How not to write a loop

[cuddy at convex.UUCP]

ok at quintus.Sun.COM (our feed trashes addreesses, that may not be right!)
> In article <832 at unmvax.unm.edu>, mike at turing.UNM.EDU (Michael I. Bushnell) writes:
> > I see NOTHING which precludes:
> > 	float x;
> > 	for (x = 0; x < 20; x += .5) printf("%f\n", x);
> > The output would, of course, be
> > 	0.0
> > 	0.5
> > 	...
> > 	19.5
> 
> Quite right, there is nothing in K&R (or dpANS) to prohibit this.
> But you have provided a good illustration of why people disparage
> the use of floating-point variables in loop control.
> THERE IS NO "OF COURSE" ABOUT THAT OUTPUT!
> 
> You should not be surprised to see as the last value
> 	19.99999

Only if you declare things as floats!

!! (from K&R:  2.4.4, pg. 181
!!   A floating constant consists of an integer part . . .
!!   . . .  may be missing.  Every floating constant is taken to be a double.
!!			     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

every "C" compiler (at least ones that conform to K&R)  will cast floats 
to doubles and sometimes they have troubles.  this has to do with your 
compiler and machine.  the above program segment ( for(x=0; x< 20; x+=5)... )
should probably be written:
	double x;
	^^^^^^  this will help you not get round off errors
	for (x = 0.0; x < 20.0; x += 0.5) printf("%f\n",x);
    		 ^^^ 	  ^^^^       ^^^
		 these don't have to be this way, but i've allways felt it
		 good programming practice.

> THERE IS NO "OF COURSE" ABOUT THAT OUTPUT!
There's lots of "of course"s  about "C" and that output if it's done right!

> I just tried the very similar loop
> 	for (x = 0; x < 21; x += .3)	/* .3 divides 21 exactly */
> By analogy with your "of course", the last output should obviously
> be 20.7.  In fact, when I tried it just now, the last output was
> 	20.999994

more float-double round off error.  This kind of thing bites you when you
use the -lm libraries without #including <math.h>. 

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The opinions stated above are barely my own, how can they be of my employers?
Mike Cuddy, 
CONVEX Computer Corporation
701 N. Plano Rd.
Richardson, TX  75081
uucp:{uiucdcs,ihnp4,sun,allegra}!convex!cuddy
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