
floats and doubles that is the time to follow the above rules closely.
-- 

    ------
    Roger L. Ferrel       (206) 356-5056
    John Fluke Mfg. Co.
    P.O. Box C9090  Everett WA  98206  
    {uw-beaver,decvax!microsof,ucbvax!lbl-csam,allegra,ssc-vax}!fluke!roger