Re: [Caml-list] float pretty-printing precision, once more.

[Brian Hurt <brian.hurt@qlogic.com>]

This looks just like rounding error to me.  I note that the string output 
has only 12 signifigant digits- and the error is, coincidentally enough, 
right about f * 10e-12.

But then, I'm not an expert in IEEE FP either...

Brian

On Tue, 10 Dec 2002 jeanmarc.eber@lexifi.com wrote:

> caml 3.06+1:  
>    
> # let f = 1. /. 86400.;;   
> val f : float = 1.15740740741e-05   
> # let s = string_of_float f;;   
> val s : string = "1.15740740741e-05"   
> # let f1 = float_of_string s;;   
> val f1 : float = 1.15740740741e-05   
> # f1 = f;;   
> - : bool = false   
> # f1 -. f;;   
> - : float = 2.59259844496e-17   
>   
> This situation may be understandable, but is unfortunate.  
>   
> Disclaimer: I'm not a specialist of the IEEE float format.  
>   
> Do I have at hand, at least on an architecture supporting the IEEE format, a  
> function that pretty-prints any valid float value (by valid I mean that I  
> exclude the "special" values like NaN, infinity, etc.) so that  
> float_of_string applied to the resulting string returns my initial value,  
> or, at least, a value that, if substracted from my initial one, returns  
> zero ?  
>   
> Background:  
>   
> In fact, my question goes a little bit further, as it concerns indeed the  
> parsing of floats in the caml compiler (that uses internally float_of_string  
> if I'm correct).  
>   
> Suppose you calculate somewhere (with an caml program, say) a float  
> constant (such a calculation may last for hours!), and you want after  
> obtaining the result to *generate* a caml source using this calculated  
> value. You will probably generate something like  
>   
> let my_const = <a float text representation>  
>   
> But my example shows that you are loosing precision and accuracy if you  
> just use string_of_float. 
>   
> Of course the goal is to incorporate this value in a caml source, not  
> to read it in binary form from a file (that would be easy!).  
>   
> Do anybody know a solution to my problem ?  
>   
> Jean-Marc Eber  
>   
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