Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: floating point multiplication BUG in C (cc compiler)
Message-ID: <1990Oct20.231726.3249@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <1348@banach.ACA.MCC.COM>
Date: Sat, 20 Oct 90 23:17:26 GMT

In article <1348@banach.ACA.MCC.COM> mishra@banach.ACA.MCC.COM (Aditya Mishra) writes:
>THIS PROGRAM DEMONSTRATES WHAT I BELIEVE IS A BUG IN FLOATING POINT
>MULTIPLICATION IN 'C' !! 

There is no bug.  You have discovered the joys of floating-point arithmetic.

>(1)   In the program, if float f = ... 0.64, 0.32, 0.16, 0.08, 0.04, 0.02, 
>	0.01, 0.005, 0.0025, ..., etc (see the pattern?), the value 
>	of i1 was one less than what it should be. 

Note that float->int conversion truncates, while %f conversion rounds.
You have come across boundary cases where this makes a difference, I'd
say.  Look hard and you will probably find more.

>(2)   Also, if float g = 1.0025, 1.32 or 9.0025 and some more values, 
>        the result in float r2
>	(r2 = factor * g) was wrong after 2 or 3 decimal places.

Wrong after the decimal point, you mean?  If so, no surprise:  `float',
as opposed to `double', often only has about seven digits of precision.
In general, you should use `double' for all floating-point arithmetic
you care about, unless speed or storage is critical and you have done
a careful analysis of the tradeoffs involved.
-- 
The type syntax for C is essentially   | Henry Spencer at U of Toronto Zoology
unparsable.             --Rob Pike     |  henry@zoo.toronto.edu   utzoo!henry