[Python-Dev] Re: [Python-checkins] CVS: python/dist/src/Doc/tut tut.tex,1.133.2.1,1.133.2.2 (original) (raw)

Guido van Rossum guido@digicool.com
Mon, 21 May 2001 15:02:43 -0400

Actually, 31.400000000000002 wasn't a true improvement over the earlier 31.4: so long as we rely on the platform C to format floats, the output isn't well-defined (the last digit or so can and will vary across boxes).

I can't check right now, but I thought that this was pretty consistent across some common platforms?

I can certainly explain that this is so, and even why, but unsure the tutorial is the right place for it. In any case the tutorial shouldn't be giving examples whose output is platform-dependent. For example, don't use 10 * 3.14, use 10 * 3.25. Want me to scour the tutorial for all such cases?

Are you serious?

This is something that the newbie wou is in the least bit adventurous will run into anyway, so I don't think that not talking about this at all in the tutorial is fair or helpful. That just perpetuates the questions from newbies about "floating point is broken" -- since none of the tutorial examples prepare them for this.

Since this is behavior that is ordinarily observed and perpetually perplexing, I think it must be treated in the tutorial. The tutorial doesn't have to have the full explanation -- maybe it's enough to say something like ``due to round-off errors you will sometimes see inexact results like 31.400000000000002; don't worry about this, you can use str() or "%g" (but not round()!) to strip redundant precision, and here's a URL for more info.''

Or maybe the full story can be an appendix.

--Guido van Rossum (home page: http://www.python.org/~guido/)